I'm not sure if I'm trying to ask an overly general question here - but thought I'd give it a go anyway.

This relates to what I'd think of as the bog standard loft conversion here in the UK, hundreds or thousands of which are done every year. Commonly they'll be to a Victorian or Edwardian terrace or semi-detached house.

A few things that most of these tend to have in common:

1) Existing, timber-rafter pitched roof planes which are to be retained
2) A new-build "dormer" element which will have vertical faces and a flat roof
3) New structural steelwork which will be by necessity bearing onto existing masonry (whether party walls or external walls)
4) The existing masonry walls (in the main body of the house) will be solid with no insulation, and insulating them externally or internally may or may not be part of the project.

With any project, I like to start out with a basic strategy for the insulation: does it go inside or outside of the main structure. And I think most people here would agree, in principle, it's usually best for it to be on the outside.

However -

With a typical loft conversion, it's not that straightforward. There is a conflict between parts of the build where the insulation is better on the inside, and parts where it's better on the outside. I'd break these down as follows:

Arguments for insulation inside the structure:
- Any *high level* steelwork, bearing into solid masonry party or gable-end walls, can do so without bridging the insulation layer.
- On any existing bits of pitched roof which are to be retained, putting insulation inside of the rafters means you can keep the external covering (tiles or slates) and you can keep the external roof plane unaltered and re-use the existing rafters.


Arguments for insulation outside the structure:
- Any *floor level* steelwork bearing onto masonry walls can do so without directly bridging the insulation layer
- It avoids most worries about compromised VCLs and condensation within the construction buildup.
- Generally works best for any new-build flat roof elements (straightforward insulation on top of deck, easier to build, no worrying about ventilation above insulation layer, etc)
- In principle it's easier to co-ordinate with any external insulation to the main walls of the house.


Some potential responses/strategies:

(A) If you want to do it properly, pay up and do it properly: remove the entire existing roof structure, rebuild the whole thing (including pitched elements) with insulation on the outside, and mitigate high level steel-masonry junctions (eg thermal breaks at junctions) or eliminate high-level steel-masonry junctions (eg by bringing load down to floor level). Then, ideally externally insulate lower walls and ensure a continuous layer.

(B) Go for an insulation-inside strategy; and accept that there's a compromise in doing flat roof elements this way. Pay maximum attention to creating a reliable VCL throughout. Eliminate floor-level steel-masonry junctions at external walls, or accept them and mitigate as best as possible.

(C) Some kind of compromise, with transitions between outside-of-structure and inside-of-structure insulation at multiple points, detailed as well as possible.


Strategy A is often beyond the budget. Strategy B feels like a "least bad" approach. I don't like the fiddly-ness of strategy C, but is it, in the end and pragmatically, just what has to happen in many cases?


Any thoughts appreciated!

Have you missed out the 'in-between" insulation strategy for insulation. What I have done is to insulate between the rafters and then increased the depth of the rafters by adding deep battens (50x100) horizontally spaced to accommodate plasterboard. Would this count as insulation inside or outside.

Posted By: Peter_in_HungaryHave you missed out the 'in-between" insulation strategy for insulation. What I have done is to insulate between the rafters and then increased the depth of the rafters by adding deep battens (50x100) horizontally spaced to accommodate plasterboard. Would this count as insulation inside or outside.

Do you mean battens fixed perpendicular to the rafters?

This I'd class as "inside" because there's insulation inboard of the rafters but not outboard of them.

Up to a certain thickness of insulation you can do away with battens and just fix direct through the insulation (eg using insulated plasterboard) to the rafters, minimising cold bridges via the battens and rafters.

Posted By: Peter_in_Hungary
Yes battens (if you can call 50x100 battens) fixed perpendicular to the rafters - rafters vertical, battens horizontal, this mitigates the cold bridge of the timbering. I have in one case full-filled the space between the rafters and in the other left a nominal 50mm gap under the roofing felt. The difference being the type of roofing felt in place. The insulation used in both cases was glass fiber wool so fixing without battens wouldn't work. The insulation thickness achieved was 200mm and 250mm respectively (150mm rafters).

What was your strategy for the flat roof portion?

Posted By: Peter_in_Hungary

Posted By: lineweightWhat was your strategy for the flat roof portion?

There is no flat portion of the roof. There are 2 dormers which have pitched roofs with ceilings the at same level as the flat portion of the attic room ceiling and here the insulation matches the loft insulation of the flat portion of the attic room ceiling (as opposed to the skeiling portion of the ceiling where the horizontal 50x100 battens described above are).

Ah, ok.

I guess it's not the kind of scenario I'm really thinking about for the purposes of this thread. I'm thinking of the type of loft conversion which is very common in the UK (resulting from a combination of the loft height of a typical Victorian/Edwardian house, and what planning/permitted development rules allow) which involves a substantial dormer which, generally, necessarily requires a flat roof as well as some substantial steel supporting structure. It's those two things that generate the tricky situations as far as insulation is concerned.

Secondly here's what I'll call the "insulation inside structure strategy".