Housing - Renovation: how to insulate bay window and porch

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- CommentAuthormattp
- CommentTimeMar 4th 2025 edited
Hello, I'm really hoping some of you might be able to offer some advice on how to proceed with the refurb of a front room and hallway of a 1930s semi. I got myself into a bit of a spiral where one thing that needed doing led to another and another and so the job's been going on for over a year now, and I'm currently a bit stuck because I'm not sure how to put back and improve the insulation.

I had the floorboards up for a new water supply pipe to be laid under the suspended floor and bought all the materials to insulate between the joists with wood fibre batts and make it airtight. There was also a bit of damp at skirting board level in the bay and when I checked inside the cavity there was very damp insulation (shredded fibreglass) that seemed to have soaked up moisture from below the DPC. I ended up removing all the insulation from the cavity in front of the window (but nowhere else) and also rebuilding most of the wall because it was all loose. While I was there I also fitted telescopic vents to connect the external air bricks to the internal holes. I think I am ready now to insulate between the joists and seal to the edges of the room, but I haven't done it yet because I want to know how I will finish the job in case this affects how I should do it in some way.

For the bay window originally I was thinking about leaving the cavity empty and putting some sort of IWI because a few tradesmen have told me how bad CWI is, but from reading, it seems it might not be advisable to use IWI to the sides of the bay where I still have insulation in the cavity because of potential condensation. The CWI might not be great, but I'm reluctant to get it removed everywhere, so now I am thinking of maybe just filling with EPS beads in front of the window (and leaving the existing insulation in place). I would then just need to get the wall re-rendered. Is this a good plan?

The second problem I'm scratching my head over is in the corner of the room where I have now placed a vertical radiator. This is a single 4.5" brick wall, but the last 40 cm or so into the corner has an uninsulated porch on the other side. There is a small amount of mould at the bottom of the wall in that area which I suspect is due to condensation. How can I insulate in this area? I could possibly put some external insulation on the other side, but it would have to be thin because there is a door in the porch that I don't want to replace, so I have only got maybe 40 mm to play with, including any render. Is insulating internally possible / advisable? I could move the radiator, but I'd rather not.

I will attach a few photos so you can see what I'm talking about. Thank you.
- CommentAuthormattp
- CommentTimeMar 4th 2025
Photo of porch with arrow indicating the wall that needs insulating
- CommentAuthormattp
- CommentTimeMar 4th 2025
corner of room. The wall in the corner behind the radiator is opposite the unheated porch.
- CommentAuthorGreenPaddy
- CommentTimeMar 5th 2025
I'll start the ball rolling...

porch - def insulate. I would keep it simple, otherwise you'll spend another year considering. Pink foam adhesive a rigid insulation board of your choice onto the wall (say 25mm PU or PI), then pink foam a sheet of 9mm cement board over that. Takes you to your 40mm thickness limit. Add half a dozen concrete screw fixings, if you really want certainty, though the pink foam will not let go if the existing surface is any good. Fill any gaps, screww heads, and paint it. All DIY. No need for rendering professionals, who will not be interested in doing 2m2 of render.

Added to that, the "ceiling" of the porch is presumably the floor of the room above it. Do the same to it, especially if you can't get to that piece of floor from the inside easily. Plus, the clad area of wall above the door is likely poorly insulated (unless that's something you've done yourself). Strip that carefully, insulate to what could be 100m or more depth, and re-clad, or cement board over. Then that's one area you can forget about.
- CommentAuthormattp
- CommentTimeMar 5th 2025
Posted By: GreenPaddyI'll start the ball rolling...

porch - def insulate. I would keep it simple, otherwise you'll spend another year considering. Pink foam adhesive a rigid insulation board of your choice onto the wall (say 25mm PU or PI), then pink foam a sheet of 9mm cement board over that. Takes you to your 40mm thickness limit. Add half a dozen concrete screw fixings, if you really want certainty, though the pink foam will not let go if the existing surface is any good. Fill any gaps, screww heads, and paint it. All DIY. No need for rendering professionals, who will not be interested in doing 2m2 of render.

Thanks, GreenPaddy. I like simple. I assume it's fine to hack off the lime render from that external wall and stick the board directly to the brickwork? Also, I guess that there's no advantage to choosing a board designed for walls (eg Kingspan K5) and I can choose any foil backed PIR-type board since I will not be applying render?

