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Background:
i'm renovating a house with sandstone and mortar walls. the joints have been re-done some time ago in, surprise surprise, ciment mortar.
so far, so crap.
no likelihood of re-pointing the entire building any time. the walls are all sound at least.
the inside wil just be the original lime-ish, soil and cowshit whatever mortar.
need to check this but highly likely.
currently, allinside walls (external and dividing) are lines with 'briques': 4-5cm thick hollow terracotta briques as used in france and spain, and rendered with a horrible ciment-hard plaster traditionally used as both morter and render for these briques. it is distinctly not breathable. there are some spots of black mold developing, and some salt-peter emerging in other places.
an ideal option would be to smash away ALL the brique and to dry-line with wood-wool, vapour membrane and plasterbaord, as i've done many times before. but here it's not really feasible, both becasue of the amount of work and beacuse the reduction in room volume; they are already tiny.
SO, i'm thinking about lime-hemp. there are plenty of downsides to this too: messy, dealing with plumbing/electrics etc etc, BUT,
WHAT I'M REALLY WANDERING IS:
given the non-breathing exterior walls, due to ciment mortar, is it a bad idea moisture-wise, to use lime-hemp render on the inside?
the added comfort form surface temp of walls would be a big bonus. i know it's nto considered a true INSULATION in some quarters, but it does some insulating nevertheless, and the current render makes the walls, and therfore the rooms, feel horrible.
so, all a bit convoluted but i'm sure there's someone who knows here somewhere
WHAT I’M REALLY WANDERING IS:
given the non-breathing exterior walls, due to ciment mortar, is it a bad idea moisture-wise, to use lime-hemp render on the inside?
Are you *sure* the mortar and render is non breathable? I used to sell Thermoplan (clay blocks, used all across Europe and far more buildings in them than our bricks) and Baumit plasters / renders / mortars. Many of them dry were rock hard feeling, looked like a gritty cement but were in fact very breathable.
I would say so. You have gradient of heat through walls, this helps 'push' moisture through breathable walls. If outer layer is non-breathable, then the moisture can't escape.
That said, volume of moisture through walls is tiny compared to what the room ventilation shifts...
The salting is common on many walls, and isn't in itself 'bad', but can indicate movement of moisture.
The black mould can be just condensation on cold inner wall, then moulding up. We have it in one of our bedrooms.
Could you just get an insulating plaster or insulating wallpaper? I'm trying the insulated wallpaper here next month...
thanks matt,
no, i'm not SURE the render stuff is non-breathable achly. it's just what i'd dedided because of the way the black mold was on the surface, paint over the top mind you, old and shiny, so perhaps not pourous. It's fairly clear the black mold is as you say, simply due to the building not being heated for 2 winters, so the int. wall sufrace being cold and casuign condensation. i'm not too worried the mold longterm therefore.
if the brique render is pourous, then i'd be expecting condensation on the inside of the stone walls. there's a small air gap there.
yes, so, you would reckon it is nto ideal to lime-hemp the inside of the stone? that what my understanding too.
You have gradient of heat through walls, this helps ‘push’ moisture through breathable walls. If outer layer is non-breathable, then the moisture can’t escape.
doesn't this depend also tho where the dew point falls? i could possibly calculate that if pushed, i've got the bumph somewhere. or does the impermeability of ext wall face make this irrelevant; dew point calc only working on fully permeable walls?
and i guess when you asy an insulating plaster, that's what the hemp lime mix is. so yes, that was my thinking
You're right, you do need to know where dew point is. The gap, if it's center, is just a cavity. With an interstitial calc, you may be able to fill this gap with rockwool or similar?