Insulation comes in many different shapes, sizes and types. Any building designer needs to decide which insulation products to specify; this will often be driven by the construction type, but there are almost always a number of different options to choose from.
Two of the main considerations are:
- thermal conductivity, how effectively the insulation prevents heat loss, and;
- practicality of installation, how easy it is for a builder to actually install the insulation neatly in the situation that arises.
Other considerations include fire performance and moisture management, but these won’t be discussed in depth here.
Insulation Options
One of the most widely used insulation types is PIR (polyisocyanurate). This is a closed-cell, rigid foam board, usually laminated with an aluminium foil facing.
It has a wide variety of applications, including walls, floors and roofs, and has one of the best thermal conductivities of any insulation type (usually around 0.021 W/mK). Practically speaking, that means less thickness is required to hit the target U-value set by UK Building Regulations Part L or Passivhaus, which in turn means more floor space or head room. It’s also easily available and cheap, so it’s no surprise that PIR is a popular choice.
Particularly for applications such as walls, roofs and suspended floors, there are alternatives. The most common is probably mineral wool, flexible rolls or batts of insulation manufactured from minerals that have been heated, spun into fibres, and compressed. Woodfibre insulation is another option, manufactured by mixing wood pulp with water before compression and drying. Woodfibre is semi-rigid, less flexible than mineral wool but with much more ‘give’ than PIR. The thermal conductivity of both mineral wool and woodfibre is around 0.040 W/mK, only half as good as PIR. That means you need double the thickness to achieve the same thermal performance.
There are also many other types of insulation – XPS, EPS, blown cellulose, sheep’s wool, straw bales, the list goes on. They all have pros and cons, but for now we will focus on PIR versus typical mineral wool or woodfibre alternatives.
Why Would Anyone Choose an Insulation With a Worse Performance than PIR?
As we’ve said, the thermal performance of PIR is almost twice as good as most other options. So this should be the product to choose, right?
Imagine you’re on the run from the law, it’s cold and dark, and you find yourself hiding out in a big insulation warehouse. On the left, you see a big pile of foil backed PIR insulation boards. On the right, you see rolls and rolls of mineral wool. Which do you go for? Do you spend the night lying under a rigid board, a cold draught swirling around you, or do you wrap yourself in the soft rolls of cosy mineral wool?
If you’ve got any sense, you’ll go for the latter. This analogy is slightly exaggerated, and I can assure you this never happened to me, but the same logic applies to buildings.
Why Thermal Bypass Matters – A Lot
Without perhaps realising it, you chose the mineral wool in the warehouse to avoid a phenomenon called thermal bypass.
Thermal bypass is when cold air flows through or around insulation, rendering it much less effective than it would otherwise be. The rigid boards wouldn’t mould to the shape of your body, so air would travel freely around you. Conversely, wrapping yourself tightly in mineral wool traps your body heat by stopping this unwanted flow of cold air.
In the context of a building, a similar situation arises in a number of places:
- When installing insulation between roof rafters
- When installing insulation between suspended floor joists
- When installing insulation in a wall cavity, either partial or full fill
When installing rigid insulation boards between rafters or joists, the challenge is in cutting the boards accurately enough to get a tight fit between the board and the structure. This is particularly difficult with old timber rafters or joists, which don’t have consistent gaps and are rarely straight.
Any tiny gap will allow the flow of air behind the boards and will significantly reduce real-life performance. Expanding foam can be used to plug the gaps, but this will degrade over time, so isn’t a robust solution. Some builders would say they can cut PIR accurately enough, but it’s very difficult and is unusual in practice.
When it comes to cavity walls, the issue is in forming a tight fit between the external face of the inner leaf and the rigid board. Any deviation from straight, or any mortar snots, mean boards won’t sit neatly on the face of the block or brick. Anything other than a perfectly straight line means small gaps and cracks, and that means thermal bypass when cold air can flow behind and around the insulation board.
All of these problems are largely resolved by mineral wool, or woodfibre where it’s appropriate. The flexibility of these insulation types allows them to be easily installed tightly between or against structure, preventing any thermal bypass.
Crucially, this means the real-life performance aligns with modelled U-values, whereas with PIR it may not.
Building Regulation Compliance Doesn’t Consider Realities of Installation
As the theoretical thermal performance of PIR is so good, it’s much easier to hit target U-values with relatively thin layers of insulation. This helps compliance with Part L of Building Regulations and any other thermal performance targets.
However, specification of PIR really opens up the performance gap due to issues with installation quality mentioned above.
The performance gap is the observation that buildings don’t perform as well in reality as they do during design. Consequently, and even though it might make theoretical U-values harder to achieve, use of flexible insulation is the best way to guarantee real-life thermal performance.
Using PIR is Choosing Petrochemicals Over Something More Natural
Even if you ignore the challenges of PIR installation, it is really nasty stuff. PIR is a petrochemical-based plastic, a material we’re increasingly trying to avoid, and isn’t pleasant to work with. It’s difficult to cut and gives off a dust that will stay on your hands and clothes for hours. Natural insulation is generally much more environmentally friendly, as well as being easier and healthier to work with.
There are times when PIR is a sensible choice. For example, insulating under a solid ground floor screed where head room is important and installation in a continuous layer is easy. Similarly, above (but not between) timber rafters. But all things considered, avoid rigid PIR boards as much as possible if you want buildings to perform in practice as well as on paper.
[…] similar pattern emerges when it comes to insulation. A cavity wall insulated with 100mm of PIR (not a great idea) has a U-value of around 0.21 W/m2K. Increasing the insulation thickness to 150mm achieves a […]