I'm renovating a 1920's semi-detached house. The plaster on the walls was slapped straight on the wall about an inch thick, and the ceilings were lath and plaster which had been overboarded. Most of this needs to come off to do repair work to the walls. I want to know what is the best way to insulate the walls and ceilings with the aim of meeting the BC regs u value. ( i believe this is 0.3 [unit needed])
All exterior walls are 9 inch solid brick. The floor joists are 30cm apart with pine floor on top which I don't plan to remove as its all sound.
I plan to insulate the loft with rock-wool/fibreglass to about 400mm and use sound deadening board on the ceilings and most internal walls.
Above the living room and kitchen are spare bedrooms which aren't really used. I'd like to insulate them so that I'm not heating unused space.
My concerns are:
To insulate external walls with insulated plasterboard would mean building a frame and losing a good 4 inches around each room. It would also create a logistical problem in mounting (future) fixtures to the wall.
I'm also concerned about condensation in the joists if I insulate the ground floor ceilings.
Best Answer
Condensation is definitely a concern in colder weather. If you create a thermal break that isn't properly sealed against air leakage, warm air from the house will seep behind the walls, bringing lots and lots of moisture with it. Meanwhile the bricks, which are now insulated, are going to be very very cold, and when warm air gets cold it lets go of its moisture - and thus we get condensation.
That condensation would then get trapped inside the walls - and long before you have a problem with brick deterioration you're going to have mold issues that lead to health issues.
The best thing to do to prevent this is to insulate with a substance that also serves as a vapor barrier, which will prevent any air (and thus any moisture) from leaking into the closed cavity. The best product for this IMO is spray foam insulation, because it fills in all the nooks and crannies and IS the vapor barrier.
Alternatively, you could use fiberglass and cover it over with plastic to form a vapor barrier, and this can work IF you properly seal it all the way around the edges, seams and any holes that occur.
Either approach, however, is going to require you to sacrifice living space, as for a proper thermal break you're going to probably want at least 4" of insulation.
Now - with regard to insulating your spare rooms. Don't insulate your interior walls. Put proper insulation in the external walls of those rooms, so that they don't lose heat to the outside. Yes you will be heating them, but if they're properly insulated they're not going to cost much to heat and there are numerous other concerns with turning an inside room into a "cold" room - it's a finished space, after all, and you don't want to risk damaging that by letting it be freezing cold in there.