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How would you insulate in and around a structural oak timber frame?

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Posts: 80
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Topic starter

With traditional Oak or Douglas fir frames the frame does all of the structural work and so the infill panels are largely just that, infill. However, this doesn't materially change the way in which they would be built up from our standard suggestions, which you can see below. The main difference is that you need to wrap the structural frame with the VCL membrane so that as this frame shrinks you do not open up huge air gaps. This takes time as there is a lot of cutting and taping of the membrane around the timbers and junctions, etc. and this should be brought round to finish flush with the internal faces of the timber frame.

Once the membrane is installed, the softwood panels and rafters can be installed in the appropriate positions and the membrane lapped on to the inside faces of these panels/rafters. This can then be taped to the air tightness layer on the inside of each section.

In terms of minimum build-ups for the panels in the walls I'd suggest the following:-

This will achieve a U-value of 0.177 W/m2K which would be required by building control.

For the roof I’d suggest the following:-

This will achieve a U-value of 0.134 W/m2K which will just pass through building regs.

Hope that helps.

Posted : 11/03/2021 11:59 am
Posts: 80
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Topic starter

Hi Dave

Breather membranes are just an easy addition to the build-up to satisfy building control without lots of justification. However, if you have bats you will need to omit the breather membrane and rely on the boards. Either is fine from a functional point of view, it just depends on the requirements of the roof. The boards are designed to cope without being covered by a membrane.

Hope that helps.

Posted : 19/01/2022 12:45 pm
Posts: 3

Hi Chris

As a follow on for this. I generally design my buildings with a solid board  fixed to the oak/ Larch frame at centeres of post (reduces the shrinkage element) with plasterboard  and skim internally when encapsulation is finished. This gives fixing options anywhere over the plastered space.

  • Is there an issue withputting the VCL outside the 11mm OSB? Much easier to apply. Also means there is no problem accessing the service void
  • Can your clay boards be used in this situation against the solid internal boarding?
  • If the foundation is built to the exterior 50mm batten line will the base of the 60/80mm Beltermo need sealing off?

Using your wall calculator I would like to use the following. Does this meet building regs? I would be happy using 80 mm Beltermo if not.


Posted : 29/06/2023 9:56 am
Posts: 80
Member Admin
Topic starter

Hi Dave

So long as the VCL remains on the warm side of the insulation, the OSB and VCL can be fixed either way round, whichever is easier for you and as you say, the OSB would protect the VCL if swapped round from the above spec.

Assuming you mean OSB/Ply when you say 'Solid Boarding' then yes, the clay boards can be fixed back to it. It is actually a lot easier to fix the clay boards back to a solid board instead of studs because you don't have to join the boards on the studs and they can be fixed across the whole boards surface.

With the external boards, you'd usually finish the base of the insulation boards with an aluminium tray called a starter track. You can see them here - . You can also see how we'd usually suggest detailing the exterior of the plinth area here - This is with a rendered finish but the line/detailing of the plinth insulation and wood fibre boards would be much the same.

Hope that helps.

Posted : 29/06/2023 12:29 pm

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