Building thermal insulation · Glass wool
SCG STAY COOL
- Thickness
- 75–150 mm
- Thermal resistance (R)
- 1.79 m²·K/W (75 mm)
How does heat get into a building?
In short
Heat enters a building in three ways: conduction through materials, convection carried by moving air, and radiation from a hot surface to a cooler one. In a house under strong sun the main paths are the roof, sun-facing walls, windows and air leaks. Each type of insulation reduces heat in a different way, so find out where the heat comes in first.
- 1Radiation: the sun heats the roof surface
- 2The hot roof radiates heat down onto the ceiling
- 3Convection: hot air builds up and moves in the roof space
- 4Conduction: heat passes through the wall material from outside to inside
- 5Sunlight comes straight through the glass
- 6Heat from every path adds up inside — each type of insulation reduces a different path
Why does the path matter?
Insulation in the wrong place can leave a house as hot as before. You might insulate the ceiling well, but if a room has a large window facing the afternoon sun, most of that room's heat still comes in through the glass. Knowing which part of the building lets the heat in lets you spend where it actually helps.
How do the three kinds of heat transfer differ?
| Mode | How it happens | Example in a house |
|---|---|---|
| --- | --- | --- |
| Conduction | Heat passes through a material, from the hot side to the cooler side | A sun-heated roof sheet passes heat through to its underside; a sunlit wall passes heat into the room |
| Convection | Warm air moves and carries heat with it | Hot air rises and collects under the roof; hot outside air leaks in around doors and windows |
| Radiation | A hot surface emits heat energy across a gap to a cooler surface, without touching it | The hot underside of the roof radiates heat down onto the ceiling; sun through glass heats floors and furniture |
In a real house all three happen at once. Take the roof: the sun heats the roof surface, heat is conducted through the sheet, its hot underside radiates down to the ceiling, hot air in the roof space convects heat onto the ceiling, and the warmer ceiling then passes the heat into the room.
Where does the heat come in?
Roof and roof space
The roof takes sun for most of the day, so the roof space can get very hot, and heat passes through the ceiling into the top-floor rooms. This is where most people start insulating, under the roof and on the ceiling. Read how to choose roof insulation and how to insulate a ceiling properly.
Walls
A wall in direct sun heats up and passes heat through into the room. Walls facing the afternoon sun are usually the ones you notice. Heat can be reduced both by shading the outside and by insulating the wall.
Windows and glass
Windows gain heat in several ways: sunlight passes through the glass and turns into heat when room surfaces absorb it, heat is conducted through the glass and frame, and air leaks through the joints. Roof or wall insulation does nothing for this part; it needs other measures, such as shading or choosing windows by their SHGC (the share of solar heat let through) and U-factor.
Air leaks
Gaps around doors, windows, pipes and cables through walls or ceilings let hot air from outside or from the roof space flow in. They also reduce the performance of the insulation you have installed.
Which kind of heat does each type of insulation reduce?
- Insulation that traps air in its structure — glass wool, stone wool, elastomeric foam and PE foam — slows heat moving through the insulation. Its performance is read from thermal conductivity (λ) and the R-value at the thickness used.
- Reflective foil (radiant barrier) mainly reduces radiant heat, and only works with an air space in front of the foil surface.
- Many products combine the two: fibrous or foam insulation with a foil facing.
Insulation reduces heat only over the area where it is installed. Gaps between pieces, areas left bare, windows and air leaks are still ways in for heat, so the result for the whole room depends on all the paths together.
If your problem is a hot house and you are not sure where to start, see Hot house / hot roof.
Sources
- U.S. Department of Energy / ENERGY STAR — Insulation Fact Sheet (three modes of heat flow; effect of gaps and air leaks)
- U.S. Department of Energy, Energy Saver — Energy Performance Ratings for Windows, Doors, and Skylights (heat through windows, SHGC and U-factor)
- Virginia Cooperative Extension — ENERGY SERIES: What about Radiant Barriers? (bulk insulation vs radiant barriers)
- ISO 7345:2018 — Thermal performance of buildings and building components — Physical quantities and definitions
Related products
Categories and example products for this topic
- SCG STAY COOL — Cuts heat passing from the roof space through the ceiling: foil-wrapped glass wool on top
- ESKIMO FOIL 101F — For radiant heat: foil surface reflectivity 97%, working when it faces an air space
- Microfiber MicroRoof FLS — For conduction through the roof: glass wool, foil one side, for metal and tiled roofs
- ROCKWOOL Rock Air — 30 kg/m³ stone wool roll for roofs and walls (2020 data sheet - ask before ordering)
Building thermal insulation · Glass wool
Microfiber MicroRoof FLS
- Thickness
- 25–75 mm
- Density
- 12–48 kg/m³
- Thermal conductivity (λ)
- 0.032–0.042 W/m·K
Building thermal insulation · Stone wool
ROCKWOOL Rock Air
- Thickness
- 50–75 mm
- Density
- 30 kg/m³
- Thermal conductivity (λ)
- 0.035 W/m·K