Added to that, the "ceiling" of the porch is presumably the floor of the room above it. Do the same to it, especially if you can't get to that piece of floor from the inside easily. Plus, the clad area of wall above the door is likely poorly insulated (unless that's something you've done yourself). Strip that carefully, insulate to what could be 100m or more depth, and re-clad, or cement board over. Then that's one area you can forget about.

You are indeed right about the top of the porch and above the front door. I just didn't want to ask 20 questions all at once! The top of the porch is directly under the bathroom and also uninsulated. I do currently have some access from above because I had a lot of new wiring put in including a porch light with sensor. Insulating from below seems much simpler, but how you would approach it from above?

Above the front door is a timber frame, and on the outside is attached is a sort of metal lath which is rendered but it has been covered with the thin PVC cladding you can see. See photo below (I removed the plasterboard on the inside because I had the door raised by a couple of inches). I was thinking of attaching insulated plasterboard directly to the battens on the inside and somehow sealing the gap at the top to stop warm air from the bathroom sinking down behind it. Any suggestions welcome.
- CommentAuthorGreenPaddy
- CommentTimeMar 6th 2025
Bathroom floor - I would fill the floor joist depth (say 200mm) with knauf frametherm 32 wool, from the external front wall till it passes over the line of the door set below, and over the hall a bit (might just be a 400mm strip). I'm going to guess your floor joists run front to back, so your flr boards will run parallel with the front of the house. Lift a couple of those boards and carefully place the wool between the joists, cutting the strips 50mm wider than the joist spacing, so it is a snug fit.

Porch Soffit - you could leave the existing soffit, and insulate externally too, as mentioned above, but take off the PVC cladding, and butt the soffit insulation tight to the old mesh render of the stud wall above the door. If you can't get into the bathroom floor because there's a WC or basin pedistal with drains/pies etc, then break out the porch soffit (likely more mesh and plaster), push the wool into the floor void from below, then replace soffit with rigid insul and cement board over, just like the porch walls.

Door over-panel - Fill between the studs on the inside with the knauf wool as mentioned for the floor void. Looks like maybe 60-70mm only. Add the rigid insul board over the outside, then cement board over, fill, paint, just like rest of porch. I'd go thicker with the external insulation, say 100mm. Will give a visual thickness to the wall above the door, which I would suggest will look better too. Internally, you could then just plasterboard over, no need for insulate p/brd. Ideally you would fit a VCL membrane on the studs, over the wool insulation, then plasterboard. However, I suspect you'll have loads of other areas that condensation will go to first, and you won't get a good seal around the edges anyway, but it won't do any harm to fit a piece of VCL over that.

Porch walls - if stripping off the old lime render gets you a bit more depth to increase the insulation thickness, and it's easy to do, then go for it.

Generally, this is a small (but important thermally) part of the house envelope. I've suggested techniques that are not "breathable", but for such a small area, it won't impact the house vapour retention. You're doing vapour open insul to the floors, and likely the loft has loftroll that's vapour open too, so you've a huge area that's being maintained as vapour open. The key for these fiddly bits in my opinion, is just to crack on and get them done, and buttoned up tight.
- CommentAuthormattp
- CommentTimeMar 6th 2025
Posted By: GreenPaddyBathroom floor - I would fill the floor joist depth (say 200mm) with knauf frametherm 32 wool, from the external front wall till it passes over the line of the door set below, and over the hall a bit (might just be a 400mm strip). I'm going to guess your floor joists run front to back, so your flr boards will run parallel with the front of the house. Lift a couple of those boards and carefully place the wool between the joists, cutting the strips 50mm wider than the joist spacing, so it is a snug fit.

Joists actually run parallel to the door. The photo below shows the space above the porch looking away from the wall I want to insulate - you can see the vertical studs and the room below. I will fill this space with insulation, like you suggest. On the right of the photo there is a joist running close to the wall. There is small gap between this and the wall. Would you try to fill this with expanding foam? Or just leave it because I will also be insulating the soffit from underneath with a thick PIR board?

Door over-panel - Fill between the studs on the inside with the knauf wool as mentioned for the floor void. Looks like maybe 60-70mm only. Add the rigid insul board over the outside, then cement board over, fill, paint, just like rest of porch. I'd go thicker with the external insulation, say 100mm. Will give a visual thickness to the wall above the door, which I would suggest will look better too. Internally, you could then just plasterboard over, no need for insulate p/brd. Ideally you would fit a VCL membrane on the studs, over the wool insulation, then plasterboard. However, I suspect you'll have loads of other areas that condensation will go to first, and you won't get a good seal around the edges anyway, but it won't do any harm to fit a piece of VCL over that.

