Hello! We are preparing to self-install (or contract out) a MVHR retrofit in the cold loft of our bungalow, but we have a few questions for the prep stage!

Quite a lot of our electrics are in the loft& attached to the current boards. These will be removed for insulating, so we aim to attach the cables to the floor joist sides instead.

Should the order of layers go:
1) membrane > insulation up to joist level > layer of ducting with insulation around it above joists > loft boards

2) insulation up to joist level > membrane across joists > layer of ducting with insulation around it > loft boards

3) ducting with insulation around within and above joists where necessary > membrane above joists and ducting ( > potentially a layer of insulation above joists and ducting so the membrane is on the warm side?) > loft boards

4) something else? We’re open to any alternative ways as we’re not sure which is correct!

We are also not sure if we need an intelligent membrane, VCL or just airtight? For reference, the joist depth is appx 8cm and the roof is ventilated at the eaves. The floor was sealed with VCL over the subfloor but under a plywood base, and the house has been replastered/painted (still some new cracks though!).


Thank you!

Hi Oliviaaa99,

Thanks for starting a new thread. It makes your needs much clearer.

One worry - reference to 80mm joists and insulating between them. (Or maybe I have misunderstood). You are required by the Building Regs to achieve a U value of 0.16W/m2K (which is reckoned to be about 270mm of 'typical' 'fluff' insulation with a lambda value (thermal conductivity) of around 0.044W/mK (note that's W/mK, not W/m2K, if you are not familiar with the units). I'd say you want 300 - 400mm (although of course that reduces as you reach the eaves - half the reason for having more where you can. Note also that your proposal re the cables will leave them 'tucked up warm' in insulation if you are using enough. That's OK if you 'up-size' your cables to allow for the heat they will be prevented from losing. Either that or you have to down-size the breakers so the cables cannot get over-warm.

You are planning a heat exchange in a cold space. I don't know how thick is the insulation (ask for the insulation value) on the insulated ducting, but pre-insulated solar water heating pipe I have seen before had a disappointingly small thickness.

If the MVHR unit is going to be in the loft void I would advise you (as I have advised many others) to build an 'MVHR 'shed' - an insulated 'building-ette' for the unit.

Posted By: djhWith MVHR running you won't need to worry about humidity building up in the house.

The MVHR ducts are full of air at room temperature, so ideally they want to be within the thermal envelope. That means putting them on top of the ceiling and then at least 300 mm of insulation above them as Nick mentions. As he also says, building a large insulated box to put the MVHR unit in will be sensible. Don't forget to think about changing the filters regularly. The intake and exhaust ducts can be run without any extra insulation, if they are themselves insulated ducts.

The electric cables want to be outside the insulation (above or below), or increased in size to allow for derating. Perhaps easiest is in conduits on top of the insulation.

If the loft is 'cold' (i.e. ventilated to the outside) then you'll want a membrane on top of the insulation to keep the draught off it. You may want another membrane below it as part of your airtightness.

Posted By: Nick ParsonsHi Oliviaaa99,

Thanks for starting a new thread. It makes your needs much clearer.

One worry - reference to 80mm joists and insulating between them. (Or maybe I have misunderstood). You are required by the Building Regs to achieve a U value of 0.16W/m2K (which is reckoned to be about 270mm of 'typical' 'fluff' insulation with a lambda value (thermal conductivity) of around 0.044W/mK (note that's W/mK, not W/m2K, if you are not familiar with the units). I'd say you want 300 - 400mm (although of course that reduces as you reach the eaves - half the reason for having more where you can. Note also that your proposal re the cables will leave them 'tucked up warm' in insulation if you are using enough. That's OK if you 'up-size' your cables to allow for the heat they will be prevented from losing. Either that or you have to down-size the breakers so the cables cannot get over-warm.

You are planning a heat exchange in a cold space. I don't know how thick is the insulation (ask for the insulation value) on the insulated ducting, but pre-insulated solar water heating pipe I have seen before had a disappointingly small thickness.

If the MVHR unit is going to be in the loft void I would advise you (as I have advised many others) to build an 'MVHR 'shed' - an insulated 'building-ette' for the unit.

Posted By: GreenPaddyA few comments, thoughts:

- you need as much insulation above the ducts as you would have for your best external wall/roof/floor, as all the warm air of the house flows in these ducts, and can quickly lose heat, dropping cold air into rooms, or failing to warm the fresh air intake. So one layer of loft roll over it, for example, won't do it.

- you mention the ceiling joists are 80mm deep. That sounds very thin, even if they're fink trusses.

