External Insulation on victorian semi with a thin cavity

Postby mcanchr » Sat Jan 18, 2014 12:05 pm

Hi, wondered if anyone can advise.

I purchased an 1898 Semi detached house last year. The house has no insulation whatsoever in the walls. It does have a thin cavity wall (around an inch air gap when I checked). However, I am reluctant to fillt this with insulation, as I have read that this can create damp issues. On the gable end of the house there has been pebble dash render appled (I presume this was put on as the brick work was probably eroded due this side facing all the weather and being exposed to the coast), which is in a bit of disrepair and requires re-applying. I was thinking of having the render removed and adding a layer of external insulation, then re-rendering over the top.

What I was wondering was would this do anything to improve heat loss from the house if there is a cavity (albeit only an inch) between the outside and inside walls? My other options would be a thin layer of insulated plasterboard insulation applied to the inside walls, after removing the old plaster.

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Simply Build It

Postby welsh brickie » Sun Jan 19, 2014 10:51 am

check with your local council there may be grants available for this work and yes it will make a big difference
welsh brickie
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Postby Perry525 » Mon Apr 21, 2014 10:37 am

Note: All insulation is based on air. With an air gap of 19mm or less the air does not produce an air circulation and does not transfer the heat from one wall to the other.
Therefore, with your set up, some of your heat escapes from the room into the first part of the wall, crosses the gap, transported by the circulating air, and enters the outer wall, heat also escapes down into the ground and up into the sky via both walls and the air space.
The problem here is, both parts of the wall stand on the ground. Heat does not just pass through a wall to the outside air, it heats the whole wall and everything connected to it. That means a lot of heat moves into the ground and other heat moves upwards into the joists, rafters and sky.
Heat always moves from hot to cold.
The perfect room is a Thermos flask. A space surrounded by a vacuum, where the only heat loss is via the neck and stopper.
Unfortunately, they do not make room sized and shaped Thermos flasks.
The nearest effective solution, is placing the insulation on the inside of the room......to stop the heat from escaping through, down and up.
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Postby mcanchr » Mon Apr 21, 2014 9:09 pm


Thanks for your post. That's very interesting of what you say about the heat loss. So effectively not a lot of the heat would reach the outside wall to be preserved by external insulation, correct?

Do you have much knowledge of possible pitfalls of cavity wall insulation on old houses like mine? For instance transferring damp from outside wall (through wind driven rain) to the inner wall via the addition of cavity wall insulation? I have been giving this method another look as I believe there are other products on the market that are better at avoiding this issue.

Thanks in advance.
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Postby Perry525 » Tue Apr 22, 2014 2:18 pm

The Building Research Establishment at Watford has done a lot of research into this, and the results have been very disappointing......external insulation, just doesn't work.
As far as cavity wall insulation goes, the only insulation that works is blown in foam, this is waterproof as well as being an effective insulation. However, in your case, most of your heat is disappearing into the ground and roof/sky from the inner wall, cavity wall insulation will not change this. Your best result will be from sticking sheets of closed cell insulation to the walls on the room side of each wall. Several insulation manufacturers make a product that has plasterboard stuck to polystyrene or similar, once in place these are skim coated and in effect disappear. There are various thicknesses, probably an inch or two will suit you, obviously you can buy up to four inches thick, which will reduce your heat loss to almost zero. These products are fixed with plasterboard adhesive.
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Postby mcanchr » Fri Apr 25, 2014 11:50 am

Thanks Perry,
I have come to that conclusion that internal insulation will be the best option. Think I will probably go for the Celotex insulated plaster board. I can then insulate as I decorate the house on a room by room basis. Too many risks with damp IMO on getting CWI in my type of house. They even have a YouTube video of how to apply it!

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