Internal wall insulation for timber frame house

corman007

Hi, I am looking to upgrade the internal wall insulation (only external facing walls) in my hall, landing, and downstairs toilet. My house was built in 2005 and not too well. Some years ago I completely gutted the box room. I found the vapour barrier was loosely hanging thin polythene crap + badly fitted yellow fibre insulation. The hole around the vent was not insulated at all and more air came in from the cavity than from the outside, also the area around the window was not insulated. I put in better insulation, added a cellulose vapour barrier and sealed around the window & floor with Siga tape. I also upgraded all windows and sealed them with the tape as well. This made a big difference.
I got a thermal scan done of the house after which showed I had air leakage at every joint between the internal and external walls - I previously found there's no insulation here.
I want to remove the all the plasterboard and install better wool insulation + vapour barrier same as before (I still have a lot left over). However this time I want to add 52.5mm insulated plasterboard on top of this. Is there an issue with this? My studs are 90mm-100mm deep. I remember researching this before and things like dew point etc came up. I understand air tightness is a key but I'd also like to add the extra bit of insulation even if its 37mm.
Thanks James.

BryanF

James

Can you give exact wall build-up

For now can I assume the following;

Brick/block outer leaf
50mm air cavity (essential this is maintained)
Plywood ‘racking board’ (should have breathed membrane on cavity side)
4’’ stud frame (some crap fitted insulation)
Vapour barrier ( albeit poorly fitted, not sealed hence not air-right)
Plasterboard

I think you already get the dew point issue (rule of thumb is to have 2/3’ insulation outside vapour barrier )

So assuming you go with 37mm plasterboard that would be ok

BUT really it would be best to remove existing plasterboard and sort stud frame insulation and seal/tape vapour barrier. Then put up the 37mm warm board on the inside

corman007

Hi BryanF, thanks for reply.
Yes that is the wall setup - I only have ~ 90mm for insulation between the studs.

I also fully removed the plasterboard and insulation the last time so would do that again and seal between studs and plywood racking board (this is very thin).

The lad I got to do the thermal scan showed me where the air was coming in and suggested spray foam - the house is very leaky for air ingress point. I'm reluctant to do this as I have many rolls of insulation and vapour barrier left over so want to get rid of this (in a good way) + I can do a lot of it myself.

kilcul123

Looking at undertaking a similar project myself. Every room probably needs doing but I'm thinking of just doing two rooms to start, as some seem better insulated than others (similar build from the same era as your house). How did this go for you?