How to insulate a 100mm cavity

Postby Adam75 » Mon Feb 01, 2010 1:25 pm


I need to insulate a 100mm gap between a breeze block wall & wood cladding.

I was thinking about taking some boards at the top off and pouring in loose fill insulation. Would this be effective? I’m not sure whether this would create enough density as it should be pressure blown in.

Alternatively I’m tempted to remove the entire cladding, pin up new 150mm vertical battens on top of the existing 100mm battens & insert 250mm compressed foam foil back board and replace the cladding. Is this the best approach or am I missing a trick?

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Simply Build It

Postby Perry525 » Fri Feb 05, 2010 6:35 pm

First of all you need to decide what you are trying to achieve.
Placing the insulation on the outside of the block wall makes the room slower to warm up and cool down and therefore costs more to heat. Placing the insulation on the inside makes the room quicker to warm up and cool down and cheaper to heat or keep cool.
If you are there all the time, and have the heating on 24/7 then having the insulation on the outside will be best.
If you like to turn the heating off, when you go out, or go to bed, then the insulation on the inside gets the quickest result.
One further problem is that using wood as you propose, will create a number of cold spots, where your heat will escape.
A better idea is to stick the foam sheets to the walls.
Question, your proposal will result in a very thick wall, is there room?
In all probability 6 inches/ 15cm of foam will do the trick. Going above this is moving to Passive House standards and the difference will be hard to measure. To what standard is the rest of the property insulated.
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Postby Adam75 » Sat Feb 06, 2010 1:18 pm

Thanks for the reply.

First of all I must apologise as I got my measurements wrong! The gap is currently 10mm & I was proposing to add 18mm battens to increase the gap to 25-28mm to allow for 25mm rigid foam insulation.

However I have just been advised that you need to leave an air gap of at least 25mm otherwise condensation will form on the form & potentially cause damp.

So I am now planning to make the gap 50mm & insert 25mm rigid foam & cover the foam in a breathable membrane.

The porch includes a cloakroom so is used regularly for short periods at a time plus out heating is only on when we are awake & at home. There is no room to insulate inside so I only have the option to attach the foam boards to the external breeze block wall.

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Joined: Tue Mar 20, 2007 12:29 pm

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