Insulating a 1930s semi

[Carwood]

Hi All

I’m in the process of renovating a 1930s semi which is currently stripped back to brick as the plaster had blown on almost every wall in the house. I’m now struggling with which insulation methods to use due to the conflicting information on both the internet and by trades. The house currently only has insulation rolls in the attic which I will be adding more layers to. The house has cavity walls with no insulation, but the cavity is quite small at 35-40mm. There is a suspended wooden floor on the ground floor which I will be insulating with PIR boards forming an airtight seal with the floor above. This leaves the external walls to deal with. I was planning on using insulated plasterboard internally but I’ve read about lots of issues using this method regarding moisture behind and at the sides of the insulation. Additionally insulated plasterboard is a pain for hanging radiators and kitchen cupboards on. What is everyone’s thoughts? Should I insulate the walls or just leave them?

 

[^woody^]

I’ve read about lots of issues using this method regarding moisture behind and at the sides of the insulation.

Best thing to do is stop reading stuff from wherever you read that nonsense.

And BTW, you don't really have a choice about whether you insulate or not, as building regulations require that you do having stripped all the walls.

Cavities that small can be insulated, but its a case of what insulation method provides the greatest payback. CWI may not be the best choice, but then losing space internally may not be a good option either

The most efficient will be to line internally with a PIR product, and not standard insulated board.

 

[JP_]

Put extra in wherever you can - I have a 1930s solid wall bungalow with suspended timber. Each room did a bit different as was learning on the job! but the last one is the cosiest - i fitted battens (some treated trellis poles I had) under the floor joists and put 50mm Kingspan on top, then 50mm rockwool on top of that. Seems better than just kingspan.

I put 50mm kingspan on my external walls, all taped up, and glued on with Everbuild Pink Dry Fix Foam, then with plasterboard glued to that. Everything is cosy and warm now.

Treat the joists wit preserve (I used Everbuild triple action) and clean out the sub-floor. I had so much crap below my floor boards, over an inch thick in some places. All that stuff collects moisture and life.

Of course, must be more sensible ways to insulated a cavity wall, although 45mm is not that thick is it? Putting loads extra in the loft helped us too.

The first winter I lived in my new house (2017/18) I had the heating on 24/7 (reduced the thermostat to 15 during the day when at work, and about 17 at night). Last winter the heating was off all night from about 10.30pm - 6am, and off when during the day too. Gas bills lower by at least £30 a month this year, so worth the investment. Still got some rooms to do!

 

[charliegolf]

If cavities are still required under BR* in order to stop damp, why is it ok to fill them? And whatever the reason is, why does that reasom not make it ok to fill timber frame cavities? Genuinely interested.

CG


*I assume

 

[JP_]

Surely the insulation injected into cavity walls is designed so that it cannot absorb any water? That is purely an assumption, never looked into it.

http://www.yougen.co.uk/blog-entry/1796/Does+cavity+wall+insulation+cause+damp+'26+condensation'3F/

according to the above, the problem is already there - maybe the insulation restricted air flow, so the snots don't dry out? They suggest a thorough inspection before pumping insulation in.

Or just insulate the internal walls. Although much more disruptive and more expensive!

 

[^woody^]

If cavities are still required under BR* in order to stop damp, why is it ok to fill them? And whatever the reason is, why does that reasom not make it ok to fill timber frame cavities? Genuinely interested.

CG


*I assume

Cavities are not required.

As for filling them, the way water transfers across walls means that a fibre filled cavity is not a conduit for moisture.

Timber frames require a ventilated cavity to prevent the frame rotting from mould - caused by humidity mainly.

 

[charliegolf]

Cheers for that.

CG

 

[Notch7]

If cavities are still required under BR* in order to stop damp, why is it ok to fill them? And whatever the reason is, why does that reasom not make it ok to fill timber frame cavities? Genuinely interested.

CG


*I assume

It isnt ok to retro fill cavities.

Do a google search and you find many problems associated with CWI.

The problems are caused partly because of the construction of some properties and some property locations arent suitable and partly because of faulty installation.

Very exposed houses, like those in north coastal areas should avoid CVI.

Install faults include pockets of missed insulation that cause damp spots.

New build construction often has full fill rockwall -I dont know if they ever suffer issues, maybe not because of larger cavities.

 

[Notch7]

There is a suspended wooden floor on the ground floor which I will be insulating with PIR boards forming an airtight seal with the floor above. This leaves the external walls to deal with. I was planning on using insulated plasterboard internally

For suspended floors, the best method is to pin sawn 38 x 19 battens dow both sides, cut PIR insulation in strips to fit (about 8mm small). Lay in place with equal gap each side and fill with spray foam (from a proper adjustable gun, turn right down). When dry cut foam off flush, foil tape the joints, goung across the joists.

A cheaper method is to lay bird netting across the joists, staple on and fill with loft roll insulation.

For walls -totally cover the walls with 50mm PIR insulation. Tape all joints. Screw on 50 x 25mm battens flat (75 × 25 battens at radiator locations. Then fit foil backed plasterboard.

 

[Leofric]

Or just insulate the internal walls.

Hate to be pedantic but presume you mean add insulation to the internal face of external walls , not insulate internal partition walls.

 

[Leofric]

It isnt ok to retro fill cavities.

Depends on the situation , some properties are not suitable for retro fill cavity wall insulation but that doesn't mean all properties are unsuitable .

 

[Leofric]

the cavity is quite small at 35-40mm.

Cavity wall insulation is not suitable for wall cavities that small.

 

[Notch7]

Depends on the situation , some properties are not suitable for retro fill cavity wall insulation but that doesn't mean all properties are unsuitable .

Yeah -apologies for misleading, I had meant to go on and say that.

 

[^woody^]

It isnt ok to retro fill cavities.

It is OK. Perfectly OK.

The problems you refer to are existing defects with the wall, and not caused by the fill.

Yes exposure and certain cavity types will make a wall not suitable, but that's all well documented as to what is a suitable wall.

And yes pockets on incorrectly pumped fill may be an issue, but that's a quality issue, in the same way as if I tiled the OP's roof and left a few roof tiles out.

 

[Carwood]

I’ve inspected the cavities at a few points where I’m moving windows and adding a new door, they are full of mortar ‘snots’ on the ties and there is mortar even bridging in some places that I’ve removed where I could reach. I think cavity insulation is a no go for me. To clarify regarding insulting internal walls, I meant add insulation to the internal face of an external wall. Would you recommend studding and adding board in between the battons or using insulated plasterboard? I’m wary of adding too much insulation as it will eat onto the living space. Additionally I can’t add any insulation next to my stairs as it would hang over them!