- are you hoping to improve the air tightness of the ceiling? If it's a ventilated attic, ie. cold air above the flooring, used for storage of the Christmas tree, then I would not fit a VCL. Use a breathable airtightness membrane, and let the humidity from the house pass into the attic (appreciate the absolute humidity will probably be reduced by virtue of the MVHR).

- was there any sort of air tighness membrane fitted in the walls, when the initial work was done. Plaster skim? What about services penetrations in the walls? Sockets, light switches, water pipes, drains from sinks. Only asking from the viewpoint of walls being an air passage up the the attic (are they plasterboard, or plaster directly on blockwork.

- are you able to access the attic joists at the eaves perimeter, as the void above the plasterbrd ceiling is where the air will tend to escape, once a membrane is laid over the 95% of the middle floor area?

thank you so much everyone - this is really helpful! I'll leave my responses to your questions below:

- We are planning on using loft legs (to create a gap above the insulation as I believe I read that this is necessary?) then overboarding, so the draught shouldn’t be a problem! Not fully sure how to do this at the eaves though, as the vents are floor level?
- Thank you so much for the input r.e electric cables - completely missed the issue of having them within insulation! We will speak to the electrician about how best to fix this, whether it’s conduits or having them above the overboarding somehow.
- We are planning on building an insulated shed for the MVHR unit that’s accessible for the filters
- Really helpful input r.e the amount of insulation for the ducting, we can use as much as necessary since due to the loft legs we will have flexibility in the height for the overboarding.
- I had my partner take a photo of the joist depth, I’m not sure if I’m just misunderstanding it but I’ll attach a photo of it and the eaves perimeter too.
- We sealed as much as we could during the initial work stage, sealing sockets/pipes penetrations etc (not to passivhaus standard though!). Plaster was a skim coat I believe, with some areas reboarded.
- For the attic joists at the eaves perimeter I am not certain, I have attached a photo of it but that area is tricky, we were planning on just sealing the membrane to the floor around the perimeter before the vents (possibly to the inside of the bricks)
- Thank you for the advice regarding whether or not to fit a VCL. Despite the humidity hopefully being lower with the MVHR I still am concerned (if we have any issues with it for example) as we definitely don’t want any moisture issues in the ceiling/loft! I had read about insulation having issues with damp, but also would be worried about trapping moisture between layers if we did use a VCL. I’m a bit unclear as to how the condensation would work in each instance.


Still a tad confused as to the order due to the variables, but leaning towards:
(non vcl) membrane > ducting > 300mm insulation > overboarding?

Or does anyone have the membrane, insulation and boarding down first with the ducting insulated separately running above the boards (for accessibility?)

Here's how I'd design this, if I were doing this for a client (which I do, it's my job)...

The objective is...seal the awkward perimeters, seal the membrane, make ceiling air tight, maintain ventilation between rafters at the eaves, keep it fairly simple.

The attached detail (hopefully) shows the following process;

1. fit 80mm PIR boards onto the plasterboard ceiling, between the joists. Don't need to be too accurate with the cuts, indeed leave 5-10mm gapsall around, for foam filling. Use what's often called 'board adhesive' which is like expanding foam, but it doesn't expand much but glues really well. Stick this PIR board to the plaster board too (set something ontop of the board for 30 mins till it sets, so the foam doesn't push the board up). Probably about 400mm depth, from eaves inwards, but 300mm might be adequate. Choose a dimension as a multiple from 1200mm which is the full board width.

2. Remove insulation from the voids where power cbles are run. I wouldn't worry about the lighting cables, as they are likely well oversized due to the use of LED lights at 5W rather than 60W bulbs.

3. Lay the airtight membrane over the joists, and stick it to the PIR board perimeter. Lay the rolls of the membrane parallel to the joists, and start the next run ontop of a joist. Fix a rip of plywood over that joint onto the joist, so the membrane is clamped mechanically. I'd use Protect BarriAir as the membrane, as it is one of the better priced air tight membranes (£1.50/m2). It is also a VCL, but that's not particularly important, as the MVHR will mitigate humidity issues, assuming it is opperated correctly.

4. Fit the ducts. Focus on how you seal around the penetrations of cables and ducts (and joist hangers - vertical timbers).

5. Fit PIR boards to the rafters, tight down onto the initial horiz PIR boards, clamping the membrane again. Adhesive foam that butt joint. Use screws with a large washer to screw the board into the rafters. I make the washers from offcuts of plywood, say 50mm x 50mm. Drill the holes in the ply first, then cut the plywood squares - saves sore fingers from holding little plywood squares whilst trying to drill. These are likely 600mm wide, to cover from eaves up the rafters to above the deck height. 500mm might be enough, but then you're wasting 100mm rip and more cuts.

