Why all insulation isn’t equal and what matters to you.
Insulation – so many choices out there from which to build from!
- Rigid boards from Celotex, Kingspan, Recticel, Unilin perhaps the most popular for the rigid high performers
- Knauff, Rockwool, Isovar in the mineral wools
- Of the wood fibre type there is Steico, Pavatex and Uditherm to name a few
- Multi foils – Actis, YBS, Superfoil
- And then the more specialist insulation materials- Sheep’s wool, Hemp, Lime, Cellulose, Straw etc
Which do you build from?
A few factors that may influence the choice of product – the construction type, for example:
- Timber frame
- Masonry,
- Cavity wall – full fill or partial fill
- Solid wall,
- New build
- Existing construction.
Then there is also:
- Cost per m2,
- Availability
- Ease of installation
- Wall, roof, floor thickness
- Healthy homes and indoor air quality
All the above is a consideration when deciding what insulation is best for your building.
But what about how thermal efficient it is? What U value will you be able to achieve and what must you to be able to achieve to meet compliance requirements?
Part L of the Building Regulations states the maximum U values for a new build dwelling are as follows.
- Roof 0.16 W/m2k
- Wall 0.26 W/m2k
- Floor 0.18 W/m2k
- Party Wall 0.2 W/m2k
However, the Notional specification, the specification that your building is actual assessed against is as follows:
- Roof 0.11 W/m2k
- Wall 0.18 W/m2k
- Floor 0.13 W/m2k
- Party Wall 0.0 W/m2k
As can be seen these values are considerably lower than the backstop values, and if you want your building to pass the Part L requirements in the SAP calculations the above values are what you need to be aiming for.
What insulation do you need to achieve these sorts of u values?
Here is a rough guide, for external walls.
Masonry External walls
- Rigid boards you can get down to 0.12/0.13 0.16 W/m2k with 150mm of insulation
- Mineral wool unlikely to get much lower than 0.19 W/m2k with the same thickness
- The best sheep’s wool down to around 0.22 W/m2k with 150mm.
- Wood fibre and multi foils are unlikely to be used in masonry walls, much more common in timber frame.
The more natural type of insulation, away from the rigid boards, is likely to need more of it, ie thicker, simply because the thermal conductivity of the product is generally higher than the petroleum-based boards.
For timber frame walls achieving around 0.18 W/m2k is usually quite easily done
- The Multifoils will achieve this when used in combination between two of their products, one between the stud, 140mm, and another internally, about 45mm.
- Wood fibre boards are similar but would need 140mm +80mm
- And the rigid boards about 140mm + 20mm internally.
There’s another consideration though
Whatever construction build up that is being used also determines what thermal bridging values are used within the SAP calculation. These are the junctions around the windows and doors and between elements, like the ground floor/wall junction and the external wall, roof junction.
Depending on the type of insulation used and the construction build up of the external element, this will influence what the psi value is (think heat loss through the junction) and where to get that psi.
Some insulation companies provide psi values for their different products and the range of thickness, Unilin for example are great. Others, like Supa foil, provide nothing. If none are available or we can’t find something suitable from another source, like Recognised Construction Details or LABC, then we must either use the default psi value in SAP or they can be bespoke calculated.
Achieving the u value is only one consideration when determining which insulation to use.
If one insulation type has been agreed at design stage and for whatever reason that insulation product is changed, even if the replacement has the same thermal conductivity and the u value remains the same, it could affect the thermal bridging psi values used and put the overall SAP calculation at risk.
Ultimately whatever insulation you choose is up to you, just bear in mind that the construction element should ideally meet the Notional specification U Value, and that you know where the thermal bridging psi values are coming from.
And finally, if the chosen product needs to change, check the impact of that against the SAP calculation.
If in doubt, please ask us, we’re here to help