Should I insulate between or above the rafters?

What this usually means

A sloping roof can be insulated in three positions relative to the rafters, and they behave differently. Between the rafters places insulation in the gaps but leaves the timber rafters themselves relatively uninsulated, so they act as repeating thermal bridges — lines of heat loss and cold surface — and the rafter depth limits how much insulation you can fit. Above the rafters (a 'warm roof') lays a continuous layer of insulation over the top of the rafters, before the roof covering, which keeps the whole roof structure, including the timbers, warm and inside the insulated envelope. Below the rafters adds a layer on the room side, increasing total depth but bringing the ceiling down into the room.

These positions have different thermal and moisture consequences. Continuous insulation above the rafters gives the best performance because there is no break in the insulation and no rafter bridging, and because the timbers stay warm they stay dry — moisture is far less likely to condense on a warm rafter. Insulating only between the rafters, by contrast, both under-performs (the bridges leak heat and create cold lines that can mark or even attract condensation on the ceiling) and, if the gap and ventilation are not detailed correctly, can leave the cold side of the insulation prone to condensation. A below-rafter layer can be added to between-rafter insulation to boost depth and cover the bridges, at the cost of headroom.

So the decision is a balance struck around the specific roof. Above-rafter insulation is often the best performer and the safest for moisture, but it usually means lifting the covering, so it suits a re-roof or a new build more than a finished room. Between-rafter alone is rarely sufficient on its own and needs careful ventilation or a vapour-open strategy to avoid trapping moisture. A combination — between plus a below-rafter (or, ideally, above-rafter) layer — frequently gives the best practical compromise of depth, continuity and headroom. Because the right answer depends on the roof construction, the headroom you can spare, and how the build-up will keep moisture out of the timber, it is a building-physics decision best made with the whole roof assembly in mind rather than from a single rule of thumb.

Common causes

Signs and symptoms

The thermal performance of an insulated roof depends on both the insulation's resistance and the continuity of the insulating layer. Insulation placed only between rafters is interrupted by the timber, which has a far lower thermal resistance, so heat short-circuits along the rafters and the effective performance falls well below the value implied by the insulation alone — and the rafter lines run cold on the room side. A continuous layer above the rafters eliminates this repeating thermal bridge, so a warm roof achieves both higher and more uniform performance for a given insulation, with no cold stripes. A below-rafter layer added to between-rafter insulation similarly covers the bridge while increasing depth, at the expense of room height.

Moisture behaviour follows from where the rafters sit relative to the insulation. In a warm roof, the timbers are on the warm side of the insulation, so their temperature stays above the dew point of the internal air and condensation against them is very unlikely — the construction is inherently robust. Where insulation is only between the rafters, the outer part of the rafter and the cold side of the build-up can fall below the dew point, so unless the assembly is either ventilated above the insulation or designed to be vapour-open and dry outward, moisture can condense within it and against the timber. The detailing of the vapour control on the warm side, any ventilation gap, and the vapour-openness of the materials therefore determines whether a between-rafter roof is safe.

Choosing the position is thus an optimisation across performance, headroom, buildability and moisture safety, specific to the roof. Above-rafter insulation is generally the best performer and the safest for the timbers but typically requires access from outside, so it aligns with a re-roof; between-rafter alone is usually insufficient and demands careful ventilation or a vapour-open strategy to avoid condensation; and a combined build-up balances depth and continuity against the headroom lost. The sound approach is to design the whole assembly — insulation position and depth, vapour control, and any ventilation — as one moisture-safe system for the actual roof, rather than defaulting to filling between the rafters. A building-physics assessment, including a condensation-risk check of the proposed build-up, is what confirms the chosen option will be both warm and dry.

How to choose where to insulate a sloping roof

Decide the build-up from the roof construction, the headroom and the moisture strategy — favouring continuous insulation that keeps the rafters warm and dry, and checking the assembly for condensation risk.

1. 01

   Assess the roof and rafter depth

   Establish how much between-rafter insulation is possible and whether the rafters will bridge.

2. 02

   Decide on continuity

   Add an above- or below-rafter layer to cover the rafter bridges and reach the target depth.

3. 03

   Prefer a warm roof where possible

   If re-roofing, insulate above the rafters to keep the timbers warm and the layer continuous.

4. 04

   Balance headroom

   Weigh the ceiling height lost to a below-rafter layer against the performance gained.

5. 05

   Design the moisture strategy

   Specify vapour control and any ventilation so the timbers stay dry in the chosen build-up.

6. 06

   Check the condensation risk

   Verify the proposed assembly against condensation risk before building it.

How to prevent it coming back

• Do not rely on between-rafter insulation alone.
• Keep the rafters warm with continuous insulation where possible.
• Match the vapour control and ventilation to the build-up.
• Run a condensation-risk check on the assembly first.