3 ms·

The one key rule that you should always follow is to have your water-impermeable layer on the warm side of the insulation. So, I'd recommend against breathable

The one key rule that you should always follow is to have your water-impermeable layer on the warm side of the insulation. So, I'd recommend against breathable internal insulation, because that will make your solid wall (water-impermeable layer) colder, but still exposed to the humid inside air, and this could cause condensation on that cold wall. As an example of this, I have rubbish double glazing, and when the temperature drops outside, if I have just the curtains drawn (which don't really provide any insulation) then I'm fine, but if I lower the thermal blackout blinds, then I get loads of condensation on the inside of the windows, which I remove with a window vac in the morning. Breathable external insulation is less of a problem, because it warms up the wall, while exposing it to the comparatively dry air from outside, so you shouldn't get any condensation.

HPsquared 5y ago
Agreed, houses with insulation have a vapour barrier behind the plasterboard for this reason: counterintuitively, the warm internal air carries a lot more moisture than the cold air outside. The partial pressure of water vapour is higher inside the house than outside, so the dew point of the internal air is higher than the outside temperature (dew point depends on water content). If there's a way for that moisture to diffuse through the wall from inside, through the insulation, to the cold structure (which would be colder than before due to the internal insulation), the conditions are right for condensation to form. Those conditions are: surface temperature ≤ dew point. Condensation can be prevented by a combination of: 1. Reduce the rate of moisture diffusion from the interior (seal the rooms and use a vapour barrier) 2. Lower the dew point of the air in the "cold" structural areas by ventilating the cold space to outside, so the moisture can exit (this is why houses have air bricks, vents in the roof, etc.) 3. Prevent the building structure from being colder than the internal dew point (i.e. use external insulation)
markvdb 5y ago
> The one key rule that you should always follow is to have your water-impermeable layer on the warm side of the insulation. This very much holds true in 99% of modern homes.. except if you build your entire wall breathable. That is how we did our walls. From inside to out: - breathable paint. Lots of conventional options available. We made a really nice casein paint with low fat cottage cheese and lime paint as the main ingredients :-) - lime plaster. 1/3 slaked lime powder, 2/3 sand. - 1.2m of straw - lime plaster

HPsquared 5y ago

Agreed, houses with insulation have a vapour barrier behind the plasterboard for this reason: counterintuitively, the warm internal air carries a lot more moisture than the cold air outside. The partial pressure of water vapour is higher inside the house than outside, so the dew point of the internal air is higher than the outside temperature (dew point depends on water content). If there's a way for that moisture to diffuse through the wall from inside, through the insulation, to the cold structure (which would be colder than before due to the internal insulation), the conditions are right for condensation to form. Those conditions are: surface temperature ≤ dew point. Condensation can be prevented by a combination of: 1. Reduce the rate of moisture diffusion from the interior (seal the rooms and use a vapour barrier) 2. Lower the dew point of the air in the "cold" structural areas by ventilating the cold space to outside, so the moisture can exit (this is why houses have air bricks, vents in the roof, etc.) 3. Prevent the building structure from being colder than the internal dew point (i.e. use external insulation)

markvdb 5y ago

> The one key rule that you should always follow is to have your water-impermeable layer on the warm side of the insulation. This very much holds true in 99% of modern homes.. except if you build your entire wall breathable. That is how we did our walls. From inside to out: - breathable paint. Lots of conventional options available. We made a really nice casein paint with low fat cottage cheese and lime paint as the main ingredients :-) - lime plaster. 1/3 slaked lime powder, 2/3 sand. - 1.2m of straw - lime plaster