I have got a roll of VCL that I bought for doing the floor (Ampatex Solero) and tape that can be plastered over (Ampacoll Fenax) so I could put that under the plasterboard if that's suitable (I know VCLs come with different diffusion rates depending on the insulation)?

Generally, this is a small (but important thermally) part of the house envelope. I've suggested techniques that are not "breathable", but for such a small area, it won't impact the house vapour retention. You're doing vapour open insul to the floors, and likely the loft has loftroll that's vapour open too, so you've a huge area that's being maintained as vapour open. The key for these fiddly bits in my opinion, is just to crack on and get them done, and buttoned up tight.

I have to say, thank you a lot for all this - it is a big relief to get this advice. Indeed I wasn't looking too much at Cellotex and the like because I was worried about breathability. But now I can go ahead and get one area ticked off the list.

Regarding breathability - should I try to get someone to lime render under the bay window? I think I would struggle to get someone just to do that little bit, but maybe if I carefully removed the gypsum (but not the underlying lime render) in the rest of the room and asked them to patch up and lime plaster the whole room it might make it worth their while?

Finally, do you have any opinion on my plan to DIY fill the bay window cavity with EPS beads?
- CommentAuthorGreenPaddy
- CommentTimeMar 7th 2025
Hi Matt,

I've attached a quick sketch of a section through the head of the porch. Should hopefully explain how I would insulate that area, summary of discussions and comments above.

My take on your bay area would be to say that yes, I would fill the cavity with EPS beads, but understand how they will be retained (what voids are there where they can escape as you fill) around air vents, through gaps in the underbuild, etc. Seal those up first. If you manage to feed the PVA glue mix "mist" witht he beads this will be a bit less of an issue, but assume the beads will escape at every opportunity. You're lifting the floors anyway, to insulate, so beads fill at that stage, when you can see the whole bay area.

You've a cavity wall, so I'd assume the dew point is going to be on either of the internal faces of the brick onto the cavity. The EPS beads will tend to push that dew point to the inner face of the outer course. That's a benefit I'd say, and much better with EPS beads than the old 1970's candy floss, that soaks up water, as I think you described having removed.

Internally, I would want to add IWI, since the wall's been stripped anyway. If you want it to breathe, wood fibre boards, or battens with a glass wool (that's my preferred route, as I don't have experience with wood fibre boards, and wool goes to 0.032 lambda versus only 0.044 ish for wood boards). I'd do the battens with wool, and add a VCL. Improve the internal ventilation to remove vapour, rather than pushing it into the brick cavities. Again, good ventilation under the floors and loft, with be the main passive dehumidifiers for the house.

I'd add a laminated plasterboard to the window boarders, wall and reveals, as these are particularly weak points, and of no benefit in terms of breatheability being small areas.

I assume the room with the bay window is an external gable, as you're concerned about lime plastering it, though the houses opposite seem to have that as a party wall (bit of guessing here from photo backgrounds)? If you particularly want all walls lime plastered then go that route. If the walls are already gypsum skim plastered, then do that.

The above works greatly improves half of the front of your house, probably about 15% of your external wall area, which is a horrible thought with the rest still as existing. But the porch is a particularly naughty heat loss area, so a good place to get sorted.
- CommentAuthorGreenPaddy
- CommentTimeMar 7th 2025
I guess it didn't attach
- CommentAuthormattp
- CommentTimeMar 7th 2025
Posted By: GreenPaddyHi Matt,

I've attached a quick sketch of a section through the head of the porch. Should hopefully explain how I would insulate that area, summary of discussions and comments above.

Brilliant. Thanks for taking the time to do this. I see that you have drawn a wooden batten attached to the bottom of the stud wall above the door, to make it easier to attach the cement board to the reveal there, so I will add that in.

My take on your bay area would be to say that yes, I would fill the cavity with EPS beads, but understand how they will be retained (what voids are there where they can escape as you fill) around air vents, through gaps in the underbuild, etc. Seal those up first. If you manage to feed the PVA glue mix "mist" witht he beads this will be a bit less of an issue, but assume the beads will escape at every opportunity. You're lifting the floors anyway, to insulate, so beads fill at that stage, when you can see the whole bay area.

