What type of insulation and why?

We recently had a client with a large house that was being built on a small budget. We suggested full fill cavity PIR insulation for the walls. This has become more common in recent years as it has become more difficult to insulate a home to the standard required by the SAP test. It used to be either 50 mm of PIR insulation in a 100 mm cavity or 100 mm of mineral wool in a wall with a 100 mm cavity.

The new full fill PIR products give 100 mm of good quality insulation in a cavity wall which is twice as effective and half the U value of the same amount of mineral wool. SAP assessors love this new insulation and some of their customers hate it. Why? Simply because it’s very expensive.

To have full fill PIR insulation you have to create space for the condensation and give the water somewhere to collect in the wall. This means that products like Xtratherm have dimples on the back and products like Celotex have wide spaces with narrow insulation. Adding these features to insulation which, for many decades, has just been a plain board of foam, increases the cost. When our customer complained that this was too expensive, we suggested external wall insulation (EWI) with mineral wool, thinking that it would be very cheap.

Mineral wool loft insulation is very cheap, but mineral wool EWI is extremely expensive. This is because when mineral wool goes on the outside of a building it has to be made waterproof. A number of companies make Rainscreen products – mineral wool boards designed to go on the outside of a building. These are very expensive because mineral wool is naturally very water absorbent and making it sufficiently waterproof to go on the exterior of a building is very difficult.

The conclusion

Our client eventually decided to use external PIR insulation which was a fraction of the cost. PIR is a lot more water resistant than mineral wool and it is easier to produce PIR products which are sufficiently waterproof to be used on the outside of a building.

One of the cheapest forms of insulation is a timber frame wall containing mineral wool on the inside of the building. This mineral wool doesn’t have to be made to the same standard as EWI Rainscreen products and is very cheap. It needs to be several layers thick in order to achieve the required U value. This is not a problem as timber frame construction allows the wall to be built with only one layer of bricks or even with no bricks at all. Just plywood and render on the outside of the house will give an attractive appearance. There is room for two or three 100 mm layers of mineral wool in a timber frame wall.

If you have a flat ceiling with nothing above it, mineral wool loft insulation will give you the same U value as much more expensive PIR as long as it is thick enough. In a loft there is room for insulation up to two foot thick. PIR is necessary when there are rooms in the roof and is good at achieving an acceptable U value in a thin sloping ceiling. It is also possible to use mineral wool in between the ceiling joists and a PIR board on top if you intend to use the loft as storage space and want to have a floor that you can walk on.

PRI can be twice as effective as mineral wool

If you are designing a house, check online stockists and look at the prices of different kinds of insulation. Remember that PIR products are generally about twice as effective as the same thickness of mineral wool. If a particular stockist doesn’t stock something, it may be because it’s very expensive. You will be able to buy it from the manufacturer’s website but it could be so expensive that few people use it, so most stockists don’t have it. Make up your mind which is the best insulation to give you a good thermally efficient house at a price you can afford.

What type of insulation?
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