Garage roof insulation
Can you insulate a garage roof when it is replaced?
Insulation can go in when the roof is replaced, but how it goes in matters. Ventilation, moisture and the energy rules each have a say.
The short answer
Yes, a garage roof can be insulated when it is replaced, and a replacement is the easiest time to do it. The method needs choosing with the roof: a cold roof needs ventilation at the eaves, and spray foam to the underside brings its own concerns. If the garage becomes living space, Part L applies and the roof works to the improved 0.16 W/m²K in Table 4.3 (threshold 0.35) where that is feasible.
4 min read · Part of Garage roofs
When Part L comes in
Part L of the Building Regulations deals with fuel and power. In AD L 2021, a retained roof above the 0.35 W/m²K threshold, for example through a loft or garage conversion, should be upgraded to the improved 0.16 W/m²K of Table 4.3. A roof that is renovated works to the same improved figure, and AD L (para 11.2(d)) treats re-covering a flat roof in a new waterproof layer as renovation. When more than 50% of the roof element's surface is renovated, all of it is improved.
Where the improved figure is neither feasible technically or in function, nor able to pay back within 15 years, para 4.13 asks for the lowest value that is both feasible and pays back within 15 years, and generally nothing above 0.7. Table 4.2, for new fabric elements in existing dwellings, gives 0.15 for a roof, so ask building control which table your build-up falls under.
Whether these rules reach a garage that is staying a garage is for building control to confirm, so ask before you design the build-up. AD L 2026 takes effect on 24 March 2027, with the tables renumbered 3.2 and 3.3, so the date of your application matters.
Ventilating a cold roof
On a pitched garage roof with the insulation at ceiling level, the roof space above stays cold and has to breathe. Marley's figures for a cold roof use a high-resistance (HR) underlay and ask for a 25 mm gap along the length of the eaves at pitches of 15° or less, and 10 mm above 15°. BS 5250 is the code for moisture control in buildings, and BSI lists its 2021 edition as current but under review.
Vapour cannot pass an HR underlay, which is why the eaves vent is not optional. A breather-type (LR) underlay can work differently: where a well-sealed ceiling sits below and the tiles or sheets above are air permeable, Marley says normal eaves construction gives fortuitous ventilation of 3 mm, which is all that is needed, so long as Marley's UKAS certification condition is fully met. Our page on insulation when re-roofing sets out the options on a house roof, and the same logic carries over to a pitched garage.
A flat garage roof is a separate case: the build-up and its ventilation are settled from the deck up at survey, and the choice of covering comes into it.
Spray foam and the lending question
Foam sprayed beneath a pitched roof has drawbacks that matter on a garage you might later convert or sell.
- RICS's guide warns that foam over the timber roof structure means the timbers cannot be fully seen or inspected.
- In March 2023 RICS reported concern within specific sectors of the lending market, such as equity release mortgages, and its guide tells you to check with your mortgage provider before the foam goes on.
- The Energy Saving Trust names no vapour permeability as the main drawback of closed-cell foam, and says the rot risk to timber should be much lower with an open-cell foam.
On mortgages, the Energy Saving Trust expects fewer problems where the foam was installed correctly, so the concern is about the work being done well and being recorded.
For homes that already have it, see the spray foam condensation page. Insulation that leaves the timbers visible keeps them open to inspection, which matters when the roof is next surveyed. A lender or buyer may ask to see the roof structure, and a garage with visible timbers can show it without anyone cutting anything away.
Planning it with the new roof
Insulation changes a roof's thickness, and thickness feeds into the build-up and the pitch. Decide it with the covering, not after the covering is chosen.
- Decide whether the garage stays a garage or becomes a room, because that sets which rules apply.
- Choose the covering and its build-up together, with a pitch that suits the covering.
- Ask building control about Part L and ventilation at the start.
- Check the written quote states the insulation and the ventilation, and whether VAT is included.
Insulation is also a reason to renew a roof instead of patching it, since the work is easiest when the old covering is off. Our guide to the best material for a garage roof compares coverings, and the garage roofs hub has the rest. The insulation is priced inside the one written total, and the quote should name what goes in as well as how thick it is. If the garage is staying a garage, building control can tell you in one call whether any insulation requirement applies at all, and the answer goes in the survey notes.
More questions
Does an unheated garage roof need insulating at all?
Not necessarily. Part L is written for dwellings, and whether it applies to an unheated garage is a point for building control to confirm. If you plan to convert the garage into living space, the roof works to the improved Table 4.3 figure of 0.16 W/m²K against a 0.35 threshold, so plan the insulation with the new roof.
Is rigid board or spray foam better for a garage roof?
Spray foam on the roof underside hides the timbers from inspection, and RICS reports concern in parts of the lending market. A board that leaves the structure visible avoids both. Either way, a cold roof needs eaves ventilation as set out above, and building control can confirm what your roof requires.
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