You've a cavity wall, so I'd assume the dew point is going to be on either of the internal faces of the brick onto the cavity. The EPS beads will tend to push that dew point to the inner face of the outer course. That's a benefit I'd say, and much better with EPS beads than the old 1970's candy floss, that soaks up water, as I think you described having removed.

Yes, I have removed all the fluffy shredded fibre glass type stuff from in front of the bay window - it was pretty damp towards the bottom. Either side of the bay I left the insulation in place, but removed it from below internal floor level. To prevent more insulation from above falling down I inserted a length of gutter brush (for preventing gutters from clogging) either side horizontally just below internal floor level. This was a recommendation made to me by a bricklayer who had a look at the internal wall of the bay (in the end he didn't want the job of rebuilding it for me). There are two air bricks in the outside wall and I have fitted plastic periscope type sleeves so the air flow won't be restricted by any cavity fill. The cavity to the left is a dead end because the porch is there, but the cavity to the right goes right through into the neighbour's side. So I would probably have to shove something that doesn't soak up water underneath the gutter brush on that side to block up that hole.

I like the idea of mixing glue with the beads so as to emulate a professional fill, but I'm not sure how I'd do that in practice. I suppose I could put very dilute PVA in a spray bottle and spray a large bucket full of beads while stirring?

Internally, I would want to add IWI, since the wall's been stripped anyway. If you want it to breathe, wood fibre boards, or battens with a glass wool (that's my preferred route, as I don't have experience with wood fibre boards, and wool goes to 0.032 lambda versus only 0.044 ish for wood boards). I'd do the battens with wool, and add a VCL. Improve the internal ventilation to remove vapour, rather than pushing it into the brick cavities. Again, good ventilation under the floors and loft, with be the main passive dehumidifiers for the house.

I was thinking of doing IWI from floorboard to ceiling level, but was a bit worried because I had read in some places that you can in some cases get condensation build up behind the insulation if you get it wrong. However battens with glass wool sounds very straightforward and doable. With your method do you simply attach plasterboard to the battens? I looked at wood fibre but it seems to need lime plaster.

Regarding passive dehumidification, my other half is dead set on putting down vinyl plank flooring. Do I need to see again if I can persuade her otherwise?! The loft is extremely vapour open (not at all airtight) and poorly insulated at the moment, but that is another story.

I'd add a laminated plasterboard to the window boarders, wall and reveals, as these are particularly weak points, and of no benefit in terms of breatheability being small areas.

Is laminated plasterboard another name for PIR backed plasterboard? I was considering hacking off the lime render from the sides of the windows to make a bit of space for insulation and to reduce cold bridging. By wall, which bit are you referring to?

I assume the room with the bay window is an external gable, as you're concerned about lime plastering it, though the houses opposite seem to have that as a party wall (bit of guessing here from photo backgrounds)? If you particularly want all walls lime plastered then go that route. If the walls are already gypsum skim plastered, then do that.

The wall is indeed a party wall. I was just talking about lime plaster because I have heard it is better for breathability and because I quite like the idea of putting things back how they were. I have no direct experience of the benefits of lime plaster though and I'm not wedded to the idea of getting it done. I also worry about getting someone in - all the plasterers I know use gypsum. I checked again and the lounge currently has lime render + lime plaster on 3 walls (but, I think, with vinyl paint). The party wall has been cement rendered.

The above works greatly improves half of the front of your house, probably about 15% of your external wall area, which is a horrible thought with the rest still as existing. But the porch is a particularly naughty heat loss area, so a good place to get sorted.

It's going to take years (hopefully not that many), but I think it will be worth it!
- CommentAuthortony
- CommentTimeMar 8th 2025
I blow the roof let full of eps beads , 20 mins , done
- CommentAuthorGreenPaddy
- CommentTimeMar 8th 2025
Hopefully I'll catch all your questions...

- you mentioned you're using wood fibre batts to insulate between the grnd flr joists. That's great being nice and vapour open, but a vinyl floor on top kind of defeats the point as you'll lose any vapour movement to the floor void. I guess the floor boards aren't good enough to relay and keep them bare? Or fit new boards? If you use the vinyl, you will retain a lot more vapour in the house, and need to find another means to compensate for that. Going to a lot of trouble and expense to make walls breathable, but then sealing the floors...??? Hopefully you're putting a good depth of insulation under the floor. I normally use glass wool in VPM hammocks, to 300mm depth, so the joists get insulation squished around them. Keeps the timber warm, and stops condensation on the under side of the joist.

