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Insulated garden rooms: what to look for

Every company says its rooms are insulated. Here's how to tell a room you'll use all winter from one you won't.

Updated 22 October 2026

What "fully insulated" should mean

"Insulated" on a product page can mean anything from a thin quilt in the roof to a building as warm as a new house. A room you'll use all year needs:

• insulation in the floor, walls and roof, not just one or two of them
• double or triple glazing in the windows and doors
• a vapour control layer on the warm side of the insulation, to stop moisture getting into the structure³
• some form of heating²

Our insulation guide explains the materials and how to stop condensation. This guide is about comparing insulated rooms before you buy.

U-values: the number to ask for

A U-value measures how much heat escapes through a part of the building. Lower is better. Most garden rooms are exempt from building regulations, so they don't have to meet any U-value. The building regulations figures are still a useful yardstick:¹

Limiting U-values in W/m²K, from Approved Document L, Tables 4.1 and 4.2.

Many companies quote insulation thickness rather than U-values. Thickness alone can mislead, because materials differ: mineral wool needs one and a half to two times the thickness of rigid PIR-type board to reach the same U-value.⁴ Ask for both.

SIPs, timber frame or log cabin?

• Timber frame with insulation between the studs is the traditional build. In house construction, a well-designed timber frame can reach a wall U-value of around 0.15.⁵ A garden room's thinner walls usually won't, so ask for the figure.
• SIPs (structural insulated panels) bond insulation between two boards. They reach similar U-values, and their joints make it easier to keep air leakage low. Pound for pound, they cost slightly more than a basic timber frame.⁵
• Log cabins rely on solid timber walls. One manufacturer's own figures put a 28mm log wall at a U-value of 2.53, a 44mm wall at 1.93 and a 70mm wall at 1.39.⁶ Even the 70mm wall lets out more than five times as much heat as the new-home wall limit of 0.26.

That's the biggest red flag in this market: an "insulated log cabin" may have an insulated roof and floor but solid log walls. If you want to use it in winter, check the walls.

Double or triple glazing

Typical double glazing has a U-value of about 1.4 and triple about 0.8.⁷ Triple glazing costs around 20% more and is mainly worth it in particularly cold or noisy places.⁸ Glazing is usually the weakest point of a garden room, so a room with a wall of glass doors loses much more heat than one with a single door, whatever the walls are made of.

Heating and running costs

• Electric panel or oil-filled heaters are cheap to buy. A 2kW heater uses 2kWh an hour, which is about 53p at the price cap rate of 26.32p per kWh for October to December 2026 (England, Scotland and Wales).⁹ Check your own tariff.
• Infrared panels are typically 350 to 900 watts, and a well-controlled system will probably cost less to run than standard panel heaters. Their cooler surfaces can increase condensation in cold spots, so ventilation matters.¹⁰
• Air-to-air heat pumps (air conditioning units that also heat) produce 2.5 to 4 units of heat for each unit of electricity, and can cool the room in summer.¹¹

The better insulated the room, the less any of these will cost to run.

Questions to ask before you buy

• What are the U-values of the walls, floor, roof and glazing?
• What insulation is used in each, and how thick is it?
• Where does the vapour control layer go?
• Is everything included in the price, or is floor insulation an extra?
• What heating comes with the room, and what would you recommend for winter use?

Get at least three itemised quotes and be wary of one that's much cheaper than the rest.² Our company comparison sets out what each company includes.

Questions people ask

What U-values should a garden room have?

Most garden rooms don't have to meet building regulations, but a new home's U-values must be no higher than 0.26 for walls, 0.18 for floors, 0.16 for roofs and 1.6 for windows.¹ Lower is better. That's a sensible yardstick for a room you'll use all year.

Is a log cabin as warm as an insulated garden room?

Not usually. One manufacturer's figures put a 44mm log wall at a U-value of 1.93 and a 70mm wall at 1.39⁶, far above the 0.26 limit for a new home's walls.¹

How much does it cost to heat a garden room?

A 2kW electric heater uses 2kWh an hour, about 53p at the price cap rate for October to December 2026.⁹ An air-to-air heat pump uses much less electricity for the same heat.¹¹

Related guides

Sources

1. HM Government, Approved Document L, Volume 1: Dwellings (2021 edition incorporating 2023 amendments).
2. Which?, May 2026, How to buy the best garden room.
3. LABC Warranty, Tech update: vapour control layers.
4. Hertsmere Borough Council Building Control, July 2011, Technical note 10: U-values.
5. Homebuilding & Renovating, May 2024, Timber frame vs SIPs.
6. Summerhouse24 (manufacturer figures), U-values of summer houses and log cabins.
7. Homebuilding & Renovating, Oct 2022, Does triple glazing make sense?.
8. Energy Saving Trust, Windows and doors.
9. Ofgem, Changes to energy price cap between 1 October and 31 December 2026.
10. Energy Saving Trust, Infrared heating explained.
11. Which?, Air source heat pumps explained.