Solid Wall Houses - Insulation + Plastering + Air Gaps ?

dlockers

dlockers

dlockers

Soldato
Joined
21 Jan 2010
Posts
24,893
Hi folk,

I have a 1930s semi as some of you are aware. To get to the end-state I have in mind, I need to lift a lot of floorboards and remove pretty much all of the skirting - upstairs, and down stairs. I am placing PIR board between the joists.

Givens:
* Air bricks will be cleaned thoroughly and ducts put in place where they are ineffective (I have some that are at skirting board level, so barely get "under the floor")

My questions relate to therefore...
* Q1 How "sealed" should the 'surface floor' (i.e. the boards I will place the proper hardwood floor on top of) be to the surrounding walls?
--- I get loads of draughts from the edges, and the current carpet is black around the perimeter due to draught (...and being 30 years old).
--- Can I butt the floor right up to the wall, or is this a problem?
--- Related to Q2 as well, but can I plaster "down to the surface" floor, or should there be a gap behind the skirting where it touches the floor?

* Q2 Plaster needs replacing -- I am reading conflicting things on how important it is for these 1930s solid walls to breath, and what types of plaster to use - as it could have been a lime+gypsum blend anyway. The walls are currently plastered in a mix of stuff, pics below.

The walls are mostly papered, but as you can see from picture 3 - the wall paper pretty much pulled away with limited effort.

T6wvi6n.jpg
CmeKGAk.jpg

Cl4FVh5.jpg


Floor pics: https://i.imgur.com/IF9zZWz.jpg

* Q3 When I do the PIR I have some areas where it just isn't practical to cut a piece of PIR and slot it in (or maybe I just need to perservere?) E.g. in this pciture the joist by the radiator is about 3cm from the wall. Can I fill this gap with expanding foam?

* Q4 I also bought silver tape --- is there any merit in taping over the top of the joists/from PIR board to PIR board? This means only "foil" would be in contact with the floor above it?

* Q5 Some of the boards are battered due to how dry they were when they came up --- I can either drill down what is left of them, replace with chipboard (like I was forced to do in the hall), or get replacement boards from Wickes (need to price up). I guess if I am doing a hardwood floor I may want to throw some giant sheets of Plywood down anyway to get a perfect surface?

Thank you!
 
Last edited:
Q1 - very sealed! get the PIR packed in tightly to the joists and tape/fill in any minor gaps to stop any draughts.

Q2 - I would advise speaking to a professional as the risk of damp and damage down the line can vary and wouldn't be trusting an online forum for answers to this.

Q3 - fill in as much as possible, expanding foam can work, or failing that get some rockwool and shove some in the smaller gaps to a thicker depth and seal it underneath and above.

Q4 - as per Q1

Q5 - Depending on flooring plywood on top of the floorboards may be beneficial to get a nice smooth flat surface.
 
For Q3, don't forget that are (at least) two types and I think you'd want the insulation one. Something like this:
www.screwfix.com

Soudal Genius Insulation Foam Hand-Held 750ml - Screwfix

Order online at Screwfix.com. Insulation foam with 60dB acoustic rating and Genius Gun applicator with accurate trigger control. Provides excellent thermal and acoustic insulation around door and window frames, pipe penetrations, insulation, roofing materials and general construction. Excellent...
www.screwfix.com www.screwfix.com
N1UHJoX.png
Think those are closed cell like PIR whereas the other foam is not.
 
The difference in thermal conductivity between the standard and the "Insulating" is negligible, standard is 37W/mK. Not sure its worth the £2 a can difference.
 
Re the insulation in the floor, the rule is simply anywhere there is an insulated cold / warm air interface you need a vapour barrier. The vapour barrier prevents the moisture in the warm air getting into the cold and condensing into water. So for PIR board between the joists, you need to tape all gaps, ie, tape between adjoining lengths of board and especially over the edge of the boards over the joist and onto the adjoining PIR (the last thing you want is water condensing between the PIR and joist !), only tape on top btw, not underneath . At the edges I would also tape up the gaps for the same reason, ie tape between the floor and the wall and fit the skirting over the top (alternatively you could lay a vapour barrier (dpm) over the whole lot and lip it up the sides of the wall, which is what you do if using rockwool between joists).
Also, where ever there is an airbrick make sure not to cover it with the PIR, I have even seen people cut the PIR in a wedge shape to guide the air from the brick down around the void under the floor. I never use squirty foam stuff, its awful.

For replacing or refitting floorboards do yourself a massive favour and secure them with tongue tite screws (taptite alternative type). These are simply great for securing boards for split free, squeak free installation. If you have failing floor boards and are covering them with a finished flooring just replace with chipboard flooring and PVA glue and screw. FYI I was always told not to use ply as floor boards, thin ply is good for overboarding to level out undulations with close screw centres however.

Regarding the walls, I have been told both re plastering too, I have renovated a good number of period properties. That I know, allowing the wall to breath externally is the most important, trouble starts when the external walls are painted with modern paint (or repointed with cement) which seals the walls, eventually moisture gets under the paint and can't escape, wet walls conduct heat better than dry, so when its cold outside the walls inside get cold and internal walls get damp. Applying the same logic to the wall as the floor, you would expect that sealing the internal wall (which the paint will to an extent anyway) should be the right thing to do. I do know I had to use distemper paint on the cellar walls of my last house (built 1889) but we used modern plaster and paint on the walls in the house and saw no issues in the 5 years before we sold.
 