- I've not done it, but for a small volume of beads that you'll be using for that bay cavity, you could do the spray bottle thing, and see if it works. It will be better than nothing.

- the area around the windows is the "reveal" which butts against the window frame, and then there's the rest of the wall that is parallel with the road. Both look to be only 250mm. You'll struggle to add timber battens to such a small area, as it will be all timber and no insulation. So for that, I'd go for the laminate p/brd, which is the combined insulation and p/brd. That can have EPS/XPS/PU as the backing. For a small area like that, I'd go for the PU version, and get the best insul property. You'll need to match the thicknesses of the laminate insul board with the timber battens and p/brd. If there's a slight mis-match, you can thicken the adhesive for the laminate board, or pack the timber battens, or get a larger size of batten and rip it down, or use a 12.5 or 15mm plasterboard. Lots of ways to adjust.

- breatheability should be done for a reason. There won't be vapour transfer through the party wall, unless that's an open vented cavity?? The lime render can absorb and release vapour as a humidity buffer, so that is sometimes stated as a reason to do it. Decide if that's useful to you.

- in terms of condensation, and damp cavity faces, walls get damp in winter and dry out over the summer, as long as there isn't too much vapour arriving. I'd prefer to see vapour being removed passively through floors/attics or mechanically, and not challenge the walls. We can do all the calcs in the world, but strange things can happen (cold draughts, bridgining, weird bricks that soak moisture like a sponge). If you don't currently have a condensation issue, the insulation items we've discussed won't make that worse. The vinyl floor covering however will, but I wouldn't begin to try to guess if it would tip the balance.

- you could, (and building regs would like say you should) add a layer of VCL over the battens, before fitting the plasterbrd. That will theoretically reduce the quantity of vapour reaching the bricks. How well you could achieve that on a piece of wall, I don't know. It would be the same process as the area above the doors. I normally create a service void over the VCL layer, by adding 25mm timber battens. Again, we need to do what's appropriate, and for a small area, where you won't want the added wall depth to be too much, I wouldn't bother.
- CommentAuthormattp
- CommentTimeMar 10th 2025
Posted By: GreenPaddyHopefully I'll catch all your questions...

- you mentioned you're using wood fibre batts to insulate between the grnd flr joists. That's great being nice and vapour open, but a vinyl floor on top kind of defeats the point as you'll lose any vapour movement to the floor void. I guess the floor boards aren't good enough to relay and keep them bare? Or fit new boards? If you use the vinyl, you will retain a lot more vapour in the house, and need to find another means to compensate for that. Going to a lot of trouble and expense to make walls breathable, but then sealing the floors...??? Hopefully you're putting a good depth of insulation under the floor. I normally use glass wool in VPM hammocks, to 300mm depth, so the joists get insulation squished around them. Keeps the timber warm, and stops condensation on the under side of the joist.

What you are saying makes sense - it is a shame to do this work and then seal the floor so maybe I need to reconsider this part. A lot of the boards are a bit damaged, some are split and some have holes / notches that are no longer required, so quite a few would need to be replaced if it's the final surface. Part of the attraction of the vinyl was that it would be an easy maintenance with no need to sand and oil/varnish every few years. Would say a 12 mm or so tongue and groove engineered floor laid on top of the boards be a better option or would that still act as a significant barrier in your view?

I have bought insulation for only 100 mm insulation to fill to the joist depths. The suggested way by the company I bought the wood fibre from was to stretch a breathable membrane tight, with light battens at the bottoms of the joists then lay the batts, then cover with a slightly less breathable membrane. I am not opposed to putting more in while I have got everything up but would need think about how to support it from underneath. With your method how do you work around the sleeper walls?

- the area around the windows is the "reveal" which butts against the window frame, and then there's the rest of the wall that is parallel with the road. Both look to be only 250mm. You'll struggle to add timber battens to such a small area, as it will be all timber and no insulation. So for that, I'd go for the laminate p/brd, which is the combined insulation and p/brd. That can have EPS/XPS/PU as the backing. For a small area like that, I'd go for the PU version, and get the best insul property. You'll need to match the thicknesses of the laminate insul board with the timber battens and p/brd. If there's a slight mis-match, you can thicken the adhesive for the laminate board, or pack the timber battens, or get a larger size of batten and rip it down, or use a 12.5 or 15mm plasterboard. Lots of ways to adjust.

