Heatloss through Lath & plaster walls

[DrewB]

Hi All,

I've been squatting on this forum for a while in the hopes that someone might cover the issue I have with an Edwardian house but no one has so here goes:

As I say, I live in an Edwardian house which is stone built. The outside walls have lath and plaster cladding on the insides with the usual gap between the laths and the stonework.
This gap goes from solum to loft. In the loft I can feel a distinct warm airflow coming out of that gap at the top of the wall which must represent a very significant heat loss.
I've lagged the loft with about 10" of glass wool but have hesitated to cover the top of that gap. I'm guessing that an air flow is essential to prevent damp since it is a solid stone outer wall. The question is how best to reduce that heat loss. I've rules out a couple of options:
- Replace the lath and plaster with modern insulation - would destroy to many original features of the house.
- Seal the top/bottom of the gap - potential of damp problem and or rot
- Line the walls on the inside - would require the removal of the cornices

I'd appreciate the groups thoughs on one idea that has been suggested. It basically the block the gap but with a twist.
How about blocking the gap top and bottom but adding vents into the house. That is to say add vents at floor level on the ground floor and ceiling level on the upper floor. The idea being that these would allow sufficient air circulation to keep the wall etc dry but keep, most of, the heat in the house.
What do you think?
I'd guess that with the heat differences in the house a draft would be set up down the gap taking the cooler bedroom air back down to the warmer living room etc. To my mind the unknowns are the extent and effectiveness of this air movement and the risk of extranious drafts, from around the sash window frames for example.

There must be a non-destructive way to reduce these current heat loss.

Thanks,
Drew

 

[masochists-r-us]

Hi

Interesting problem. Hope this helps. My view is that the air flow is essential to prevent damp. Vents could be the answer but you run the risk of damp if the vents are not big enough to allow sufficient air flow. To be certain you have sufficient air flow the area of the vents would need to match the total area of the gap at the top and at the bottom of the walls. However, even then unless the vent runs the total preimeter of the walls I think there is a risk of local areas not receiving sufficient ventilation leading to damp spots. This I have to say is all theoretical and in practice small vents may be sufficient. But this is a risk I would not take. I put forward an alternative idea below.

From the description you give, you have insulated the loft horizontally and therefore the heat escapes into the roof space but is then able to escape unchecked by the insulation you have added. Have you considered insulating the sloping part of the roof. This way the heat will not be lost. The house will then be isulated by the original insulation, the warm air bubble and the second layer of insulation.

 

[MdB]

Welcome to the forums Drew. Unfortunately, I don't see any easy answers. You have come up with the obvious solutions and the problems with them. Hopefully someone else will have a good solution for you.

 

[ljn]

I have a similar setup- solid brick walls lined on the inside with lath and plaster, leaving a gap for ventilation.
On the positive side, the gap prevents the thermal mass of the outside wall cold-bridging into the rooms inside.
On the negative side, hot air is drawn up through floorboards etc and goes up the inside of the wall lining.
Another good thing is that any damp coming from inside, dries out before reaching the inside, saving the decoration.
In a couple of small areas, someone has removed the lath and plaster and cemented and skimmed directly on the bricks and then applied wallpaper. When the rainwater hopper got blocked and water poured down the building, the cement bit got black mould and had to be stripped back while the traditional bit was totally immune.


The roof also works on the same principle. Lots of ventilation. There is no membrane and some tiles are cracked. Also the internal gulley overflowed in one of last year's flash floods. but it just dries out before causing damage.
The roofer said that in, the new waterproof layers they put under the slates are breathable, so in theory, I could insulate the roof and have a warm roof.
In practice, I think the breathability could only cope with a bit of condensation, and if for some reason water got in, there would be trapped damp and then mould and rot.

So I'm leaving as it is as the system seems to work. I might seal the floorboards, definitely draft seal the windows and put a ball of newspaper up unused chimneys ( chimney sweep recommended) That should improve things.

