If your summer house turns into an icebox the moment October arrives, you are not alone. It is one of the most searched questions every autumn: how do you get garden room heating working in a building that was never designed to hold heat?
The good news is that you can improve things yourself. The other thing worth knowing, before you spend a weekend with a staple gun, is where a DIY job hits its limit, and what a building looks like when insulation is built in from day one rather than added later.
Why a summer house gets so cold
A standard summer house is what is called a single skin building. There is one layer of timber cladding, a light timber frame behind it, and nothing in between to stop heat moving straight through the wall. From the inside, you are usually looking at the back of the cladding boards.
Our own range of Summer Houses is built this way on purpose. It keeps the price down and the building light and airy, which is exactly right for a space you use from spring to autumn. It is just not built to fight a British winter. Windows are single-glazed, and there is no lining or insulation anywhere in the structure.
That is why, on a frosty morning, a summer house is roughly the same temperature inside as it is outside.
What you can do yourself
There is a fair amount you can do to close the gap. None of it turns a summer house into a warm room overnight, but done properly it makes a real difference.
| Area | DIY option | What it does | Watch out for |
|---|---|---|---|
| Floor | Rigid insulation boards under a raised floor, or thick underlay and rugs | Cuts heat loss through the base, which is often the coldest part | Boards mean lifting the existing floor first |
| Walls | Insulated plasterboard, or a timber batten frame filled with mineral wool and covered with OSB or ply | Usually the biggest single improvement | Eats into floor space and needs a vapour barrier fitted the right way round |
| Roof | Insulation boards fitted between and under the roof timbers | Stops warm air escaping upward, where most heat loss happens | Trapping moisture without a breather membrane can lead to damp and rot |
| Windows and doors | Secondary glazing film, draught excluder strips, thicker curtains | Cuts cold draughts and cold glass | Does not fix a frame that no longer fits properly |
| Gaps and draughts | Foam strip and silicone sealant around frames and skirting | A cheap, quick win | Limited on its own if the walls themselves stay cold |
Work through the list roughly in that order. There is little point draught-proofing a window if the wall next to it is still letting heat straight through.
For heating itself, a small electric panel heater or oil-filled radiator is usually enough once the building has some insulation in it. Without insulation, a heater is just warming the garden.
Where DIY insulation runs into problems
Retrofitting insulation into a building that was not designed for it has real limits, and it is worth knowing them before you start.
The timber frame in a summer house is lighter than the frame in an insulated building, so there is less depth to fit proper insulation into without building the walls out further. Fitting insulation without a correctly placed vapour barrier can trap moisture inside the wall, which leads to condensation and eventually rot, the opposite of what you are trying to achieve. Windows and doors are usually single glazed and hard to upgrade without replacing them outright.
As the Energy Saving Trust explains, the lower a building’s U-value, the more slowly heat escapes through its walls, roof and floor together, not through any single part on its own. A summer house was never designed with that fabric-first approach in mind, so a DIY retrofit is always working around the original build rather than with it.
Or skip the retrofit and have it built insulated from the start
If you are at the start of your search rather than trying to rescue a building you already own, it is worth thinking about this the other way round.
Our Heavy Duty Lined & Insulated Buildings are built on a heavier 38mm x 75mm frame, with 100mm Earthwool insulation packed into the walls and roof, a lined and insulated floor, and double glazed doors and windows as standard. The wall performs about as well as 300mm of solid timber, and there is no vapour barrier or moisture risk to work around because it is all fitted correctly at the factory, not retrofitted in the garden on a wet Saturday.
Send an enquiry through our contact form and choose the lined and insulated option from the product list, and we will talk you through sizes and pricing for your garden.
If you want to see exactly what goes into a factory insulated building before you decide, our Lined and Insulated Building Specification sets out the full build, from the frame to the felt.
The bottom line
A DIY insulation job will take some of the chill off a summer house, and for light use through the shoulder seasons that might be all you need. If you want a space you can genuinely work, relax or entertain in through a full winter, building insulation in from the start almost always works out cheaper and warmer than trying to add it later.
We have been building timber garden buildings in Danbury, Essex since 1951, so whichever route suits you, we would rather give you an honest answer than sell you the wrong building.