Extensions Question

Cavity walls or external insulation?

On a proposed extension which will be rendered, can I ask the pro's and cons of my 2 options? And of course, how it will affect my budget. Thanks v.much

Gareth, if you mean is it currently being built, no, it is at proposal stage. It would be an either/or situation not both, although the part of the building it will be going on top of is cavity.

3 Answers

Best Answer

Is this new build or current?

External insulation will give much better insulation values as it will fully envelope both skins of the walls. The idea is to allow the walls to absorb the internal heat and disapate it later on. New building technology is pointing this way now.

If it was my build, i would do both.

The downside is, 1 cost, 2, depending on the thickness of the external insulation used, it will increase the depth of your window reveals.

Im pretty sure theres more pro's and cons to be added. " You can never have too much insulation"

HTH Gareth

Answered 9th Jul 2011

AGL Heating & Building

Member since 1 Oct 2009

Your Question is easy the answer not so, Factors involve
Use of extension
Are the walls equally exposed to the weather?
How heavy is the roof
You can choose timber frame for optimum insulation effect. However a sheetrock wall has limitations when fixing things to it. And can create nightmares for bathrooms if not installed correctly, leading to damp.
Thermal blocks with added insulation as an internal wall with a cavity and a common brick outer wall rendered, is a good strong and energy efficient wall. But you lose on internal dimensions.
Other options are also possible and you must choose.
Costs vary greatly depending on supplier, contractor and market changes.
Don’t just discuss cost of materials with your builder but also consider time, as some methods cost less because they are easier and less weather sensitive. Methods and material costs often balance out. The cheaper materials don’t mean a cheaper build.
How much insulation and where, is less important than good method. Good cavity ventilation is vital to effective walls.
Good hunting.

Answered 10th Jul 2011

Alan Bentley Property Services

Member since 10 Jul 2011

i will try to keep it really simple
external insulation - most expensive way to insulate as the tech is new and not common in small extensions
cavity construction- allows you the most versitility and will cost less than external
there is a third option that is
internal insulation- this is usally the cheapest option.

Answered 11th Jul 2011


Member since 27 May 2010

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