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increase depth of rafters or use a combination of PIR boards and insulated plasterboard

Postby igtrier » Mon Apr 11, 2022 5:10 pm

I need to upgrade the existing pitched (loft) roof insulation. Rafters are 100mm in depth and there are between-rafters insulation boards installed (50mm), which is inadequate. As a layman, I can think of two options: (1) install Kingspan Kooltherm 107 (75mm) between rafters, then a vapor membrane under rafters, then 25-29mm PIR insulated plasterboard; (2) increase depth of rafters (by adding 20mm planks udert rafters), then install 100mm Kingspan boards between rafters, then a vapor membrane, then 12.5mm plasterboard. Which of these two options will result in better insulation? Or should I think of something else? Thanks.
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Simply Build It

Postby stoneyboy » Wed Apr 13, 2022 9:47 pm

Hi igtrier,
Suggest you leave the existing insulation in place, overboard with 50/75/100mm insulation and overboard with plasterboard.
Regards S
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