Thanks for the explanation. The wall either side is around 300 mm, but I think your point still stands. So I would probably hack off the render from the reveals to make space for the laminated boards, patch up the two walls with maybe bonding to give an even surface to glue insulated boards. With the bay I would batten, fill with glass wool, then plasterboard. I assume you would glue a thin strip of plasterboard to the edge of the insulation where the two boards meet between the wall and the reveal?

- breatheability should be done for a reason. There won't be vapour transfer through the party wall, unless that's an open vented cavity?? The lime render can absorb and release vapour as a humidity buffer, so that is sometimes stated as a reason to do it. Decide if that's useful to you.

The party wall seems to be two bricks thick with no cavity. I was talking about getting the room lime plastered mainly in order to make it worth their while for a plasterer to patch up the lime render while they are there. From what you have said it seems I need to be thinking more about the floor than the walls.

Once again, thanks for your detailed advice.
- CommentAuthormattp
- CommentTimeMar 10th 2025
Posted By: tonyI blow the roof let full of eps beads , 20 mins , done

At the moment I have no felt under the slates and I suspect a lot of gaps round the eaves, so I might not be popular with the neighbours if I did this!
- CommentAuthortony
- CommentTimeMar 10th 2025
They tend not to come out after installation, big gaps in the eaves need to be sealed, the ones I have done didnâ€™t have sarking felt either
- CommentAuthorGreenPaddy
- CommentTimeMar 11th 2025
Floor finish
- I don't know how vapour open engineer'd flooring is. Same as plywood I guess. Better than vinyl of course.

Floor insulation
- for all the effort of lifting and relaying a floor, I'd want a LOT better than 100mm of wood fibre. I'll add a part drawing of the system I use. Can be done from under the floor, if there's space. If done from above, then drape the membrane over the joists, not just up the sides. I use a board cut to the joist gap width, and use it to push the membrane to the correct depth repeatably (like a template/former). Then staple. I've shown filled with wool, but since you've already got the wood fibre, I think I'd ut the wood board on the bottom of the "hammock", making sure each board was touching it's neighbour, then fill on top with 200mm wool. It's not ideal, but you are where you are.

Laminate boards
- when making an external corner with the laminate board, cut off a slice of the insulation board from the plasterboard, so the P/brd projects past its insul board, over the full length of the edge where it will meet the other board. That projection shd be the depth of the board+insul that it will meet. That gives a p/brd to p/brd external corner. I'd run a bead of pink foam up that edge to get a nice firm joint, and minimise air passage.
- CommentAuthorGreenPaddy
- CommentTimeMar 11th 2025
U/flr insulation detail
- 75 Insulation details W2.pdf
- CommentAuthormattp
- CommentTimeMar 11th 2025
You've given me a bit to think about. I had previously considered replacing all the floorboards with ~ 20 mm thick engineered wood, but that gets complicated round the stairs in the hall because they are attached to them!

For the floor insulation I think I will do what you suggest and try to fit more in. I need to check whether I have got enough depth to allow airflow under 300 mm insulation as shown in your document. I also need to think about how to connect some sort of periscopic sleeve to the periscopic sleeves I have already got poking out under the floor. I would have to do it from above because there are sleeper walls in the way preventing access from below. I guess the membrane will be a bit crinkled where it goes up over the sleeper wall plates. I suppose that as long as I overlap the membrane I don't need to worry too much about air gaps? Stapling seems like an easier way to go than battening - I'll have to hire a stapler.

Thanks for the laminated plasterboard tip. I was imagining doing an external mitre join which would obviously be really hard to get right!
- CommentAuthorGreenPaddy
- CommentTimeMar 12th 2025
Sleeper walls - run the membrane from wall to wall. There's no need to bridge with the membrane. Just fill the gap at the wall head with wool. If you don't have enough depth, you can use a more dense wool (frametherm 32), which is 25% better than the regular loft roll 44. Let's say the floor joist is 175 deep, put the wood fibre board in to go just 100mm below the joist, then add 2 layers of 90mm FT32.

Periscopes - could you add a photo of what you've done for the vent periscopes please? I sometimes just form a tunnel with rigid board off-cuts and foam, to direct the air downwards into the void, as the vent openings in the masonry aren't always suitable to take the off-the-shelf plastic periscopes. Plus it adds insulation to the coldest spot which may be close to the finished floor.

