Advice on internally insulating cavity filled external walls

[Beete]

Hi,

I have a 1940s house which I believe has had its cavity wall insulated at some point after it was build (I suspect by last owners in the last 20 years). These walls are extremely cold to touch and after checking and sealing draughts and gaps it is no different. Have decided to look into insulating internally some of the external walls in the house.

Based on my reading of various articles/sites my thoughts on how to go about this:

- Walls are uneven by 15-20mm so I was planning on using 50mm battens and packing out to level

- 50mm Celotex/Kingspan fitted between the battens

- All gaps sealed between batten and insulation with aluminium foil tape.

- Plasterboard over the top.

I do have a couple of questions/reservations though:

1. Is there any addition risk for moisture/mould given the cavity walls have been filled? Is there a maximum thickness of insulation I should have i.e. is 50mm too thick? To be clear, the walls are dry and no signs of damp or anything like that.

2a. Since I need to pack out the battens, there will be a gap between the batten and the wall. Does this also need to be sealed or is it just the top layer behind the plasterboard?

2b. Would it be easier to stick the insulated boards directly to the wall instead of battens despite the wall being out by 15-20mm? There is currently a radiator on some walls so perhaps batten is the best way?

3. I've read about using a damp proof membrane to wrap around the timber instead of using treated battens and this makes sense if I am packing out using wooden shims. Just wanted to check there are no easier alternatives.

4. Any else I need to take into account before I set about doing this?

Any thoughts on the above would be much appreciated.

 

[motorbiking]

2b and use plasterboard foam adhesive. If it’s not too big a job, take the plaster back to brick first. Avoid battens. You then use foam to fill all the gaps and attach the plasterboard the same way. For belt and braces a few frame fixers will hold everything in place. The foam should help you sort the levels out. 50mm will reduce the U value from around 1 to 0.25. Radiators can be mounted with longer frame fixers in to the brick.

 

[JP_]

Yep, I stuck 50mm Kingspan to walls with Everbuild Dryfix Foam, silver taped all joins, then stuck plasterboard on top of that with same foam.
The downside of this method is that there is a lot of time spent waiting for the foam to cure (4 minutes) and then applying pressure to the board so the foam does not push out when still expanding (varies depending on how much foam or space for it to expand into).

But, for a DIYer with no experience of using other materials it was great. I didn't use mechanical fixings and everything is still standing 4 years later. No cold, mould, no damp, lower bills.

I was lucky in that there were no electrics on outside walls, and was having some rooms renovated completely with new radiators etc which made it easier. In my front rooms with bay windows, I didn't insulate the bay / radiator area. Too tricky! I am considering having external insulation just on the front of the bungalow, but a job for later.

My bedroom - I didn't stripe plaster off the walls, too dusty and messy!:

A room with new window, no rad etc. (there was a chimney there before)

 

[Beete]

Cheers both, sounds like the foam and plasterboard is the way to go and saves me an effort trying to level and plumb the battens at least!

@JP I'm not planning on taking back the plaster either as some of the rooms are already decorated and the missus is not happy with that option so will just do the same as you. Have you noticed a considerable difference in heat retention since installing the insulation like over the winter period?

Quick question for you motorbiking, I've never used frame fixings before and one or two of the radiators are double panel type 22 (1200x600) which are currently held in by M8 screws and are pretty heavy. Will these frame fixings adequately hold these radiators up? What size should I be looking for as I had a quick look on Screwfix/Toolstation and looks like they go up to 180mm in size - is that sufficient or just overkill?

 

[JP_]

Have you noticed a considerable difference in heat retention since installing the insulation like over the winter period?

Yes, made a huge difference. When we moved in, we had black mould in every outside corner, and had to keep the heating on 24/7 in winter - would set thermostat to 22 during the day and 17 at night, and that kept the house OK. Now the rooms are warm and stay warm, no condensation (other than one window reveal on the bay window that I didn't insulate) and no mould. Also, no matter how cold it is, the heating is off at night from 11.30pm til 5.30am, comes on from 5.30am til 8am, then comes on again at 3pm. The rooms do get colder in the day with the heating off, but not too cold really - but they do not hold the heat as well as the extension that is south facing.

Overall, very happy. I also insulated the floor with kingspan & rockwool. More than anything really is how much more comfortable the rooms are, it is nicer to not need the radiators blasting out heat all the time. And handy as gas going up again!

