Heat
House too hot or a hot roof — what can you do?
Most of the heat in a house — especially in upstairs rooms — comes from a roof that sits in the sun all day and passes the heat down through the ceiling. The usual fix is insulation under the roof or laid on the ceiling, together with reflective foil and ventilation of the roof space. Sun-facing walls and windows also play a part.
How does heat get in?
- The roof — roof sheets get very hot in the day and radiate heat into the roof space and onto the ceiling
- A closed roof space — hot air builds up, warming the ceiling and the rooms below
- Sun-facing walls — especially west walls in the afternoon
- Windows and glass — direct sunlight
Hot roof, hot upstairs rooms: start with the roof and ceiling
- Insulate under the roof sheet or on the roof structure — reduces heat before it reaches the roof space
- Lay insulation on the ceiling — easy for finished houses; reduces heat coming down into the rooms
- Reflective foil — works well when there is an air space in front of the foil
- Ventilate the roof space — reduces the heat that builds up there
How to choose the insulation
| What to check | What it means |
|---|---|
| --- | --- |
| Thermal conductivity (λ) | Lower is better at the same thickness |
| Thickness | Thicker gives higher thermal resistance (R) |
| Where it goes | Under the roof, on the structure or on the ceiling — use the product the maker specifies for that position |
| Facing | Foil reflects heat and resists moisture |
Read more: How to choose roof insulation · Stone wool vs glass wool
Products used for this problem
SCG STAY COOL ceiling insulation
Building thermal insulationWhy it fits: Glass-wool insulation laid on the ceiling — suits finished houses
SCG SUPER COOL-G roof insulation
Building thermal insulationWhy it fits: Foil-wrapped glass-wool insulation for roofs
ROCKWOOL Cool 'n' Comfort RL stone wool roll
Building thermal insulationWhy it fits: Stone-wool roll for roofs and ceilings
Microfiber MicroRoof FLS roof insulation
Building thermal insulationWhy it fits: Glass-wool insulation for roofs
ESKIMO FOIL 101F double-sided reflective foil
Building thermal insulationWhy it fits: Reflective foil for under the roof
Good to know
- How much cooler it gets depends on the roof, ventilation and where the insulation goes — not on the insulation alone
- Cover the whole area; gaps let heat through
- Do not cover hot light fittings or electrical equipment — follow the maker's instructions
- Roof work is work at height and needs proper safety equipment
Frequently asked questions
Is insulation on the ceiling enough?
For a finished house, laying insulation on the ceiling is easy and reduces the heat reaching the rooms. If the roof space gets very hot, ventilating it adds to the effect.
The ground floor is hot too — what else?
Look at afternoon-sun walls and the windows. Send us photos and the direction the rooms face and we'll help assess it.
Common problems
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Want us to look at your case?
Send photos, the area size and a description — we'll suggest the approach and materials
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