I am about to insulate my solid walled extension (of a 1936 cavity walled house – not sure of the date of the extension). The solid wall is twin skin, around 9" thick.
The Options
I had set my heart on using wood fibre boards (i.e. PavaWall, Pavatherm, Pavatex, Diffutherm, etc), but in order to get the desired U-value (arounf 0.35) these would have to be 100mm thick with an additional 15-20mm clay plaster coat on top. The room is not that big now so losing 120mm off 3 walls would lose me nearly 1m2 of floor space. Good thing about this approach however is that it is environmentally friendly (recycled), non-toxic and breathable. Cost is quite high though.
So I think I have now decided on going for optimum insulation to reduce the thickness required. However, because a vapour barrier is needed (non-breathing material which degrades with mositure content) I am thinking a cavity is needed between the barrier and plasterboard to prevent the barrier being punctured if driving nails through the wall. This will add to the depth and negate a lot if the benefit of the "better" insulation performance. (I was thinking of 40-50mm of Celotex tuff-R GA30**, 25x50mm treated battens (producing a 25mm cavity), 10mm plasterboard or Fermacell).
An alternative would be to use the laminated plasterboards such as Kingspan K17/k18. However, although these have an integrated vapour check how effective is this considering this will be broken at each of the boards' joints? Also, I take it no nails are allowed as this will also comporimise the vapour barrier?
Some (lots of!) questions:
1) 3 walls are external solid walls. The 4th wall is a cavity wall from the main house - presumably I DON'T need to insulate this wall (even though the cavity is not insulated)?
2) Do all walls have to be done the same - i.e. same material, construction, thickness? Or could I leave one wall as breathable to ensure I don't get a condensation/moisure problem that will require lots of ventilation to fix?
3) Can I put insulation directly against the wall? I don't believe I have damp walls but if damp was a problem would that water seal solution you can buy be sufficient to stop penetrating damp (applied externally)?
4) I have removed the plaster from 2 of the 3 walls so I can claim back some of the depth. However I was going to keep the plaster on the 3rd wall - is there any problem with installing insulation on top of an existing plastered wall?
5) Does anyone know of the difference between Kingspan K17/18? From what I can tell, one can be adhesively bonded to the wall but both can be mechanically fixed. I guess going for the one that can do both gives me more options. Would adhesive bonding be a bad idea for a solid wall (it kind of implies this in the literature, but if I don't have a damp problem would it be OK?).
6) Would anyone recommend using these Kingspan k17/18 boards? I have concerns about the vapour barrier - how do you make it continuous? The insulation is phenolic which I read is better with moisture because any ingress due to punctures will remain localised. Any comments?
6a) What do we do at corners with laminate boards because there will definitely be a large break in the barrier here and potentially the plasterboard will create a thermal bridge?
7) If I went the Celotex separate route instead how could I fix the battens (over the insulation) to the wall? Should I use frame fixings and should these be of the hammer-in kind? Although this will reduce thermal bridging won't this just puncture the vapour barrier (foil face of insulation)? Are there any steps that can be taken to reduce this effect? Is there anything better that Celotex tuff-R?
8 ) I have depth issues around the sash window and door frame on 2 of the walls (the ones I have removed the plaster from). Rather than reduce the depth of insulation on the whole wall I was thinking of tapering the plasterboard lining near these points. Has anyone got any experience of doing this type of thing?
9) I need to fix a radiator back to the wall afterwards. Can I just use long screws and screw into the brick or do I need to use wooden noggings fixied to the appropriate points on the wall first then reduce the thickness of insulation over these?
10) If I go the separates route I may opt for Fermacell 10mm board instead of plasterboard as this is much stronger and I could get away without a plaster coat (apparently). It is also more moisture resistant apparently. Any comments on using Fermacell?
11) I have a single glazed sash window. With all this extra insulation I guess I am likely to get much more condensation forming on this. Should I double-glaze this at the same time? Maybe add a trickle vent as well?
12) If I manage to form a pretty good vapour barrier do I need to worry about the dew point? I guess if I get a dew point calculation and it turns out OK, I won't have to obsess so much about the barrier (or have I got this wrong?).
