Insulating a lath and plaster wall...

[Reiver]

Sorry for the long absence but COVID has rather messed life up as I'm sure it has for a lot of folk. I haven't managed to find anywhere to do a new build yet, but happened to come across a rather interesting possible renovation project last week which I'm kind of tempted to have a go at if I can make some sense of it. One of the things I'm struggling with is getting any insulation into the walls which are typical hard stone/lime mortar (600mm or so) with lath and plaster on the inside. I'm aware of the breathability issues and the only potential solution I've seen mooted is to blow either cellulose fibre or EPS beads into the cavity behind the lath and plaster. Does anyone have any experience of this please? and/or know of a reputable installer North of the Border.

[Mr Punter]

I don't think you will get a meaningful amount of insulation in there.  How wide is the cavity, and is it clear from obstructions?

[Reiver]

On the wall I looked at there appeared to be 60...70mm of space. Not room for a lot I know, but considerably better than none.

[scottishjohn]

maybe time to think about getting rid of the lathe and plaster and build stud walls -providing rooms are not very small

 

 maybe a bigger job then you wanted but is the right way to go long term  and also a chance to replace electrics etc at same time  and have house with modern insulation stds 

 if rooms are big enough you don,t even need to rip out old plaster +lathes-just build a modern house inside it inside it ?

 apply vapour barrier to the frames before you lift them up into place and you got full damp proofing at same  time and a gap for the old walls to breath into the roof space 

a house that old will be built with a cold roof and lots of drafts in the loft area -so you kill all problems at same time

also will be a job you can do while still living in it ,a bit at a time if you wish 

 filling the gap with anything will stop the air flow to the roof you must have with this type of old house -you will make more damp problems 

Edited July 26, 2021 by scottishjohn

[joe90]

Or remove the lathe and plaster, replace any defective timbers, batten out the existing studs to the desired depth then fill with insulation .

[Reiver]

14 hours ago, scottishjohn said:

maybe time to think about getting rid of the lathe and plaster and build stud walls -providing rooms are not very small

 

 maybe a bigger job then you wanted but is the right way to go long term  and also a chance to replace electrics etc at same time  and have house with modern insulation stds 

 if rooms are big enough you don,t even need to rip out old plaster +lathes-just build a modern house inside it inside it ?

 apply vapour barrier to the frames before you lift them up into place and you got full damp proofing at same  time and a gap for the old walls to breath into the roof space 

a house that old will be built with a cold roof and lots of drafts in the loft area -so you kill all problems at same time

also will be a job you can do while still living in it ,a bit at a time if you wish 

 filling the gap with anything will stop the air flow to the roof you must have with this type of old house -you will make more damp problems 

Thanks for that @scottishjohn . Apart from a box room and a wee bathroom adding some inches to the exterior walls shouldn't be a problem in most rooms, though as you say it will be a lot of work: there will be a fair bit of stuff like cornices, architraves & fire surrounds to replace/move. The vapour barrier idea is interesting, but I'm wondering would there be any long term issues with the plaster which would have been intended to be kept warmish & dry by the heating and would now be outside the insulation envelope?

[TonyT]

pretty sure there is historic scotland  reports on 5 or 6 difficult period properties that the helped insulate.

I have the same build as you, and its the only thing left  to do without removing period features, I have insulated the suspended timber floor from below but left the space at the bottom of the wall open- I may insulate this before the winter

[Gone West]

On 26/07/2021 at 15:16, Reiver said:

Does anyone have any experience of this please?

I don't have personal experience but I remember reading on the Green Building Forum several years ago that a chap in Canada insulated an old house behind lath and plaster walls and it was effective. IIRC the person was Paul-in-Montreal.

[Reiver]

OK thanks for those comments I'll try and check them out.

20 hours ago, TonyT said:

I  have insulated the suspended timber floor from below but left the space at the bottom of the wall open- I may insulate this before the winter

How did you do that please Tony? I've seen something about little robots spraying foam from below, but other than that it seemed to be a matter of ripping the floor up!

[TonyT]

I have a about 2ft -4ft depth under the floorboards, just crawled under the floor and fitted rock wool slabs friction fit ie cut just a bit bigger than the width and it holds it self in, it’s made a big difference to the room 

[scottishjohn]

10 hours ago, TonyT said:

I have a about 2ft -4ft depth under the floorboards, just crawled under the floor and fitted rock wool slabs friction fit ie cut just a bit bigger than the width and it holds it self in, it’s made a big difference to the room 

you mean you have sealed the gap between the outer wall and the back of the lathe +plaster?

 

 

[TonyT]

No not yet, just under the floorboards.

Imagine rock wool cut into sausage shapes and stuffed up the gap will help a bit, at least stop mice running down/up the cavity. And stop some air infiltration from outside air/air bricks going up the wall too