Stapler - just buy one of these
https://www.toolstation.com/stanley-heavy-duty-staple-gun/p52027
If you're draping OVER the joists, you won't need much to hold it, just enough to stop it moving as you insert the insulation. If membraned to the sides only, add the felt tacks, as detailed on the drwg.

Flooring - you'd need to check that an engineered finished floor board can replace the existing "structural" floor board. I would always lay the eng'd board over the original flooring. Is relaying the original flooring (albeit with replacement boards where damaged) not an option? Yes you'd need to sand and oil when first laid, but I'm not sure you'd repeat that every few years. More like 10 years, or never. If there's a damage, or highly traffic'd area, a very light sand and brush on a coat of oil is very easy (note - oil not varnish)
- CommentAuthordjh
- CommentTimeMar 12th 2025
For floor coverings you might consider lino instead of vinyl. Similar in many ways but more breathable. Not quite as many patterns unfortunately.
- CommentAuthormattp
- CommentTimeMar 12th 2025
Periscopes - Thanks, I'm away from home at the moment but I will take some photos on Saturday. Boxing them in with rigid insulation boards seems like a good idea to me.

Sleeper walls - the membrane will run across the joists parallel with the sleeper walls (there's a wall roughly every 1 m). Some guides for doing this advise making the breathable membrane airtight and taping it along the walls and overlaps to prevent wind wash, while others only emphasise making the VCL on top airtight. Either way, since my joists are only 95 mm deep, the insulation will drop down below the level of the damp proof course on the sleeper walls and on the walls of the room. I don't know whether it's a problem to have the insulation touching this very slightly damp brickwork.

I have been thinking also about possibly a slight modification of your method - what about attaching a bit of 2x1 to the underside of the joists? These could be held at the right depth by using M10 threaded rods with screw on backplates. Then I would be able to attach the membrane taught between the bits of wood between the two joists. If there is enough slack in the membrane on the vertical sections and the insulation is wide enough it would still squash around the joist from either side. Maybe I am worrying about a non-existent problem with this, but I am thinking about avoiding any sagging over time.

Floor - I can see oiled floor boards would be the best in terms of breathability. I'll have to discuss with my partner and see what she thinks. I'm glad you don't advocate replacing the structural floor with engineered wood - it would be a hell of a game (and beyond my capabilities) to replace the wood under the stairs.
- CommentAuthorGreenPaddy
- CommentTimeMar 13th 2025
That's a lot of sleeper walls, every 1m. I don't think it changes the approach however. I suspect you may be overthinking this whole thing, and taking to heart every potential detail you've read on the web. Get the membrane looped over the joists (wall-to-wall), get the insulation suffed in there, and get on with the fun things in life, knowing you've a lovely warm floor.

If you're still worried about draughts getting past the first VPM, and the wool, and the bats, then you could put another layer of VPM on top of the joists, and tuck it up the back of the skirting boards. It will help, and not be detrimental, but think diminishing returns.

Unless you increase the air change rate (vapour removal from the house) I don't advocate a VCL on the floor. I've built and refurb'd houses and commercial buildings from fully breathable old stone, thru to air tight, 3G, with MVHR. Choose an approach and stick with it. You shouldn't pick and choose what sounds best from all of them.
- CommentAuthormattp
- CommentTimeMar 13th 2025
I'm trying to avoid too many mistakes I might later regret but you're probably right about me overthinking things.

I already bought some membranes when I bought the insulation. I have a more vapour open one for the bottom (sd is 0.04 m) and more closed one for on top (sd = 5 m). Do you think the top membrane will close things up too much because I don't have mechanical ventilation? I guess a lot of guides on the web / suppliers are geared towards renovation to near passivhaus standards?
- CommentAuthorGreenPaddy
- CommentTimeMar 14th 2025
The SD of 5 is still relaitvely vapour open, so probably fine for the inner layer, per the supplier's recommendation. It's very hard to say exactly what will happen within any paticular house, without lots of modelling beyond most peoples ability, as there are so many variables, but in simple terms, if you make it harder for the vapour to exit over very large areas, that were previously very vapour open (through carpet and leaky floor boards), the vapour will have to seek other routes, which may concentrate the vapour in localaised areas, unless there is another mechanism to reduce the Absolute Humidity.
- CommentAuthormattp
- CommentTimeMar 17th 2025
Hi GreenPaddy, here are some photos of the lounge showing the periscopes I fitted. There is one in the bay and another tucked right in the far corner. The top of the vents is not far from the top of the joists (50 mm in one case, 80 mm in the corner). With hindsight it would have been better to remove bricks to have the vents a couple of courses lower, although this would have been a little tricky due to the sleeper walls in the way. I suppose the best I can do here is box them in with 3 bits of insulation board and adhesive foam?

