Roof insulation options for traditional Scottish homes

Why Edinburgh roofs need a different approach

Most traditional roofs across Edinburgh and the Lothians were built with sarking boards — overlapping timber boards laid across the rafters — rather than the modern breathable underlays we use today. That single detail changes everything. A sarked roof with no ventilation path below it behaves very differently from a twentieth-century roof with a felt membrane and a clear eaves-to-eaves airflow.

Add slate coverings, steep pitches, exposed coastal sites in East Lothian and Fife-facing suburbs, and chimney stacks that are still in use, and you have a building fabric that is far less forgiving than a modern timber frame. Insulating it well is entirely possible, but the design choice between a warm roof and a cold roof has to be made deliberately, not assumed.

The cold roof: ventilation is everything

A cold roof keeps the insulation at ceiling joist level and leaves the roof space itself cold and ventilated. It is the traditional arrangement in Scotland and it still works well — provided the ventilation is genuinely continuous.

• Keep a clear 50mm gap between the top of the insulation and any roof underlay or sarking, running without interruption from one eaves to the other.
• Fit eaves vents, tile vents or mushroom vents to give cross-ventilation. A roof space with air entering at one gable and nowhere to leave is not ventilated.
• Use insulation trays or baffles at the eaves so that pushing mineral wool into the last 300mm does not block the airflow you have just created.
• Insulate between and then across the joists — typically 100mm between, then 170mm or more over the top — to reduce cold bridging through the timber.
• Insulate the hatch to the same standard as the rest of the ceiling and draught-seal the frame.

Eliminate the ventilation and you will get condensation on the underside of the sarking, wet rafters and, eventually, rot. That is not a theoretical risk on a west-facing Edinburgh roof.

The warm roof: moving the insulation upwards

A warm roof places insulation above or between the rafters so the roof structure itself stays warm and the dew point is kept outside the timber. The condensation risk moves to the outer face, which is where it belongs.

In sarked roofs this usually means lifting the slates and sarking, laying rigid insulation such as PIR boards directly on the rafters or on a deck, and re-covering. Because you are closing the roof up, the build-up must be vapour-tight internally and water-and-wind-tight externally.

• A continuous layer matters more than total thickness. 100mm of PIR between rafters plus a 50mm continuous layer above is significantly better than 150mm stuffed between timbers with cold bridges either side.
• Add a vapour control layer on the warm side, sealed at laps and around penetrations, then a plasterboard ceiling.
• Never mix a warm roof with the old leaky ceiling. Warm roofs rely on airtightness, so gaps around downlights, pipework and loft hatches undermine the whole specification.
• Check headroom and eaves detail. Raising the roof plane by 75–100mm can affect gutters, skews, dormers and how the roof meets a neighbouring tenement.

Warm roofs suit dormer conversions, attic rooms and properties where the roof space is already habitable or is about to be.

Hybrid roofs and the tenement question

Many Edinburgh tenements and Victorian terraces end up with a hybrid: rigid insulation between the rafters plus mineral wool laid flat at ceiling level. This can work, but it needs careful calculation of where the dew point falls. A hybrid without a designed ventilation gap and a properly sealed vapour control layer is the classic recipe for interstitial condensation.

In a tenement, remember that the roof is common property under the Tenement Management Scheme. A warm roof conversion affects every flat below it, so you will need the agreement of the other owners and, in most cases, a building warrant. Budget for a surveyor or architect to handle the notices as well as the technical design.

Materials that cope with Scottish weather

Driving rain, salt-laden air along the Forth and hard frosts all take their toll. Wood fibre, hemp and sheepswool boards perform well in breathable sarked roofs because they buffer moisture rather than trapping it, though they need more thickness for the same U-value as PIR. Rigid foam boards give the best thickness-to-performance ratio where headroom is tight. Whichever you choose, do not block the eaves, and keep gutters, skew putts and flashings in good order before adding insulation — damp walls and a warm roof make poor companions.

Regulations, listed buildings and paperwork

In Scotland, Section 6 of the Building Standards sets a target U-value of around 0.11 W/m²K for a roof, with a technical feasibility caveat for existing buildings where achieving that is genuinely impractical. That caveat is not a free pass; you need to show you have done what the structure reasonably allows.

• Building warrant: most insulation upgrades, and any re-roofing, require one. Alterations to the roof covering of a flat or maisonette also affect the common property.
• Listed buildings and conservation areas: much of central Edinburgh is covered. Changing a roof covering, adding roof lights or altering the profile may need listed building consent or planning permission, even where the work is invisible from the street.
• Ventilation of the dwelling: as you make the roof tighter, kitchens and bathrooms need mechanical extract that actually terminates outside. Tenement flues and shared ducts are a common failure point.
• Insurance and mortgage: keep completion certificates and warranties together — surveyors ask for them at the point of sale.

Get the design right at the start and a traditional Scottish roof will be warmer, drier and quieter for decades. Guess at it and you will be paying twice.