Hi
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:
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.
Ta
Ben
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):

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.
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.
Ta
Ben
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):