Last edited:
Orifice said:
The difference in thermal conductivity between the standard and the "Insulating" is negligible, standard is 37W/mK. Not sure its worth the £2 a can difference.
Click to expand...
I though it was the fact it is closed cell so (slightly) less likely to get and retain moisture? And the closed-cell part is probably where the better thermal resistance comes from? As open cell would just be air. All leak eventually anyhow. Those Soudal Genuis cans are great for small jobs as you can leave them for a few days and come back but I think their non insulation ones does that too.
 
For this application it would make barely any difference, the insulating foam is up to 80% closed cell but that will be based on a perfect installation, mostly people massively overdo it not realising how much it actually expands. The thermal value is a combination of the cell structure, blowing agent, and whatever additives they put in to improve the lambda.

Getting as much rigid PIR board in the hole and filling around it with foam is the better option, but more hassle.
 
Amazing - cheers lads, digesting! This has cured several of my sleepless nights already. I'll start thinking about follow-up questions :D
 
I'm in a similar position (except it's 1950s so a cavity wall).

There's loads of good videos out there and everyone seemingly has their own method. This video is the method I plan on following for insulating floor joists on a suspended floor with air bricks:


I'm still debating whether or not to replace the floorboards or removing them and using plywood sheets on the joists instead. The idea of perfectly level plywood boards is appealing but there's something about floorboards. Plus keeping floorboards would be quite a bit cheaper.

You could also chipboard over the floorboards bit of you're going for the full reno, it seems like a bit of a cop out.

No idea about plastering down to the floor with solid walls. I'm planning on plastering as low as possible and packing any space with foam or similar.

The current skirting is nailed into wooden chocks between the brick, which was apparently a common way of doing it. Once I remove the living room skirting, I'm going to try and replace using adhesive if possible, asit just seems much easier and less scope for damage.
 
-strip the walls back to brick,
-stud them with 2x3 CSL
-finish your rewire/plum
-stick 50mm PIR in the stud with a small breather gap (ensuring plumbing is on the warm side)
-tape and foam gaps
-board and plaster

having insulated my solid walls through a variety of different methods this has proved the best really

The other include foaming thin insulated boards on to brick - cost > benefit
and dot and dabbing insulated boards onto brick - biggest risk of damp
 
Last edited:
eviled said:
-strip the walls back to brick,
-stud them with 2x3 CSL
-finish your rewire/plum
-stick 50mm PIR in the stud with a small breather gap (ensuring plumbing is on the warm side)
-tape and foam gaps
-board and plaster

having insulated my solid walls through a variety of different methods this has proved the best really

The other include foaming thin insulated boards on to brick - cost > benefit
and dot and dabbing insulated boards onto brick - biggest risk of damp
Click to expand...
See, what I don't get it how this is any different than stopping the wall from breathing?

Quite happy to do this approach if it is possible. Why is this better than dot and dab as well?

Edit: ah you mean a breather gap between the wall and the PIR? Not the PIR and the board?
 
Last edited:
I went with the warm batten method when I did mine. Not sure a breather gap achieves anything if its not then vented out.

www.mannokbuild.com

Insulating solid walls with internal dry lining – insulation boards fixed by battens - Mannok Insulation

Wall Insulation: Insulating solid walls with internal dry lining – Mannok's Therm Wall insulation boards are high performance PIR insulation boards with a rigid thermoset insulation core, faced on both sides with a low emissivity composite foil facing. These foil facings increase the thermal...
www.mannokbuild.com www.mannokbuild.com

And used these things to hold the battens, which don't crush the insulation.

thorhelical.com

Batten Frame Fixings - Thor Helical | Wall Ties | Helical Bars | Insulation Fixings | Europe | Asia | America | Australasia | South Africa

Batten frame fixings are high speed hammer driven fasteners. The metal fixings secure timber battens and frames to brick, block and concrete
thorhelical.com thorhelical.com
 
dLockers said:
See, what I don't get it how this is any different than stopping the wall from breathing?

Quite happy to do this approach if it is possible. Why is this better than dot and dab as well?

Edit: ah you mean a breather gap between the wall and the PIR? Not the PIR and the board?
Click to expand...
yes 3" CSL is ~63mm if my memory serves me, which gives a nice gap beteween the PIR and brick (also for cables)
 
eviled said:
to be honest if the walls are in good nick outside and face south-ish I would dot and bad 50mm insulated board and just stud the walls at high risk of damp
Click to expand...
Is there an idiots guide to determining risk of damp?

I had some visible damp but this was due to the door frame wicking moisture in.
 
From what I've briefly read for a solid wall

Batten studwork to the the brick with a DPC on the connection between brick and studwork (to prevent the studwork getting wet through any capillary action) . I believe at around 400mm or 600mm centres

Then you can just drywall screw (might need long screws) insulated plasterboard over the internal walls. (e.g. 112. 5mm will be 100mm PIR with 12.5mm plasterboard on it.

The problem with this is you could end up losing quite a bit of internal space.


Some more info on it here

GreenSpec: Housing Retrofit: Solid Wall Insulation: Internal lining

Internal retrofit of insulation is more difficult than external application. There are various methods- but each should be modelled to check for condensation
www.greenspec.co.uk www.greenspec.co.uk
 
You must log in or register to reply here.
Back
Top Bottom