6. Fit 3 layers of wool. Happens to be my choice to use 3 layers at 140/150, but you could do 2 at 200mm. What ever, just make sure there is plenty over the ducts, and that you run the rolls perpendicular to the one below. 3 layers means better gap minimising as have 3 passes.

7. I would fit the deck as I roll out the insulation, to minimise treading on the wool. I'm still concerned about the 80mm deep joists. They were sized to carry the plasterboard ceiling, and an occasional tradesman. The chipboard deck alone will be quite a load, plus a person or two, plus all the really valuable junk we all store in attics, before taking it to the skip 20 years later . Perhaps there are lots of joist hangers to support them? I don't recall seeing 80mm ceiling joists in 40 years on sites (I started VERY young).

Posted By: djhOne other consideration to add to the list - rodents. Design whatever you end up with to be resistant to rodents. Maybe mice, or in our last house's loft we had squirrels for a while!

What type of ducting are you planning to use? Plug-together semi-rigid radial ducting is very easy to use and is unlikely to need access afterwards.

I agree the joists look like 80 mm. Just be careful about putting point loads on them.

great point! we had rats in our old loft and it was horrendous, will definitely look into rodent-proofing! We have semi-rigid radial ducting, unsure on if it's plug together but I will check!
Didn't realise the joists were smaller than usual - bit nervous about that! We will need to research how to strengthen it as that was going to be our main storage space!

edit - just seen your comment about strengthening joists, may do that whilst the boards and insulation are up? At least in the 'main' area!

My opinion is you'd have to be anally meticulous to get any membrane sealed 100% around the perimeter of the roof. I'm not saying it's impossible but very challenging to do a good job.

Even the smallest unsealed section will allow air to flow from the unsealed bit and under the membrane to any part of the ceiling below. To get it sealed around the ends of ceiling joists and hip beams will take so much time. If you fix to the wall plate and there's any gaps under the wall plate that the rafters sit on there'll just be a constant draught under the membrane through any cracks or gaps in the ceiling.

As its a bungalow with easier access and if you've got cavity walls, my view would be you'd get better results stripping off the lower perimeter of the roof covering to allow you to fully seal the edge of the ceiling and extend the loft insulation to reach the cavity insulation so you've got the previously inaccessible perimeter fully insulated. Straight forward to strip the roof from an alloy tower if it's a tiled roof.....a slate roof is a bit more involved!

When you come to insulate the loft fill between the ceiling joists so the first layer is levelish with the top of the joists. The next layer gets laid so it rests on the first layer AND the top of the ceiling joists. If you don't get the first layer level with the joists then the second layer will have lots of tunnels under it

GreenPaddy said: ''I don't recall seeing 80mm ceiling joists in 40 years on sites''

That's a pretty standard size for Victorian terraced houses in Yorkshire. Even ones built with rooms-in-the-roof! My chest of drawers used to lean forward to greet me when I came into the attic room at the other end! It is very common to see Victorian terrace attic rooms with a 'hump' in the middle where the joists dip either side of the central spine wall! (I'm saying it's common, not good!). It's also fairly common to see 65-80mm as the bottom chord of 1960s trusses.

Posted By: GreenPaddywould you have a closer/brighter photo of the eaves? I'm not clear why the air would not vent up between the rafters at the wall head? The photo you added earlier seems to show this, but it's tricky to see.

The reason I'm asking about this is that the edges at the ceiling to walls are very weak points thermally, and so you want to get insualation right out to the bricks if you can. I appreciate that the vent line may not be at the base of the rafters, but presumably it can be shepherded to the rafter gaps as it passes over the wall head, since the rafters pass over the wall head (or appear to).

If necessary, add 50mm timber battens over the underside face of the rafters, to deepen them, then do the angled PIR board, but I can't YET see why it would be necessary. A photo will no doubt reveal why it is.

The middle bit of the ceiling is easy, it's the edges that are tricky.

Not sure (from your sketch) if you're thinking of PIR boards over the whole ceiling in the 80mm joist void? That's not necessary with the wool depth above. The joist voids can either be empty with cables, or leave the old insulation. The PIR boards only fill the joist void around the perimeter for say 300mm. Maybe I'm mis-reading your sketch?

Not keen on the stepped insulation around the edge, for the reasons mentioned above.

