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Advice on How to Achieve a U Value of 0.13 for my Pitched Roof Loft Conversion

Postby » Mon Feb 29, 2016 4:32 pm

I am looking for advice on how to achieve a U Value of 0.13 for my pitched roof.

I plan to use the loft as a playroom and storage but still need to comply to Scottish building regulations - the planning department at the council said the i need a U Value of 0.13 for my 1962 detached bungalow

I can see the Sarking board and the rafters are 100 mm deep, 450 mm apart.

I am trying to work out what materials i need to achieve the 0.13 U Value - it looks like rockwool will be the cheapest option will eat heavily into my head height but the foil insulation appears a lot more expensive

There is also mention of ventilation gaps

I would just like someone to explain to me in laymans term on how i do it with both products (Wool or foil) (or both together or what ever is the best way to achieve the U Value of 0.13) I need step by step instructions like say Sarking board then 50 mm ventilation gap then Kingspan (Whatever relevant Number) then 25 mm Baton then 12.5 mm Plasterboard or however

Sorry to be so vague i have looked at so many sites and U Value calculators online my brain is mush now and i dont understand them. This "cheap playroom" is turning out be dearer and dearer

Thank you in advance for any help you can give me

Please if there is something important that i am missing - let me know
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Postby ONLINE INSULATION » Wed Mar 02, 2016 1:43 pm


Take a look a the Celotex U-value calculator online. Rockwool and Kingspan also have online versions.

Celotex tends to be a little cheaper than Kingspan and this will give you an idea of all of the components to achieve you target U-value. However you may be able to get this from you architect if you used one for the loft conversion as they would have probably done the calculation to get planning consents if needed.

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Postby MikeGrahamT21 » Tue Jun 28, 2016 10:08 am

0.13U is a low value, and would be almost impossible to achieve within a 50mm space.

Calculating a u-value is very easy, simply take the Lambda value, i.e. Thermal Conductivity: 0.036 W/mK and divide it by the thickness in meters.

So the above was for rockwool, 100mm would be 0.036/0.1 giving 0.36U.

Having 50mm in between the rafters and a further 50mm below would give around 0.21U using PIR (the most common and cost effective insulant). Having the rafters covered, would also stop the effect of cold bridging (heat being transferred through the timber), which would give a better overall effect.

I would very much doubt you will have to achieve 0.13 to get it passed, have a word with your building control department, they usually have maximum values, which will likely be around the 0.25-0.3U range, though scotland could be different!
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