Lots of questions. Any help would be most appreciated.
The Options
I had set my heart on using wood fibre boards (i.e. PavaWall, Pavatherm, Pavatex, Diffutherm, etc), but in order to get the desired U-value (arounf 0.35) these would have to be 100mm thick with an additional 15-20mm clay plaster coat on top. The room is not that big now so losing 120mm off 3 walls would lose me nearly 1m2 of floor space. Good thing about this approach however is that it is environmentally friendly (recycled), non-toxic and breathable. Cost is quite high though.
So I think I have now decided on going for optimum insulation to reduce the thickness required. However, because a vapour barrier is needed (non-breathing material which degrades with mositure content) I am thinking a cavity is needed between the barrier and plasterboard to prevent the barrier being punctured if driving nails through the wall. This will add to the depth and negate a lot if the benefit of the "better" insulation performance. (I was thinking of 40-50mm of Celotex tuff-R GA30**, 25x50mm treated battens (producing a 25mm cavity), 10mm plasterboard or Fermacell).
An alternative would be to use the laminated plasterboards such as Kingspan K17/k18. However, although these have an integrated vapour check how effective is this considering this will be broken at each of the boards' joints? Also, I take it no nails are allowed as this will also comporimise the vapour barrier?
Some (lots of!) questions:
1) 3 walls are external solid walls. The 4th wall is a cavity wall from the main house - presumably I DON'T need to insulate this wall (even though the cavity is not insulated)?
2) Do all walls have to be done the same - i.e. same material, construction, thickness? Or could I leave one wall as breathable to ensure I don't get a condensation/moisure problem that will require lots of ventilation to fix?
3) Can I put insulation directly against the wall? I don't believe I have damp walls but if damp was a problem would that water seal solution you can buy be sufficient to stop penetrating damp (applied externally)?
4) I have removed the plaster from 2 of the 3 walls so I can claim back some of the depth. However I was going to keep the plaster on the 3rd wall - is there any problem with installing insulation on top of an existing plastered wall?
5) Does anyone know of the difference between Kingspan K17/18? From what I can tell, one can be adhesively bonded to the wall but both can be mechanically fixed. I guess going for the one that can do both gives me more options. Would adhesive bonding be a bad idea for a solid wall (it kind of implies this in the literature, but if I don't have a damp problem would it be OK?).
6) Would anyone recommend using these Kingspan k17/18 boards? I have concerns about the vapour barrier - how do you make it continuous? The insulation is phenolic which I read is better with moisture because any ingress due to punctures will remain localised. Any comments?
6a) What do we do at corners with laminate boards because there will definitely be a large break in the barrier here and potentially the plasterboard will create a thermal bridge?
7) If I went the Celotex separate route instead how could I fix the battens (over the insulation) to the wall? Should I use frame fixings and should these be of the hammer-in kind? Although this will reduce thermal bridging won't this just puncture the vapour barrier (foil face of insulation)? Are there any steps that can be taken to reduce this effect? Is there anything better that Celotex tuff-R?
8 ) I have depth issues around the sash window and door frame on 2 of the walls (the ones I have removed the plaster from). Rather than reduce the depth of insulation on the whole wall I was thinking of tapering the plasterboard lining near these points. Has anyone got any experience of doing this type of thing?
9) I need to fix a radiator back to the wall afterwards. Can I just use long screws and screw into the brick or do I need to use wooden noggings fixied to the appropriate points on the wall first then reduce the thickness of insulation over these?
10) If I go the separates route I may opt for Fermacell 10mm board instead of plasterboard as this is much stronger and I could get away without a plaster coat (apparently). It is also more moisture resistant apparently. Any comments on using Fermacell?
11) I have a single glazed sash window. With all this extra insulation I guess I am likely to get much more condensation forming on this. Should I double-glaze this at the same time? Maybe add a trickle vent as well?
12) If I manage to form a pretty good vapour barrier do I need to worry about the dew point? I guess if I get a dew point calculation and it turns out OK, I won't have to obsess so much about the barrier (or have I got this wrong?).
Lots of questions. Any help would be most appreciated.
