Hello all
My understanding from a bit of searching is that there has been a fair few posts about dwarf walls on this forum but I couldn’t find any that were directly relevant to my situation. Apologies if I’ve missed something.

Context: Victorian terraced house with room in roof that gets freezing in winter and baking in summer. We had quotes for room in roof insulation, so insulating from the inside, but it would mean losing 50mm all around and it was £4,500 so think of option options. I was thinking to DIY on the walls externally improving what’s there. This would mean the sloping roof and the flat roof bit at the top of the room are left uninsulated, but I figure if I do a good job on the walls it should still improve the situation.

I have two questions, the first concerns a build in wardrobe, the second regarding the dwarf walls.

This is on one side of this room which runs almost the length of the room (~5m) (with 1m on one side of it and .5 on the other). It runs floor to ceiling with 300mm off wall on top before the slope (in the room). This wardrobe is freezing cold. Attached is pic of the middle section (on the sides you can see the wardrobe doors).

Also attached are two pics of wardrobe in loft space (back and side top).

My plan is to cover as much of this with 100mm Celotex GA4000 as I can by screwing it into the structural parts of the frame (edges or where the shelves are) (will using metal screws create thermal bridges – is there a better way?). There is a gap between the wardrobe and the floor as it’s sat on a piece of wood. I could stuff this with rockwool to close the air gap so help with the cold floor of the wardrobe (would this create any damp issues?). The sides are fine as its just one but sheet of wood. For the back, near the top there is a ceiling joist in the way so I guess I will either have to leave this or perhaps wedge rockwool as best as I can. For the top again the joist is in the way, and on the other small pieces of plastic are in the way so I guess laying a sheet of Celotex on top isn’t going to work. Again, I could use rockwool instead for the top.

My other question concerns the dwarf walls. Currently there is around 100mm of glass wool stuffed between the dwarf wool studs as per attached pic from the other side of the room (yellow material).

I’m thinking about the best strategy to improve this.

1. Leave the glass wool and sheath the whole wall, studs and all with say 100mm Celetox (leaving 50mm gap for roof airflow at top).
2. Take off the tool, use it to top up the loft insulation, and place Celotex between the studs. Note the studs are 75mm. Would it matter if the depth of the insulation is greater than the studs?
3. Use 70mm Celotex in between the studs, then overlay with 23mm Celotex across the whole wall to create a total of 100mm.
For any option, do I need to think about moisture control?

For 2 and 3, as you can see in the attached pic there is grey filling between the wooden slats of the wall. Sometimes this sticks out a bit from the wall. It’s very brittle and falls off easily – can I just remove this so I can get the Celotex flush with the wall?

Apologies for the long post. Any advice on any part of it would be very gratefully received.

top of wardrobe

inside wall

Our first foray into 'green building' was a task just like this! Your plans sound good and will make a huge difference, not just to the wardrobe.

Wardrobe: cut the celotex to fit the sides and back boards and overlap so it meets at the corners. Glue it onto the outside of the wardrobe and fill any gaps with expanding foam. Slide a thin piece underneath, and into the gap by the purlin timber.

Air does need to circulate above the wardrobe to get up the sloping ceiling section, so don't stuff it with wool. Cut rectangles of cellotex the right size to slide between the joists and wriggle them into place on top of the wardrobe, fill gaps between with offcuts of cellotex and expanding foam, do the back of the vertical bit of wall at the same time.

Vertical wall: any of your 3 plans would work. Make sure there's no gap at the bottom where it meets the loft insulation.

Sloping ceiling: this is DIY-able. One option is to cut rectangle pieces of thin cellotex, the right width to slide up between the joists from below, leaving a gap above for air to circulate. I found cutting each rectangle diagonally into 2 triangles, made it easier to wriggle in round the woodwork. It won't sit flush against the plaster.

A better (bit more committed) option is to remove the plaster off the sloping section from the inside, and replace it with insulation and plasterboard fixed to the inside of the joists. This brings the sloping ceiling into the room by a few inches but you don't notice. Fill and tape the joins and repaint.

Flat ceiling in middle of room: cut out a square of the plaster inbetween two joists to make an opening big enough to get your head and arms through. Feed lengths of rockwool in and use long broom handles or similar to manoeuvre them to lie inbetween the joists, as far as you can reach in both directions from your opening. Do the same again length-wise along the loft space. Repeat with more holes as needed, then patch them up and redecorate.

