150mm rafter choices

One of the most frequent requests for build ups is where there are 150mm rafters with 400mm centres. As always, the choice is determined by cost and labour, and how much you can go up or down.

Although at 600mm centres it’s possible to achieve 0.18 W/m2K with TLX Gold and 110mm PIR in 150mm deep rafters:

at 400mm centres this isn’t possible unless you use the more expensive Phenolic board.

So, the options become:

1) Batten the rafters out to 165mm and use TLX Gold insulating breather membrane taut and counter battened with 125mm PIR board – you would need to decide if you could raise the roof to do so. Alternatively, you could batten out to 180mm and use TLX Gold draped with 125mm PIR.

2) Use 100mm PIR between the rafters and an underdraw of 20mm PIR with TLX Gold fitted over the rafters.

3) Use TLX Gold taut and counter battened with 110mm of Kingspan Kooltherm K107 phenolic board between the rafters.

If there is a preference to use mineral wool, then TLX Gold combined with 100mm 0.036 W/mK or better mineral wool would need 40mm of PIR beneath to achieve 0.18 W/m2K.

When using TLX Gold to reroof or with new build, it can be useful where there is an existing ceiling. (And there is unlikely to be a vapour barrier), as TLX Gold prevents interstitial condensation risk.

For a loft conversion or a new build where there is a breather membrane above, use TLX Silver under rafters. Though this does mean a 40mm loss of headroom.

You only require 85mm PIR for 0.18 W/m2K. Though since 100mm PIR is more readily obtainable, and quite possibly cheaper, you can use it in 150mm rafters, when it will achieve 0.16 W/m2K. Set the PIR board 30mm up from the bottom of the rafters. Friction fit or support with a batten or nail. Staple TLX Silver to the underside, then secure with a 38mm deep batten set crosswise.

TLX Silver compresses to 2mm under battens. This creates sufficient space to accommodate the product thickness of 30mm plus 2x20mm unventilated air gaps. Heat reflects across these gaps, contributing to the thermal performance.
If there is a bitumen felt in place then this requires a 50mm ventilated space under it, and 100mm PIR board set 25mm up from the bottom of the rafters plus TLX Silver under as before. Batten the rafters out by 25mm to 175mm, resulting in a total depth of 215mm plus plasterboard.

Loss of headroom may be unacceptable, then Building Control may accept a worse U-value, such as the 0.21 W/m2K achieved if the 100mm is set flush with the bottom of the 150mm rafters, with the TLX being applied directly and secured with a 38mm batten. If they are insistent on achieving 0.18 W/m2K then using 100mm Kooltherm K107 and TLX Silver multifoil secured with a 50mm deep batten will lose 50mm of headroom plus plasterboard.

If none of the above meet your needs, then give us a call on the Thermal Hotline – 01204 674730.

Read the TLX Silver case study related to this post HERE

Read the TLX Gold case study related to this post HERE