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Integrating External Wall Insulation with Loft Insulation Specs
This is my site Written by admin on June 12, 2009 – 4:39 pm

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.

roof3

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.

roof21

This diagram shows the integration of external wall insulation and the above, which raises a number of points :-

  1. The Barge board (A) become bigger to cover both the existing joist (D) and accomodate the additional insulation (C).
  2. 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.
  3. 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).

roof211

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.

roof4

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 ?

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2 Responses »

  1. Hi, I have a solid walled property and I am hoping to insulate externally and internally in the loft as set out here. One question I have is in relation to ventilation. Our property is from 1924 and the loft is pretty draughty. Won’t sealing it up cause warm damp conditions, I thought this was to be avoided. Would welcome you thoughts.

  2. I sealed my roof by putting insulation between the rafters so the loft space is what is classed as a “warm roof”. Hence no condensation forms due to cold air. The wall insulation meets the roof insulation so the house becomes completely sealed. However, you then need to install a heat recovery system to ensure you get fresh air to breath.

    The trick I missed for the heat recovery system was to bury the air intake below the earth in the garden i.e. the entry point is above ground but the rest of the tubing is buried underground as much as possible before entering the house. The cold incoming air absorbs the ground heat and gives you an extra boost of heat before heat exchanging with the stale air. Very similar to a ground source heat pump but heating up the air not water / gas.

    I now have 5 years of data (taken weekly), and we’ve cut our emissions by about 80%. We burn logs in the evening and that heats the whole house up for the next day. If I was installing the log burner again, I would put super insulation on the walls and full any voids behind the burner with concrete blocks to give more thermal mass. But what we have works reasonably well.

    Hope this helps.
    Rob

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