Leaving big areas uninsulated like the flat and sloping sections of the ceiling will still leave the room cold. If theres a staircase serving the room youll be able to get plasterboard up there so probably best to bite the bullet and take down the plaster covering the inaccesible areas to do a proper job. Unless youve got the talents of a genealogist, trying to fit insulation to inaccessible areas is very likely to lead to gaps all over the place which will allow warmth to by pass the insulation. Fitting insulation, VCL and boarding out is all DIYable with a pasterer to skim the lot.

Where youre fitting insulation against lath and plaster, rigid insulation is going to leave pretty big gaps so you might be better putting a layer of 80mm quilt between 75mm rafters/studs and celotex over the top of the quilt.

One thing to look out for is a cold draught blowing from one eaves to the opposite side under the rooms floor. Youll still get air movement through a full depth of quilt so best way is to close the gaps between floor joists with thin ply between the joists and fixed to the bottom of the stud wall. Mastic the perimeter to seal all the gaps.

Unless the plaster is overlaid with plasterboard then it will delaminate and fall off but maybe not for a few years, I wouldn’t chance it

<blockquote><cite>Posted By: philedge</cite>Leaving big areas uninsulated like the flat and sloping sections of the ceiling will still leave the room cold. If theres a staircase serving the room youll be able to get plasterboard up there so probably best to bite the bullet and take down the plaster covering the inaccesible areas to do a proper job. Unless youve got the talents of a genealogist, trying to fit insulation to inaccessible areas is very likely to lead to gaps all over the place which will allow warmth to by pass the insulation. Fitting insulation, VCL and boarding out is all DIYable with a pasterer to skim the lot.

Where youre fitting insulation against lath and plaster, rigid insulation is going to leave pretty big gaps so you might be better putting a layer of 80mm quilt between 75mm rafters/studs and celotex over the top of the quilt.

One thing to look out for is a cold draught blowing from one eaves to the opposite side under the rooms floor. Youll still get air movement through a full depth of quilt so best way is to close the gaps between floor joists with thin ply between the joists and fixed to the bottom of the stud wall. Mastic the perimeter to seal all the gaps.</blockquote>

Thanks for the advice. The quilt sounds like it might be a good idea to counteract the gaps. Noted on the insulated plasterboard being ideal. It's just not something I feel comfortable doing and I don't want to make a mess. So perhaps I do the others measures I've mentioned then see if I can get someone in to do those bits. I hadn't considered a draught blowing under the floor. Something else I'll have a look at but I'm not sure how possible this will be to do because the wardrobe side has a chipboard floor so I don't think there is a gap, but I'll take a look.

Dan - I had almost exactly the same situation as you, as we have a dormer bungalow (with Velux windows rather than “true” dormers”).

Above the flat part of the ceilings we have put extra fibreglass quilt between the rafters and also over the rafters at right angles. To do this I did as Will has suggested – I cut holes in the ceiling to make entrances into the upper attic spaces. I made mine wide enough that I could actually get up into the space. I have fitted trap doors so I can get up there again if necessary.

We also put extra quilt between and over the joists behind the dwarf walls.

When the place was built (about 10 years before we bought it) 50mm of Celotex was placed between the rafters behind the sloping ceilings and between the purlin supports in the dwarf walls. What I did subsequently was to put 50mm of Celotex on the inside of the sloping ceilings and the dwarf walls and held in place with horizontal 50 x 25mm battens screwed through into the rafters/purlin supports. I then put 25mm Celotex between the battens, a VCL over that, and then plasterboard over that (total depth of Celotex = 75mm). The plasterboard was secured with screws into the battens. I used taper-edge PB so I could do a DIY job to fill the gaps with “mud” rather than paying a plasterer to skim coat.

Later I went back and cut and fitted small rectangular sections of Celotex between the floor joists under the dwarf walls (a very tedious job!) to prevent the cold draught which otherwise blows across from one side of the attic to the other – effectively underfloor cooling! I first nailed short lengths of 25 x 25 mm battens to the joists so the Celotex had something to butt up against. The Celotex pieces were then foamed in place using low expansion foam. I also foamed the edges of the existing Celotex which was between the purlin supports as they were a loose fit.

All these jobs took an age but fortunately as there are only the two of us living here it was not too disruptive, as we have bedrooms downstairs as well. I must admit they were very fiddly, tedious jobs but have made a big difference to the heat retention of the upstairs rooms. In fact the rads are not on up there at all, as there is sufficient heat coming up from the ground floor below. I am a bit disappointed that the insulation has not made a big a difference as I had hoped regarding heat absorption in the summer months – it still gets very hot up there unless we open the Velux windows and get a cross-breeze.

Re: the your built-in cupboard: I would do as Will has suggested.

