Roof Insulation in Garden Shed -- Closed or Open Ventilation System

Hello,

Shed roof insulation has been discussed on the forum before, but I'm stuck on one bit:

Air Flow

Please bear with me

The shed is going to be a fully finished space for daily office and studio use.
5.5m x 2.5m -- fairly large
Sloped roof, felted
Interior is a shell - we're saving by doing the interior ourselves.

Roof insulation is the next step.

The felt is already on so we're looking at a Cold Roof style, which I believe is:

- Rigid insulation between the rafters
- Choose a thickness of insulation that leaves 50mm air gap above
- Air flows through vents from the front, through the gap, and out vents at the back

Our rafters are 65mm and the insulation is sold in 25mm thickness, so I'm hoping a 40mm gap rather than 50 is ok...

Our issue is that this is no air-flow:

- the front soffit is completely sealed - no vents or air gaps.
- the rear overhang is tiny and you can see it's not sealed well - light and air come into the roof cavity


It seems the choices are:

(A) Open the system - add vents at the front and back for each pair of rafters (that would need 9 vents in the front and 9 in the back - but there's no room in the back anyway!)

(B) Close the system - seal up the rear where some air is getting in. Hopefully stop damp air getting into a place it can't escape from.

(C) Leave it as it

I just want to protect my investment from damp and keep it warm. Any thoughts?


Many Thanks

p.s. there's a great video here explaining both warm and damp roofs.

Relevant Cold roofs are covered from the 7 minute point and beyond

Don't know much about ventilation but I would be inclined to fill the void completely, thereby eliminating the risk of cold, damp air become trapped between the insulation and underside of roof. If there is no wind/breeze blowing then there would ne no airflow to remove damp air in the void.

Others with better knowledge may think differently.

Just for anyone visiting this thread in the future, we ended with advice from an insulation specialist who advised:

- Rockwool in both the walls and roof
- Intello as a smart VCL (vapour control layer)
- closing off the soffit areas with OSB+sealant because they were very airtight (the soffits were left as an empty space)
- prior to insulating we sealed off all the edges as well such as where the floor meets the wall, etc, to ensure it was as airtight as possible

This solution gave good insulation whilst also providing vapour management (drying to the interior) given the outside and inside cladding is thin and probably not perfectly airtight.

It wasn't cheap but we were able to get everything we needed from baunativ in germany including instaabox's for the electrical outlets, etc. We could have put in a service void but decided it was more important to have the space.

conny said:

Don't know much about ventilation but I would be inclined to fill the void completely, thereby eliminating the risk of cold, damp air become trapped between the insulation and underside of roof

Click to expand...

I like your train of thought and I had considered the same, with something like Celotex. However celotex themselves told us not to use their product in this situation. (probably because if anything does get damp, it cant breath/dry anywhere). From an effort and cost perspective, I understand it's fairly easy to cut celotex to fit and to seal with tape provided you get the really good aluminium tape. But having looked into it in detail, I feel the insulation specialist gave us a really thought-through solution for a building which we hope to have for many many years.