Insulating Under a Slate Roof Without Causing Damp

Warm air carries water vapour. Cooking, washing, drying clothes and breathing all add it to the air in a house, and it drifts upward. Where that vapour meets something cold it condenses into water. In a roof that surface might be the underside of the slates, a batten, a rafter or a nail.

Insulation does not create moisture, but it changes where the cold surface is. A roof with nothing in it is leaky and cold, and moisture escapes with the heat. A roof insulated to a good standard is warm below and cold above, and the line between the two moves into the roof space. If vapour reaches it and cannot leave, the timber stays damp.

What does BS 5250 have to do with it?

BS 5250 sets out how moisture is handled inside buildings, and BSI lists the 2021 edition as current but under review. It does not give one fix for every roof. It asks the person designing the insulation to think about where the vapour comes from, where it can go, and what the roof can dry out through. Manufacturers turn that thinking into numbers for their products, such as the eaves ventilation figures below.

Damp from this cause is slow. The first signs are often a musty smell in the loft, black spotting on the underside of the roof boards or battens, or rusty nail tips and wet insulation. Slates themselves look fine. That is why the choice has to be right at the start rather than put right after the damage shows. A loft that smells of damp in a cold spell, or insulation that feels clammy when you lift it, is the cue to look harder.

A cold roof is the common way to insulate a pitched slate roof. The quilt is laid across the loft floor, between and over the ceiling joists, and the roof space above it stays cold and ventilated. The slates, battens and rafters sit outside the insulation, so they stay close to the outside temperature and dry out the same way they always did.

This works as long as air can pass through the loft. Under a non-breathable HR underlay, Marley wants the eaves left open by 25mm if the roof is pitched at 15° or less, and by 10mm if it is steeper. BS 5534 indicates a minimum of 20 degrees for natural slate, so a slate roof lands in the 10mm band, although the exact figure depends on the product and its data sheet.

What goes wrong with loft insulation?

The usual faults are blocked eaves and unsealed ceilings. Insulation pushed hard into the eaves cuts off the air at the very place it should enter. A loft hatch that does not shut tight, spotlights cut into the ceiling and gaps round pipes send warm, damp air up into the loft faster than it can leave.

• Keep the insulation back from the eaves, or use a baffle to hold an air path open.
• Seal the hatch, light fittings and pipe holes.
• Do not stack stored boxes against the underside of the slates.

Our guide to slate roof vents explains how vents at the eaves and ridge work with this route. Where the ceiling below is old lath and plaster, gaps around the edges and the hatch are the places to check first.

The underlay is the membrane laid beneath the slating battens, below the slates. It catches any water driven past the slates and sends it into the gutter. Underlays come in two families, and the family you have decides what ventilation you need.

An HR underlay is non-breathable. It stops vapour passing through, so the loft must be ventilated to carry moisture away. An LR underlay, the breather kind, allows vapour through. Marley lets a breather underlay with a sealed ceiling depend on the 3mm gap that standard eaves detailing already leaves, so long as the product is third-party certified.

Does a breather membrane mean you can skip ventilation?

No. It means the ventilation requirement is changed and, with a sealed ceiling, can be much smaller. The product certificate states the conditions, and the ceiling has to be sealed as the certificate asks. Breather membranes are not a reason to pack insulation hard up against the underlay, because contact can leave moisture nowhere to go. Follow the product certificate for the gap it asks for between the two.

When the slates are off, that is the time to fit a membrane that suits the plan. When they are on, the existing underlay is what you have, and the insulation has to suit it. If the roof has no underlay at all, as some older slate roofs have none, the insulation route needs more thought, because there is nothing between the slates and the space below to catch wind-blown rain or snow.

Our page on membranes under slates goes into the types in more depth.

If the loft is a living space, the cold roof route is not open, because the insulation has to follow the sloping ceiling. Two methods are used. Insulation goes between the rafters, which run up the slope under the battens and slates, with an air gap left above it so the underside of the roof can still dry. Or it goes above the rafters, which is a warm roof.

Can you insulate between rafters without lifting the slates?

Yes, from inside, with the insulation fitted between the timbers and the ceiling lined below. The risk is that boards pushed hard against the underlay or the underside of the slates remove the air space. A roof with no underlay is a harder case, because the slates and battens then sit directly against the insulation, so a roofer should look before anything is fitted.

A warm roof puts the insulation above the rafters, under the battens, so the timber sits inside the warm part of the building. That needs the slates off. New battens and the slates go back on over the new layer. This is a re-slate in all but name, so it makes sense when the roof is due for new battens and underlay in any case. Our guide to re-slating describes that job.

With either route, the insulation should not be allowed to alter the roof lines in a way that matters to planning. A warm roof can raise the roof level slightly at eaves and verge, which is worth mentioning to Somerset Council's planning team for a listed home or one in a conservation area, before you order.

The Building Regulations come into it in two ways: the share of the roof you are redoing, and the figure the insulation has to meet.

Does insulating a slate roof need building control?

The Planning Portal's rule of thumb: stay under 25 per cent of the building envelope and under 50 per cent of the roof, and normally no building control application is needed. Go past that and the work is notifiable. Somerset Building Control lists no percentage for re-roofing. Its fee calculator lists a category for renovating a thermal element in one dwelling, and a re-roof with new insulation falls under it. Speak to your building control body before work starts.

The renovation rule is in AD L Volume 1 (2021 edition, 2023 amendments). Table 4.3 gives 0.35 as the threshold U-value for an existing roof and 0.16 as the improved target when it is renovated. Paragraph 4.13 lets you fall back to the best upgrade that is feasible and repays itself within 15 years, if 0.16 cannot be reached for technical or functional reasons; once upgraded, the element should not exceed 0.7. A new roof element is different again, at 0.15.

Until 24 March 2027 the 2021 edition applies. A 2026 edition takes effect from that date, with the tables renumbered 3.2 and 3.3 and the payback rule at paragraph 3.14.

With the covering off anyway, insulation belongs in the plan, and your building control body can say what evidence it wants for the figure. Keep the product data sheets and any certificates, since your building control body or a later buyer may ask for them.

Spray foam is the shortcut that causes the most trouble on a pitched roof. It goes on wet, then swells to fill the space beneath the slates and battens. It seals gaps quickly, but it can also seal timber and slate fixings against any air, so moisture that gets in cannot dry out. Foam also makes later repairs awkward, since slates cannot easily be lifted without disturbing it.

Is spray foam under slates a good idea?

Spraying foam over a roof that leaks draws a flat answer from the Energy Saving Trust: "We don't recommend doing this." We would add that old slate roofs often have small leaks, which is the case where trapped moisture does most harm. If foam is already up there, ask your mortgage provider whether it accepts it, as RICS's consumer guide advises. We cover removal on our spray foam pages.

Other shortcuts to avoid:

• Filling the gap above insulation with a second layer, which kills the air path.
• Pushing loft insulation over the eaves.
• Blocking vents because the loft feels draughty.
• Leaving a gap-filled ceiling that still has an open hatch.

A new roof does not attract a national grant. Insulation schemes run separately and change often, so confirm eligibility with the scheme itself before you commit.

Tell us what you have and what you want to achieve. We look at the loft and the underside of the slates, photograph the timbers, and write one price for the route that keeps your roof dry. Start by sending us the age of the roof and whether there is an underlay.