How do I insulate between rafters without causing condensation?
The ventilated air gap, where the insulation and the vapour barrier each sit, and why the rafter depth usually dictates the whole design.
- Difficulty
- advanced
- Time
- 4 hr
- Read
- 4 min
- Safety
- warning
Short answer
Leave a continuous 50mm ventilated air gap between the insulation and the roofing underlay unless the underlay is certified breathable and the design says otherwise. Fit rigid insulation between the rafters, then a continuous layer over them to break the cold bridge, then a taped vapour barrier on the room side.
Insulating at rafter level converts a cold roof into a warm one, and it changes where the condensation risk sits. It is the domestic detail with the biggest gap between how simple it looks and how easily it goes wrong.
Safety
What you'll need
- rigid PIR insulation boards
- insulated plasterboard or a second insulation layer
- vapour control layer and tape
- counter-battens or rafter extensions (optional)
- expanding foam or tape for edges
Step by step
- Find out what the roof covering and underlay are.Traditional bitumen felt is not vapour permeable and requires a ventilated gap above the insulation. Modern breathable underlay may allow an unventilated design, but only where its certificate says so.
- Measure the rafter depth.This usually decides the whole design. A 100mm rafter with a 50mm ventilation gap leaves only 50mm for insulation, which is nowhere near enough on its own.
- Plan for a continuous layer under the rafters.Insulation only between the rafters leaves the timber as a cold bridge across the whole roof. A continuous layer of insulated plasterboard or rigid board beneath solves that and makes up the depth.
- Maintain the ventilation gap.Where it is required, 50mm clear from the top of the insulation to the underlay, running continuously from eaves to ridge, with ventilation openings at both.
- Use proprietary ventilation trays if needed.Rigid trays fixed against the underlay guarantee the gap is maintained and stop insulation being pushed up against the felt.
- Cut rigid board tightly.Rafters are rarely evenly spaced. Measure each bay individually, cut a millimetre oversize and friction-fit, then foam or tape the edges.
- Seal every gap.Air moving round the edges of insulation carries moisture with it and largely defeats the insulation. Expanding foam or foil tape at every perimeter.
- Fit the vapour control layer on the room side.Continuous, taped at every lap, sealed around every penetration, and continued into the wall and ceiling junctions.
- Detail the eaves and the ridge.Ventilation has to get in at the eaves and out at the ridge or high level. Blocked eaves are the commonest reason a ventilated roof fails.
- Deal with services deliberately.Downlights, cables and pipes puncturing the vapour layer are the usual weak points. Plan a service void beneath the vapour barrier so cables run in front of it rather than through it.
- Check what approval the work needs.Insulating a roof as part of a habitable room is notifiable work with requirements for insulation, fire safety, escape and structure. Find that out before, not after.
Tips
- A service void beneath the vapour barrier lets you run cables and fit downlights without ever puncturing it. It is the single most useful detail in the whole build-up.
- Blocked eaves ventilation is the most common cause of a rafter-level insulation failure. Check you can see daylight at the eaves.
- Insulation between rafters alone leaves the rafters as continuous cold bridges. The continuous layer beneath is not an optional extra.
Common mistakes
- Pushing insulation tight up against the underlay — It removes the ventilation path that carries away moisture. Condensation then forms on the underside of the underlay and drips into the insulation and the timber.
- Omitting the vapour control layer because the insulation is foil-faced — Foil facings help but the joints between boards are the leak path. Without a continuous taped layer, moist air moves through the gaps and condenses in the cold zone.
If it doesn't work
Damp patches and staining on new plasterboard
Cause: Condensation in the roof build-up — Fix: Open up and check the ventilation gap, the vapour barrier continuity and the eaves ventilation. Wet insulation must be replaced.
Mould in the corners of a converted loft room
Cause: Cold bridging at rafters or at the junctions — Fix: Ensure a continuous insulation layer beneath the rafters and insulate the junctions with walls and ceilings.
Water dripping from the underlay
Cause: Condensation on the cold underside of an impermeable underlay — Fix: Restore the ventilation gap and the eaves and ridge ventilation, and improve the vapour barrier below.
Rafters feel soft when probed
Cause: Long-term hidden condensation causing rot — Fix: Stop and get the roof structure assessed. This is beyond a DIY repair and the extent is usually greater than it appears.
Questions people ask
How much insulation do I actually need?
It is set by the U-value the regulations require for the work being done, which changes over time. The insulation manufacturer's calculator, or the approved design, gives the thickness for a given product.
Can I use mineral wool instead of rigid board?
Yes, and it is easier to fit around irregular rafters, but it takes more depth for the same performance and it must not be compressed. The ventilation gap and vapour control requirements are identical.
What if my rafters are too shallow?
The usual answers are extending the rafters with battens, or making up the depth with a continuous insulated layer beneath. Both are common; the second also solves the cold bridging.