Thickness of Insulation boards

[rizzir]

Hi,

I am building rear extension.
Walls will be double cavity walls 30cm thick
10 cm for Brick / 10 Cm for Insulation / 10 CM for Blocks.
To save some space i am think of using 50 mm insulation boards.
so this way i will have double cavity wall 25cm thick.

10 cm for Brick / 5 Cm for Insulation / 10 CM for Blocks.

But i need your help to decide appropriate thickness of insulation.
I want to save space but i also want good insulation.
is 50mm thick insulation boards are sufficient for the walls?

 

[tony1851]

is 50mm thick insulation boards are sufficient for the walls?

Probably not. Kingspan also recommend a minimum cavity of 25mm. There is a new rigid foam product which can fully-fill the cavity, but not sure if 50mm would be enough.

 

[excited]

"There is a new rigid foam product which can fully-fill the cavity, but not sure if 50mm would be enough."

Tony what's this product called please...

 

[tony1851]

It was this stuff, but I think it's too thick for your requirement (min 100mm?)

http://cavitytherm.com/wp-content/themes/cavitytherm/downloads/Cavitytherm-Brochure-2013.pdf

 

[damo152003]

Im juat about to start my extension and 75mm has been speced with breeze block walls with render this has passed building regs etc.

 

[cjard]

If 5 cm loss of space bothers you, build the extension 5cm bgger all round

50mm of PIR will attain 0.27 U and the regs requirement is 0.30 but this is only the walls. You must then add in the windows (typically 1.2-1.6 for normal DG) and average it all out..

sO 50mm might be pushing it. See how friendly your building inspector is

That said, you're only looking at an extra £6 a square metre or so for an extra 50mm so taking a 5x4 extension it's got a wall area of about 35 sqm, i.e. £210 worth of insulation... peanuts in the grand scheme of things

 

[noseall]

Unless the potential loss of space is extraordinarily relevant, I would discourage veering away from a standard full fill 100mm - 100mm - 100mm scenario.

Whilst the loss of space is annoying, the alternatives can be more costly to install. These systems require a conscientious builder to install properly, as they are a right faff.

Rigid foam board full fill sounds like an absolute nightmare to fit and partial fill is not much better.

As said, realistically all you are likely going to gain is 50mm i.e. 25mm each side. Is it worth the extra cost and construction hassles?

 

[champagnecharly]

Have you thought of Insulated Concrete Formwork instead of traditional brick & block?