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.

  • I'm assuming the walls are plaster directly on the brick? Sep 10, 2012 at 21:05
  • currently yes Chris, but this is 60+ years old and mainly crumbling away
    – Charlie B
    Sep 11, 2012 at 8:29

4 Answers 4


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.

  • apologies, wasnt quite clear, with the spare rooms i meant insulate the floors so that the heat from underneath isnt lost in to unused floors.
    – Charlie B
    Sep 12, 2012 at 7:41

I own a semi detached brick building in Boston. I did extensive research and it seems like its not recommended to insulate masonry structures from the inside. Condensation becomes a big issue and the brick detiorates.

  • thanks Vitaliy, quick check though, are you in Boston USA or UK.
    – Charlie B
    Sep 11, 2012 at 8:27
  • I don't agree. Proper insulation of brick can be done but the work has to be done PROPERLY with sound vapor barriers to prevent condensation concerns. Sep 11, 2012 at 12:28

I was going to tell you not to worry about insulating, as current thinking leans toward stopping air flow through exterior walls instead of stopping heat transfer, but then I did a little research on brick. To say the least, it's complicated. Here's one article that speaks to the complexity: http://www.greenbuildingadvisor.com/blogs/dept/musings/insulating-old-brick-building. It seems like your best bet is still to limit your air infiltration through windows and other wall penetrations, but leaving your brick alone. If you have your heart set on insulating the interior walls, I'd recommend having a consultant review your home to give you a response tailored to your complete situation.

  • worth noting that had a chat with local building control yesterday who advised the best way to insulate the walls is with 27mm insulation backed plasterboard, and the ceilings of the ground floor with 100mm of rockwool
    – Charlie B
    Sep 12, 2012 at 7:44

I'll add another permutation to the mix.

Spark's answer was probably the most correct when he said "it's complicated". As most others have pointed out, dealing with moisture issues is going to be your biggest concern. However, vapor barriers are not always the best solution. In fact, there are different levels of moisture "permeability" that depend on a variety of factors such as climate, amount of insulation, and even the temperature and humidity you keep inside your house.

For a detailed research paper on this topic with a variety of options check out the RR-1105 report from Building Science Corp.

Lastly, to address your concern about losing interior space: One approach to take would be to insulate from the outside instead of from the inside. You'd get to keep your interior space. However, the tradeoff would obviously be a changed aesthetic from the outside.

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