Shed roof insulation - condensation problem

13 Jan 2009
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United Kingdom

My parents had a kind of custom shed constructed on part of their land. During the construction of the shed they decided to change the purpose of the shed from being just a shed to more of a snug, so they asked the builder (who built it all from timber frame) to install insulation in the walls, roof and floor.

The insulation in the roof is Recticel foil-faced (PIR) boards, which has then been over-boarded with T&G pine boards.

In some parts the insulation boards are sandwiched tightly between the T&G and the outer roof skin (which is felted ply). In other places there are gaps on both the warm and cold side. There is some eaves ventilation, but there isn't a full cross-ventilation path on the cold side.

The problem is - that in warm weather, condensation forms in the apex of the roof on the foil facing of the warm (internal) side of the insulation boards.

My guess as to what's going on is:
  • Shed is unheated
  • During a warm day the ambient air warms up quickly, contains more moisture
  • Inside the shed it's still cool, the warm side of the insulation board is at shed temperature, below the dew point
  • Warm air from outside the shed with higher RH condenses on cold warm-side of insulation boards
  • T&G pine boards get wet.
I've added some pictures below.

The question is how to solve this issue? I have thought about vapour barriers, etc, but I don't think this would solve the problem - because then vapour may condense on the VB if the temperature of that is below the dew point.

The only thing I could think of was to add some ridge vents that vented the internal space, but this somewhat defeats the purpose of the insulation.

Any ideas/comments welcome.


The shed/snug from outside:


Internal roof structure showing T&G boards and embedded PIR insulation:


Internal roof structure with PIR removed, showing ply outer skin (which is felted on the external face):
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9 Feb 2010
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United Kingdom
The foil is a vapour barrier, heating and ventilation together have a positive impact on condensation when the surface that is subject to condensation is cold is exposed like plaster.

I am surprised you get condensation on the foil but if the foil is cold enough and the air next to it warmer and humid enough it will happen. With timber in front of your V.B (foil). you are not going to get a flow of air across the surface of the foil so you have to stop the moist air getting to it. The moisture can quite possibly be coming from the timber itself. So perhaps you need space between your insulation and the boards to allow the air to move, together with heating and venting the shed.

The logic of it happening on a warm day seems back to front unless there are people in the shed more on warm days, in which case tell them to stop breathing or open a window!
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