 

[Hatster]

This is a very long term "solution", but I recently met some University researchers who are conducting field trials of a system that 'collects' the rising hot air and push it down to the bottom of the building again. Perhaps in about 20 years, you'll be able to buy their product in B&Q? :wink:

 

[chuckey]

This is an unusual form of construction. You say "with the usual gap between the laths and the stonework." Do you mean that you have a single sided stud wall standing close to the outside wall? In this case the "continous gap" is actually the closeness of the floorboards to the stone wall and a 4" cavity between the back of the L&P and the stonework. What one gets with sort of wall, is that the plaster "curls" break off and fill the bottom of the wall cavity*, so if there was any dampness, you would see this at the bottom of the wall on each storey where the bits of plaster get stuck on the floorboards and joists.
I would lift a downstairs floorboard and with bits of garden cane and bent coat hangers actually measure the depth of the continous cavity and the depth of the cavity behind the plastered panels and also see if a load of plaster bits fall down when you start to rattle about in the cavity.
Frank
*In a Victorian house I worked on, the cavities at the bottom of a standard L&P stud wall was filled to over a foot high with plaster "shrapnel".

 

[DrewB]

Hi

I had considered lagging the roof space as masochists-r-us suggested but have the same concerns as ljn expressed. The roof is slate on timber, there must be some form of sarking but it will also be 90+ years old. The roof is sound but I have had a few small leaks over the years from gullies or slipped slates and would not like such leaks to go un-noticed as I'm sure that would cause greater problems over time. So lagging the roof space does not seem an option to me but perhaps I should investigate further.

Heat pumps, now there's an idea.

chuckey: The construction seems fairly standard around here. The l&p is held about 1.5" off the solid stone outer wall by wood strapping etc. I know the bottom of the gap between the stone and l&p is clear all round as it stops a floor level above a the sole plates (?) on which the joist rest. Since the solum is 4 to 8 feet deep access and inspection is easy (once the hatch is cleared of furnature )

Thanks for the thoughs folks, keep them coming

Drew

 

[chuckey]

Very interesting. Lets think about this, if the wall was leaking water to any extent, then your "sole plate" which I think is a wall plate - a wall stands on a sole plate, would be rotten? and so would your joist ends? so you have a decent waterproof wall. I take your point about ventilation, but your wall is like one of ten million solid brick walls that are in our cities, Victorian builds and right up to 1930s semis. In the majority of these houses, there is no thought to ventilate the wall, just slap on the plaster and plastic wallpaper.
I would have thought that using something like loose fill, will provide a measure of insulation and still preserve some ventilation. Again if the wall was wet, then the fixings for the wall batens would have rotted out, leaving a very springy P&B wall.
There is a good argument for not trying to insulate the gap too well, otherwise the dew point will appear within the insulation, causing intersticial condensation which will lead to nail and wood rot. ( unless you paper with plastic paper )
My $.02 worth
Frank

 

[Lime]

I think this is a difficult one to answer.
Obviously the OP knows the set up better than most householders would so has been through all the possibilities.

I would not fill between the stone and the laths with anything.
Too many problems could ensue from that.
if there is a lot of detritus and broken plaster in the cavity that is best cleared out if possible.

I would experiment by restricting the upward air flow by some means.
This could be by either layering a thin fibrous material over the loft gaps or short pieces of a more solid construction spead out at intervals so as to reduce the total air gap.
I would not stop the air flow completely.
It would need to be monitored.

 

[michael]

Just a thought but are we all sure that blocking the top of the cavity in the loft would stop the air flow? Surely as long as the bottom of the cavity has somewhere for warm air from the living rooms to get in (which it seems it has) then you would get warm air from downstairs rising and cool air or 'cooled off ' air from the lesser heated top of the house falling, a kind of rising and falling cycle? Also when the heating goes on the air rises and when the heating goes off and the house cools down the air falls.There is it would seem air movement of some kind as long as the air can get in. As I say just a thought.

 

[worms]

In Michael's rising and falling cycle above, is there not a problem that the warm rising air would carry more moisture which would condense out as the air cools and be adding to the dampness in the gap between plaster and wall? In other words, the warm air going in would carry more water vapour than could be carried by the cold air going back out. The moisture has to go somewhere. In most cases that goes into the larger area of the loft space which tends to be reasonably well ventilated. The heat loss is a problem, but the condensation might be a greater one.

 

[Lime]

I presumed that the warm air coming up through the gap in the loft was solum air heated by conduction from off the laths and plaster inner wall.
If there are holes through the plaster and laths then they need to be sealed in some way as they would introduce moist air from the heated rooms into the cooler cavity.