Posted By: Jeff BI am a bit disappointed that the insulation has not made a big a difference as I had hoped regarding heat absorption in the summer months – it still gets very hot up there unless we open the Velux windows and get a cross-breeze.

Is the heat coming through the insulation, through the windows or up from the floor below? An infrared thermometer would probably help to identify the source.

Posted By: daninsulgreenJeff, interesting that you put the VCL over the Celotex. My understanding was the VCL goes first, but maybe I have that wrong.

The VCL always goes on the warm side of the insulation.

Any takers on which quilt product to use?

Also wondering whether for dwarf walls better to have (1) the first layer of PIR sit just shy of the studs so second layer would sit tightly on studs and leave a small gap with first later of PIR, or (2) have the first layer of PIR sit just proud of the studs so the second layer sits tight on the first, but there are gaps between it and the studs. I guess the ideal is to have the first layer perfectly flush, but that's going to be difficult given the lath and plaster walls.

Posted By: daninsulgreen

Ok thank you, in my case for the main part of the plan that would be between the dwarf wall and the Celotex.

Im no authority on intersitial condensation but I dont think a VCL between the wooden studs and celotex is going to be of much help. Any moisture in the wall is going to be held in the wall and not able to get out into the ventilated roof space. The foil faced celotex is going to do more or less the same so seems a waste of time putting a VCL in unless you can get it inboard of the studs to stop room moisture getting into the wall ie beteween plaster and studs??

Posted By: WillInAberdeenMineral wool comes with different insulation values, often mentioned in the name, if it is '32' then it's the best stuff, better than '38' and much better than '44' which is what my local DIY place sells.

You can also get 'acoustic', but that's not what you need here.

I would fit each layer of insulating board as tight as possible to the layer below, if there is a slight gap then put criss cross beads of expanding foam to stop air flowing through.

Ah get it now thanks, I was getting side tracked with this stuff as its what comes up when I google insulation quilt:

https://www.screwfix.com/p/ybs-superquilt-multi-layer-reflective-foil-insulation-10-x-1-5m/68120

But its actually the more conventional stuff.

I originally assumed @philedge meant to do this for the sloping ceiling (on the outside) but actually I think he meant the dwarf walls as there will be a gap there too. Probably getting to the territory of splitting hairs now... it depends on the gap and what's possible for the sloping section. I'm gonna get a bore camera up there and have a look.

Posted By: daninsulgreen

Posted By: Jeff BDan - I had almost exactly the same situation as you, as we have a dormer bungalow (with Velux windows rather than “true” dormers”).

Thanks Jeff, appreciate you sharing your experience.

Just out of interest, when you say fibreglass quilt, when I google that I get something that looks like a large roll of shiny foil. Is it that, or do you mean conventional fibreglass like what is used to insulated lofts?

Also mentioned by philedge, should have asked earlier...

Posted By: philedge
Where youre fitting insulation against lath and plaster, rigid insulation is going to leave pretty big gaps so you might be better putting a layer of 80mm quilt between 75mm rafters/studs and celotex over the top of the quilt.

Jeff, interesting that you put the VCL over the Celotex. My understanding was the VCL goes first, but maybe I have that wrong.

Regarding the tediousness yes I realise this is going to be a big job and is going to take a long time... I am trying to brace myself for that... I want to have the patience to do it slow and steady, and properly. So this first stage is trying to do my research as best as I can so I know I'm not wasting my time! This forum has proved very helpfulhttp:///newforum/extensions/Vanillacons/smilies/standard/smile.gif" alt="" title="" >

daninsulgreen: apologies for the delay in replying but I have been away from the keyboard for a few days!

Re: fibreglass quilt. To be honest I used whatever was on offer at the time, so there is a mixture of types here. I have used some of the stuff you mention i.e. fibreglass enveloped inside aluminium foil on one side and polythene on the other, also bog-standard naked fibreglass (the itchy variety!) and also the much nicer recycled plastic bottle stuff which is like the soft material used to line quilted jackets - lovely to handle.

The VCL should be on the warm side of the insulation. I did debate whether this was necessary at all as the aluminium foil on the Celotex should do the job by itself. I did tape all the joins with self adhesive aluminium tape. I also used foil backed plasterboard and with vinyl paint on top of that, I think it would be amazing if water vapour managed to penetrate that lot!

Good luck with the work. It will be worth it.

Posted By: Jeff BI did tape all the joins with self adhesive aluminium tape.

Note that most aluminium tapes have a pretty poor reputation in terms of their ability to maintain an airtight seal over the long term. I think it's mostly to do with the adhesives that are used and the specialist tapes use specialist adhesive. Some people here don't trust any tapes!