Insulation materials in the frame

The four considerations

Thermal performance

How much it gives per unit thickness. The most obvious consideration, but not the only one.

Fire class

In many places this is the strongest constraint: on façades, in cavities and along escape routes there are specific requirements on the class of the material. Fire questions around insulation materials.

Behaviour in moisture

Whether it takes up water, whether it dries out, what happens to it once it has got wet. In a closed structure this is a question of service life.

Ability to fill

How completely it fills the structure without gaps. A good insulant badly installed gives a worse result than a more modest material installed carefully.

The two main routes in practice

Foam insulation in the structure

The foam expands everywhere, so quality does not hang on how carefully the gaps are stuffed — and for its thickness it gives the best thermal value. It completes the wall cavities and the outside of the house in one trade.

In fire terms, though, it is more constrained.

Mineral wool

Where the regulations do not allow organic foam, mineral wool gives just as good a result — but there the careful filling of the gaps is critical, because a board material does not fill them by itself.

Its advantages are the fire class and the good acoustic behaviour.

What to avoid

Do not put water-absorbing organic material into an external wall. In a light gauge steel wall moisture cannot “get lost” as it can in a thick brick wall — whatever goes in has to be lived with for the whole service life of the structure. Organic and absorbent materials in the wall.

The role in the build-up

Where the insulation sits matters:

• between the sections: it fills the structural space but carries the thermal bridge,
• outside the frame: this is what eliminates the thermal bridge, Continuous external insulation,
• on the inside: risky in vapour terms, only with a check.

The acoustic role

Fibrous insulation in the cavity of the structure improves the sound insulation as well — rigid foams less so. Sound insulation basics.

The point

Decide first where the insulation will sit and what the regulations allow there. Comparing thermal figures comes after that.