What is perimeter insulation?
Perimeter insulation is insulation applied to the outside of a building on the parts that sit below ground level: basement walls, foundation strips and the edge of a ground-bearing slab. The insulation is therefore permanently in contact with soil and moisture.
That imposes requirements which do not apply to ordinary wall insulation. The material must be load-bearing under compression (it carries earth pressure and sometimes the paving above), absorb almost no water and not decay in the ground.
Material choice
- XPS (extruded polystyrene) is the standard. Its closed-cell structure absorbs virtually no water, and compressive strength is high — boards in the order of 300 to 500 kPa are common for loaded applications.
- Cellular glass (foamed glass) is stronger still and completely waterproof and vapour tight, but more expensive.
- Specially water-repellent grades of EPS are also used; ordinary facade-grade EPS is not suitable.
Mineral wool and wood fibre are not suitable below ground: they retain water and then lose their insulating effect.
| XPS (extruded polystyrene) | The standard choice: closed-cell structure absorbs virtually no water, high compressive strength. |
|---|---|
| Cellular glass (foamed glass) | Stronger still, completely waterproof and vapour tight, but more expensive. |
| Water-repellent EPS | Specially water-repellent grades are also used. |
| Ordinary facade-grade EPS | Not suitable below ground. |
| Mineral wool and wood fibre | Not suitable below ground: they retain water and then lose their insulating effect. |
Why do it
- Remove the thermal bridge at the slab edge. Even with a well-insulated floor, heat escapes outwards through the foundation beam. That costs energy and produces cold floor edges and sometimes mould in the corner.
- Make the basement usable as living or working space, without reducing the internal dimensions and without the interstitial condensation risk of internal insulation.
- Protect the tanking. The insulation board shields the bitumen or polymer waterproofing from damage during backfilling.
Execution
The correct sequence is: clean the wall, apply the waterproofing, bond the insulation boards onto it with bitumen adhesive (do not fix mechanically — every plug is a hole in the waterproofing), then place a dimpled membrane or drainage mat against it, and only then backfill with free-draining material. Carry the insulation above ground level up behind the facade finish, and detail the plinth transition neatly and weather-resistantly; that transition is the most vulnerable point.
In new build, perimeter insulation is a modest extra cost because the excavation is open anyway. Retrofitting means digging out all round, which raises costs sharply — usually only worthwhile if you are already excavating for drainage or basement tanking.
Related terms
- Rigid insulation
- EPS
- Floor insulation
- Ground level
- Drainage
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