Definitely do not want the membrane above the insulation, unless it's very breathable. You're also trying to seal a 3D box, which is much trickier than a single plane. Don't under estimate how difficult this will be, especially in a dusty attic. Granted it eliminates most of the duct penetrations, but there are solutions to make sealing those robust.

I would pay for a structural engineer to assess the ceiling joists, (spans versus load bearing walls below), and the load your planning to put up there. One approach can be to build the raised deck as suspended with horiz timbers that fix to the rafters and joist hangers like a new floor, with supports down onto the joists at load bearing wall head points, so at least some load is carried by the rafters and transmitted to the supporting walls. It will likely cost you £250 to get them to come out, but I'd say it's money well spent.

Thank you so much, that's really helpful! Here is a link to the eaves/loft photos https://imgbox.com/g/0iaW5KKPdv.
I think I understand your initial diagram more now, I had completely misunderstood a couple of aspects!

So the ‘angled’ and ‘joist fill’ PIR both only go up to the underside face of the rafters? Leaving the gaps between/behind rafters for the air flow/ventilation tunnels? Then we work within the new sealed loft space created by the PIR wall and floor around the perimeter, insulating the whole inside and avoiding the stepped insulation issue.

I had originally thought the PIR would go between the rafters all the way to the soffit/wallhead (brick wall?) which I thought would block the ventilation, but I think I understand it better now, hopefully! Please let me know if I still have the wrong end of the stick haha. Again, a really helpful suggestion for deepening the underside face of the rafters, but hopefully I have just misunderstood how it works and your original angled PIR boards will work!

Yes, I had thought the PIR boards would cover the whole ceiling in the joist void, much easier if it’s just the 300mm outer perimeter! Then the joist voids can act almost as a service void for all the cabling (will need to potentially notch the top of some joists for the cables for them to stay within the joist height, unless they can sit on top and the membrane would just go over these?)
Will the membrane be okay if pulled taut on a non-solid surface, as we were originally planning on creating a flat layer via the PIR board. I was a bit worried about tunnels/airgaps as mentioned in the post above, but is it okay since it’s not between insulation layers? Or did you mean lay the membrane flat across the top of the plasterboard and above the joists in the U shape, if so, we still may need to rethink the cabling!

Note taken about the membrane above the insulation - thank you so much, we can definitely rule that out. I will also look into the structural engineer for the idea of a suspended deck as I was thinking something similar, plus we could always have that as a separate project once the MVHR is installed (and we can finally shower here!).

@oliviaaa99

The PIR strip between the joists insulates from the wall head to in-board of the wall below. It forms a flat surface which is flush with the top of the joists, and is sealed by foam adhering it to the plasterboard/wall head and the joist sides. That gives a fully sealed perimiter to the attic, which is the starting point, and gets rid of trying to seal up and down around the joists.

It's like a picture frame onto which you'll fit the canvas (membrane). You can now just adhere the membrane in the 1.5m runs, starting at the very edge of the junction of rafter and joist. There's no problem having the void below the membrane to the plasterboard (joist depth) when there are cables, as the perimter PIR seals that whole area (the main reason for doing the PIR picture frame).

Start the next strip of membrane ontop of a joist, so that joining of this strip and the previous are over a solid, and can be clamped as described above. If a few cables hop over the joists the membrane simply passes over them. I would not notch if at all possible.

Angled PIR is again just high enough to reach above the deck, forming a sort of tray (with slopey sides). Fit the PIR hard down onto the strip of PIR that's flat, and this will trap the membrane around the perimeter. Foam adhere it to that horiz perimeter, and fix to the rafters. The wool will now be held in place with no side penetration of the eaves vented air, and also the wool will not block the very important eaves vent air from passing up the back of the roof felt, and removing condensation. (side note - condensation in the loft will not just/even be from the house, but instead from the roof surfaces getting very cold with say 3 or 4 days of low temps, followed by the sudden temp swings we regularly get, and so that warmer/damp air from outside meets the cold inner roof surfaces, and condenses. It dries out as the temps balance out).

So, I think you've understood what I was proposing, and hopefully this description confirms that. This is a simple and robust detail, which minimises taped joints, other than around ducts, which are easy to tape up well. Consider adding a square of plywood (say 600x600 or 450x450 depending on joist spacing) on top of the joists where a duct penetrates the ceiling. That gives a secure restraint for the duct but more importantly gives you something to press against when sealing around the duct. The problem with trying to tape to a membrane is that you are often pushing against a flexing surface. Could even add another patch of plywood over the membrane seal around the duct, to again clamp the two layers.