Advice on Roof Insulation and Construction for Timber Framed Garden Office


Postby stuartkey » Sat Feb 20, 2016 11:18 pm

Hi, I'm building a timber frame garden office and I have a question regarding insulation and condensation control of the roof.

It's a shallow pitch roof, with the rafters meeting a ridge beam and overhanging the wall plates on both sides. We've chosen to use Eco Slates, which need to be installed on a boarded surface, on top of a breathable membrane. I'd like to keep some rafter exposed internally, so was considering fitting insulation boards between the rafters directly in contact with the underside of the roof board and fitting plasterboard on top, leaving around 2 inches of rafter exposed internally. However, after LOTS of reading online, I am concerned about the rafters creating thermal bridges from the board and causing condensation.

Does anyone have any experience or advice, including better ways to go about this?
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Postby thedoctor » Mon Mar 14, 2016 10:07 am

The reality is Stuart that cold bridging is extremely unlikley as (with the depth of the timber) the internal section is likely to get far warmer than the cold passing through it from the outside. Essentially any cold is going to have to pass through the slates, the membrane, the board then through the full depth of the rafters in order to be cold enough for warm air to condense on the exposed sections. As 50% of the rafters will be warmed by insulation on either side plus the warmth of the room will be focussed (as the ceiing recesses between the rafters will focus heat on them) on the rafters. My bet would be that the rafters will always be at room temperature.

If you are still worried however, you can cut strips of foil insulation to lay on top of the rafters before boarding out. The foil is only a few mm thick and will provide a thermal barrier between the boards and rafter.

Hope this helps
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