To improve a cavity wall insulation

[amstel]

Hello, I live in a 50's bungalow, with 11" cavity walls, stock on the outside, commons inner leaf, I would just like to improve the insulation while I am doing some decorating etc.
The whole bungalow has had that fluffy white stuff injected, it must of been 30 years ago approx, ( we have been here 22years) and it looks and feels rubbish.
It is the living room I want to do, it has quite a large outside wall, Iv'e been reading insulation threads all afternoon and would just like your opinion please.
Do you think 25mm pir Iko enertherm would be plenty, or maybe 50mm? should I fix inbetween battens or without battens, and then 12.5 plasterboard.
Thank you.

 

[DAZB]

50mm is preferable to 25mm and battens will give you something to fix to for the plasterboards. Bear in mind that if you use 50mm PIR make sure you measure the timbers you buy because although it is often sold as 2 X 2 it will measure less than 50mm and if your timbers are fixed directly to the wall the insulation will sit proud. If you can afford the space then leave a gap behind the studwork and the PIR can sit flush on the face.

 

[amstel]

Thanks Dazb, I will go with the 50mm pirboards, useing battens, it means because the the plasterboard and insulation are the same widths (1200 x 2400) I will have to trim the insulation to fit between the battens, is that right?
It will certainly make this wall warmer even if if it is a cavity wall.

 

[cdbe]

I would foam 50mm insulation backed plasterboard direct to the wall to avoid thermal bridging caused by battens, and look into "topping up" the cavity insulation - it's a pretty effective product but but does settle over time.

 

[amstel]

Hi Cdbe, is it possible to use expanding foam on 50mm pir boards, and then foam the plasterboard to the pir insulation? do you just squirt the foam randomely and the press against the brickwork?

 

[cdbe]

Hi Cdbe, is it possible to use expanding foam on 50mm pir boards, and then foam the plasterboard to the pir insulation? do you just squirt the foam randomely and the press against the brickwork?

Have a search on here, lots have done this and you'll be able to consider the pros and cons - bonded Vs separate PIR and boards and techniques for fixing.

 

[mrrusty]

There is a potential problem. If you internally insulate, the inner brick skin will be much colder. It is likely to then have interstitial condensation, and because the cavity is no longer ventilated (because it's filled) there will be no air movement to evaporate any dampness that does occur. Proceed with caution.

Dew point of room air (typically 20-22 degrees at 50-60% humidity) will be as high as 12-13 degrees. If that inner brick is below that temperature it will be subject to condensation.

Insulated PB does usually include a VCL which can assist in controlling the risk.

 

[amstel]

Hi MRRusty, thanks for the warning, where would this condensation appear? I was thinking of fitting a vcl or dpm membrane after the insulation/before the 12.5 plasterboard?
This white fluffy stuff in the cavity looks really past it to me, all the bungalows here have solid floors and no air bricks to seen which I think is correct for solid floors, thanks.

 

[mrrusty]

It would appear in the brick itself if warm moist room air is allowed to reach cold brick behind insulation. IMHO if insulating internally you also need to pay attention to air tightness. I am not a fan of cavity wall insulation because the ventilation in the cavity helps to keep the wall dry.

 

[amstel]

Thanks, I can follow that now, another consideration is forget battens and use foam, silver tape all joints, then taper edge 12.5 plasterboards, skim joints, sand down, Paint. then possibly a air brick on outside a course up from the double slate dpm?
If I do the above should I fit a vcl or similar just before the plasterboard? Thanks.

 

[mrrusty]

Personally I'd just use insulated PB - it already has a VCL. It fixes with either foam or dab adhesive. I think some air bricks in to the cavity is a good idea, but there are so many different opinions, others might have different ideas...

 

[^woody^]

25mm batten.
25mm PIR between battens.
25mm PIR across the face.
Foil tape all joints.
Plasterboard and skim.

You can do this with 50mm PIR, and cut a strip to cover the battens.

 

[amstel]

Thanks Woody, that's another very good option, I'll take a trip to Selco and price it up, they seem the best bet rather than Wickes/b&q, I allready have a roll of 3" foil and some 25mm pir to cover the battens.