The gaps between the sleeper walls is actually more like 1.2 - 1.3 m.
- CommentAuthormattp
- CommentTimeMar 17th 2025
Vent in corner against the party wall.
- CommentAuthormattp
- CommentTimeMar 17th 2025
whole room with floorboards up.
- CommentAuthorGreenPaddy
- CommentTimeMar 17th 2025 edited
I'd use 50mm kingspan (or similar, but not EPS/XPS as mice love it) to form a box around the vent, creating a tunnel to pass over the sleeper wall, then turn down to point towards the solum. You can buy the plastic periscopes sleeves, but you need to do the same thing with the kingspan outside of that anyway, so not that useful in this instance. It will be a weakish spot, having only 50mm air-to-floor board, but you can only do what you can do. The corner one is a sod, but just be creative with your LEGO building, and get past the joist.

Really simple to drape the VPM and fill with your insulation, as you've total access. Use 1200/1300 wide strips, fix the first end, push the wood fibre onto the VPM, pushing it down, till it slips under the joist. Pull the membrane tight, whilst holding the woodfibre board horizontal. Then staple the loose end to the adjacent joist, and continue. the woodfibre boards would be cut joist gap PLUS joist thickness, so they are butting to each other under the joist. Then Knauf FT32 wool on top. Other brands avail of course, but I find Knauf the best to work with in terms of itchyness. There's also Superglass, which is cheaper but itchyer. That said, it's below you, not over head, so maybe the cost saving is worth it.

There look to be vent gaps in the dwarf/sleeper walls, that are a brick plu wall plate below the u/s of the joist, so you'll not block air flow across the floor, with 100mm woodfibre board under the joists.

Each dwarf wall clearly limits the insulation depth to 100mm, along the top of the wall. You could just run the FT32 wool over that, but when I do that (or often clients choose to do it themselves), I always use kingspan for that bit, foamed in place. So, you'd have 4 runs of kingspan, 100mm thick, and say 120mm wide, so the wool butts nicely against it. At the walls, butt and foam the kingspan to the face of the brick walls (external and internal walls.

You could also slip some rigid insulation down between the outer dwarf walls and the house walls (both exterior and interior walls). Maybe go down say 200mm below the top of the dwarf wall. You could slice some of your 100mm woodfibre boards, and foam them in place. Or, ideally use up your off-cuts from making the vents. If you get a sheet of 50mm kingspan to make the vent boxes, you'll likely have enough to take 200mm rips of it to slide down vertically.

Think I'm seeing some MDPE water pipe under there too. That's you're most tricky problem. Ideally you'd have that inside the new insulation layers, but going to be tricky. Otherwise, you need some thick pipe insulation. This stuff is not tasty for vermine.
https://www.pipelagging.com/rockwool-rocklap-1m-foil-backed-pipe-insulation-lagging
Select pipe ext diameter, and I'd go for 50mm wall thickness. It has self adhesive foil wrap, but need to tape the butt joints.
- CommentAuthordjh
- CommentTimeMar 17th 2025
That whole area looks dry enough that it might be worth considering @fostertom's scheme of just filling the whole underfloor space with EPS beads or similar and blocking off the ventilation.
- CommentAuthorGreenPaddy
- CommentTimeMar 18th 2025
Whilst filling with EPS beads is very likely a good solution functionally, there are issues...

- you'd be contravening building regs, unless you can prove with certifiable data that it can be an alternative.
- assuming you don't have that evidence, your mortgage or future house sale could be at risk
- you'll get a shock at the cost of the beads, for that volume (a lot of the cost is the shipping as bulky)
- the volume of vapour removal from the house would be reduced, as there'd be no ventilation sub floor, so back to the earlier discussion re. alternative vapour removal systems.

Whilst I dislike the thought of suspended floors, and cold air blowing around under the insulated envelope, be careful about solving one problem, only to create another.

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