The Energy Saving Trust best kept secret is a booklet entitled “Energy Efficiency is best practice” which lists a whole set of technical guides that are appropriate to different audiences. Really useful reference material at a relatively high level.
Although I do not have a solid wall, CE184 Pratical refurbishment of Solid-walled houses is a useful guide for the insulation work that is being considered for my property. In particular, I am looking at loft insulation, and External Wall Insulation using rainscreen cladding and how the 2 come together. I am making the assumption that both sets of insulation need to meet inorder to remove any gaps that could leak heat.
The diagram above is an extract from the EST guide that shows a solution with “Insulation between and below the rafters”. The reason for picking this solution is to avoid disrupting the existing rooms – we do not want to remove the existing inside plasterboard to insert insulation between the rafters and then put up new plasterboard (possibly with additional insulation).
This increase the roof height and also the size of the boarding. It looks like the guttering needs repositioning and a longer downpipe will be required.
This diagram shows the integration of external wall insulation and the above, which raises a number of points :-
- The Barge board (A) become bigger to cover both the existing joist (D) and accomodate the additional insulation (C).
- Due to the external wall insulation (E) the size of the soffit board (H) has reduced i.e. the overhang has reduced. Previously it would have covered the existing joist (D) and given more overhang.
- The rainscreen cladding (G) cannot go any higher because the barge & soffit boards (A, H) accomodate the extra insulation and the remaining protuberance of the existing joist (D).
This option involves cutting back the existing joist to allow the soffit board to be placed higher up and the rainscreen cladding to meet it.This keeps the size of the barge board (A) the same size as before, but higher than previous.
However the overhand is relatively small – does this matter ? Increasing the overhang would involve extending the roof further out in front.
So option 2 lengthens the Timber Stop (H) causing the roof to overhang further than previous. It still requires the existing joist (D) to be shortened, but enables the soffit board (H) to overhang further. Also the Barge board is approximately the same size as before, but higher due to the additional roof insulation.
What is the right approach ?