A cold roof is a roof construction in which the insulation sits below the structural deck or between the rafters, leaving the structure and the sarking on the cold side of the insulation. In a warm roof the insulation sits above the structure, so the whole structural element stays warm and dry.
The build-up from inside out
- Ceiling finish
- Vapour control layer on the warm inner side
- Structure (rafters or joists) with insulation between
- Ventilated air space
- Roof deck or sarking
- Roof covering
In this type of roof, the ventilated air space is not optional but essential.
Why it is risky in building physics terms
The cold side of the structure lies inside the build-up, and warm indoor air carries water vapour. As soon as that vapour reaches the cold deck it cools below the dew point and condenses. The result: wet insulation that loses its performance, and in time rot in the deck and the rafters. A cold roof does not forgive detailing errors.
The three places it usually goes wrong:
- A leaky vapour control layer. Every penetration for a pipe, downlight, rooflight or flue is a hole through which warm, moist air enters the construction. One untaped penetration can introduce more moisture than the entire roof area does by diffusion.
- Insufficient or blocked ventilation. The air space must be able to draw air in at the eaves and discharge it at the ridge. Insulation pushed tight against the deck closes that path off.
- Insulation carried through into the gutter. That blocks the inlet and the roof stops working.
For these reasons cold roofs are no longer used in new build. You mainly encounter them in existing buildings.
When renovating
- Converting to a warm roof is the safest option in building physics terms: insulation on the outside, above the structure. That is usually only feasible when the roof covering is being replaced anyway.
- If it stays a cold roof, keep the ventilation gap clear (in the order of a continuous 30 to 50 mm air space) and provide a genuinely airtight, continuous vapour control layer on the inside, including all junctions and penetrations.
- Check the existing timber first before adding insulation. If moisture is already present, you will seal it in.
Keeping a cold roof when renovating: checks
- Check the existing timber first: Do this before adding insulation: if moisture is already present, you will seal it in.
- Keep the ventilation gap clear: In the order of a continuous 30 to 50 mm air space.
- Airtight vapour control layer on the inside: Genuinely airtight and continuous, including all junctions and penetrations.
Related terms
- Vapour control layer
- Vapour barrier
- Roof deck
- Roof covering
- Condensation
- Insulation
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