How to Insulate Garage

A poorly insulated garage loses heat rapidly, making it cold in winter, uncomfortably hot in summer, and wasteful if you use it as anything other than basic car storage. Whether you’re converting a garage to living space, setting up a home gym or workshop, or simply want to stop heat bleeding out through an attached garage wall, insulation makes a real difference.

This guide covers the three main zones, roof, walls, and floor, their respective material options, 2026 costs, and the Building Regulations position.

Do You Need Building Regulations Approval?

This depends on what you’re doing:

• Insulating an attached garage that’s already in use as part of the house (e.g., a garage room): there is no change of use, but adding insulation to an outside wall, floor or roof is treated as renovating a thermal element, as described for the house wall below.
• Converting a garage to a habitable room: this will normally require Building Regulations approval, and the converted room has to meet the relevant requirements, including Part L (thermal performance), Part F (ventilation) and Part B (fire safety).¹
• Insulating a detached garage: no Building Regulations required unless you’re converting it to living space.
• Insulating the wall between an attached garage and the house: that wall separates the heated house from an unheated garage, so the Building Regulations treat it as a thermal element.² Adding a layer of insulation counts as renovating it, and where more than 50% of the wall’s area is renovated the whole wall has to meet Approved Document L and the work is notifiable building work, so check with building control before starting.³⁴

If in doubt, contact your local authority building control (LABC) before starting.

Zone 1: Roof / Ceiling Insulation

The roof is usually where most heat is lost, especially in a single-skin concrete or metal-roofed garage. It’s also typically the easiest place to add insulation without major structural work.

Option A: Rafter/joist insulation above a flat ceiling

If the garage has a flat ceiling (plasterboard or similar) with a void above, you can lay mineral wool quilt or rigid board in the void.

• 100 mm mineral wool quilt (e.g., Knauf Earthwool, Rockwool RollBatt): £2.50–£4/m²
• 100 mm rigid PIR board (Celotex, Kingspan): about £16–£20/m²; Wickes sells a 2400 × 1200 × 100 mm Kingspan TP10 board, covering 2.88 m², for £55.⁵⁶

A garage ceiling of 20 m² fitted with 150 mm mineral wool costs approximately £80–£120 in materials, easily a half-day DIY job.

Option B: Rafters lined with rigid insulation (pitched roof or directly under roof covering)

Where there’s no ceiling void, rigid PIR boards can be fitted between or below roof timbers.

• 50 mm PIR board (between rafters): approximately £8–£14/m²
• 25 mm PIR board (continuous layer under rafters, to eliminate cold bridges): £5–£10/m²

For best performance, a two-layer approach (between rafters + a continuous layer below) minimises cold bridging.

Option C: Spray foam (not recommended)

Spray foam has been used for garage roof insulation but has significant downsides: it can trap moisture in timber, is difficult to remove, and, importantly for UK homeowners, some mortgage lenders will not advance funds on properties where spray foam has been used.⁷ Avoid for any space that may eventually be mortgaged or sold.

Zone 2: Wall Insulation

Garage walls are typically:

• Single-skin brickwork (common in older attached and integral garages)
• Concrete block (common in detached garages)
• Timber frame with cladding (some modern garages)

For all types, the practical options are internal insulation (losing a small amount of internal space) or, where accessible, external insulation.

Internal wall insulation options:

For a non-habitable garage workshop or gym, you don’t need to skim to a domestic finish, foil-faced PIR board taped at joints can be left exposed. This saves cost and keeps the job DIY-accessible.

For a garage that will be converted to a habitable room (bedroom, office, playroom), the walls must achieve the standards in Approved Document L, 0.30 W/m²K for a renovated wall insulated internally or externally in England, under Table 4.3 of Approved Document L⁸, which typically requires 80–100 mm of PIR or 125 mm of mineral wool.

Zone 3: Floor Insulation

Concrete garage floors are a significant source of cold in winter. If the floor will see regular use (workshop, gym, home office), floor insulation makes a noticeable difference in comfort.

Option A: Rigid insulation under a new screed

This is the highest-performance solution but requires raising the floor level by at least 100–150 mm (insulation + screed) and is only practical if ceiling height allows.

• 75 mm PIR board under 65 mm screed: materials approximately £40–£65/m²
• Total inc. labour (if screeding professionally): £80–£140/m²

Option B: Insulation board under a floating chipboard deck (e.g., Jablite EPS board)

Rigid EPS or PIR insulation boards laid directly on the existing slab, with a floating chipboard deck (or chipboard-faced insulated panels) on top, then finished with flooring. The rise is typically 45–100 mm, depending on the insulation thickness.

• Cost: £15–£30/m² for panels + chosen floor finish
• DIY-friendly; achievable over a weekend for a standard single garage

Option C: None / accept cold floor

For a garage used purely for car storage, floor insulation is rarely worthwhile. Thermal mats or rubber gym flooring offer minimal insulation but improve comfort without significant cost.

Garage Door Insulation

An uninsulated steel garage door is the biggest thermal weak point in most garages. If replacing the door is not in the budget, a lined insulation kit is available:

• Reflective foil-faced bubble wrap insulation kits (DIY, fits between door ribs): £30–£90 per door
• PIR board cut-to-fit (better performance, more work): £50–£150 per door

A replacement insulated sectional garage door for a single garage typically costs about £1,900–£3,500 supplied and fitted, the upper end with an electric operator. Hörmann’s recommended retail price for its RenoMatic insulated sectional door with an electric operator starts at £1,847, excluding VAT and fitting, in its campaign sizes.⁹ Good ones insulate well: Hörmann quotes a U-value of 1.0 W/m²K for its LPU 67 Thermo sectional door, measured on a 5000 × 2125 mm door, whereas a single-skin steel door has no insulation at all.¹⁰ In a heated garage it cuts heat loss, although at this price the energy saving alone is unlikely to repay it quickly.

Cost Summary: Typical Single Garage

Practical Tips

• Ventilation: insulating a garage tightly without ventilation can lead to condensation problems. Add a trickle vent or background ventilation if the garage will be regularly occupied or if moisture-generating activities (washing, painting) will happen inside.
• Vapour control: in most garage-to-habitable-room conversions, a vapour control layer (VCL) should be installed on the warm side of the insulation to prevent condensation within the insulation layer.
• Work from the outside in: always insulate the roof/ceiling first (heat rises), then walls, then floor. Doing it in this order lets you assess the improvement before spending money on each subsequent zone.
• Material choice for damp garages: if the garage is prone to damp, cure the damp first, then prefer foil-faced PIR board, which resists moisture, to mineral wool, which loses much of its performance if it gets wet.