 

[motorbiking]

140mm should be fine. With your 50mm celotex and plasterboard and foam thickness, you will be losing around 80-90mm. You will also need extra long Patrice screws for any electrical points.

yes the frame fixers will hold well.

worth also mentioning that you can use the PL celotex (pre bonded insulation an plasterboard )if the budget will stretch which will be easer, though more expensive

 

[Beete]

Thanks, I'm aware of the space I'd lose which I can afford to do just nothing much larger than that. I did see the foam backed plasterboard which I'm considering but as I'm doing a few walls the cost does add up compared to buying separate.

Re electrical points, I've managed to source some back box extenders as I will need to extend the wiring on some of the sockets since they're extremely short, so should have that covered hopefully.

May report back with questions when I get round to doing it but if I don't thens safe to assume it's gone fine.

Cheers for the advice.

 

[JP_]

For my first room I bought thin foam backed plasterboard and it was flimsy (I think only 25mm insulation) and I didn't feet comfortable trying to get it on the walls! But was total DIY noob then. One consideration is that you can so easily create a vapour barrier as you can by taping insulation joins then boarding over.

 

[jacko555]

. I didn't use mechanical fixings and everything is still standing 4 years later

I think the mechanical fixings are for fire safety, rather than to hold them up.
If there is a fire, it's to stop heavy plasterboard sheets and violently flammable celotex boards peeling off the walls and blocking escape routes or hitting people - especially important if you are trying to exit the building or the fireservice is trying to rescue someone mid blaze...

 

[JP_]

I think the mechanical fixings are for fire safety, rather than to hold them up.
If there is a fire, it's to stop heavy plasterboard sheets and violently flammable celotex boards peeling off the walls and blocking escape routes or hitting people - especially important if you are trying to exit the building or the fireservice is trying to rescue someone mid blaze...

Yeah, I read that too, which is partly why I didn't bother. If a fire is strong enough to get through the plasterboard it will probably already be burning the timber floors and timber roof, so be past salvaging anyway.

 

[noseall]

are pretty heavy. Will these frame fixings adequately hold these radiators up?

You should make provision for any future fixing by including say some 18mm ply or additional timbers, within the framework.
I wanted the freedom to fix anywhere I liked on a stud wall at home (telly, kitchen units etc)...

View media item 107584View media item 107583

 

[Deleted member 292770]

You should make provision for any future fixing by including say some 18mm ply or additional timbers, within the framework.
I wanted the freedom to fix anywhere I liked on a stud wall at home (telly, kitchen units etc)...

View media item 107584View media item 107583

what framework? I think they intend to bond straight to the wall

 

[mrrusty]

In my recent house project we used insulated PB (60mm +12.5) fixed directly to the brick with plasterboard dab adhesive which easily took up the variations in the wall smoothness. Radiators fixed through with long fixings back to brick which has been fine, but cutting away the insulation and fixing in some timber behind could be helpful if you know exactly where it is going. There seems to be an obsession with battening first but for ease and speed, direct to the wall works as well.

You'll have to take a view on the already filled cavities - ours are open and ventilated and no issues 3+ years later.

 

[Beete]

Thanks mrrusty, I've always always assumed battening is the preferred option but understand direct fixing can also work. I'm planning on doing a retro fit in the kitchen too though I probably will batten out there since we have a few heavy wall units but for the living/dining room I will just stick directly to the wall. Also I did put in some air vents earlier in the year as the only vents we had on our house are just above the ground windows and that's when I realised they may have been filled.

About to start the project this either this weekend/next weekend and I have one final question. When I purchased the house last year we noticed there was a stack of insulation in the loft, foil-backed on both sides though I can't see a brand on it - I assume it was for insulating the roof but previous owners didn't get round to it.

Would there be any issue with recycling these boards instead of buying fresh ones i.e. they don't have a shelf life on them in terms of lying around gathering dust? Granted they're only 500mm x 250mm and that would mean a lot of time spent sticking each one to the wall but they are 50mm thick and it does mean I clear out some loft space. Most of them aside from dust seem to be in decent condition with not broken foil or anything like that.

 

[mrrusty]

seeing as they're intended to be installed for many years, they'll be fine to use.