Roof Rethink

An Eccentric Anomaly: Ed Davies's Blog

Over the last few months I've been having a little re-think on the structure of the roof - not a trivial aspect of an A-frame house.

Here's the “current” design - as in, the basis for my initial approach to the planners:

Inside to out it's:

Internally the rafters would be a “feature” with shelves and other storage space between in places and providing space for discrete routing of services in boxed in parts at the ends of rooms. The OSB would be left as is in places (e.g., in the loft and behind the thermal store), painted (living room, study, bedrooms) or covered (kitchen and bathroom).

I was originally quite happy with this design but the more I've thought about it the less I like it:

From the point of view of embodied energy it's probably best to just use mineral wool all the way through the roof. (Well, one could use sheep's wool or recycled newspaper or something but that'd be more expensive and I'd rather spend my limited funds elsewhere.) But that would result in a very thick roof which would make small windows even less effective and generally be a pain to engineer.

What I now have in mind, therefore, is a combination with mineral wool on the outside and a significantly smaller amount of PUR on the inside. This loses the internally visible rafters, which I slightly regret, and also the available space between them. It also make the outside envelope of the house slightly smaller.

Now the structure comes from I-beams a bit further out in the roof:

Now, inside to out:

The foil faced PUR does extra duty as the vapour barrier and also to provide mechanical support for the mineral wool. Inside there would, again, be different finishes (or not) on top in different parts of the house.

In practice, of course, the I-beams would need some sort of noggins between them to stop them twisting over, probably sitting on top of the hangers over the main beams in the middle. The sarking layer could be T&G OSB appropriately glued for good wind-tightness or it could be a more traditional Scottish timber-plank deck with 2 or 3 mm gaps for breathing in which case the breathable membrane would need to be well taped for wind-tightness.

The ground floor would be a similar structure though I haven't worked through the details of how that'll be made up. Ditto the gable ends though they'll be more like the roof than the floor can be.

As described above, there are a number of advantages to this scheme but, to me, the main one is the relative ease of construction. Once the posts and beams are in place the I-beams can be added progressively with the sarking layer being attached and providing rigidity in stages. The result is a house-shaped thing which is reasonably weather resistant in the short term (weeks).

Adding the breathable membrane and counter battens produces something with more long-term weather resistance (months but not a winter). After that work can go on on the inside in bad weather and the outside in good more-or-less independently. In particular, all of the insulation can be added in the dry with no pressure to get to any particular stage before the weather changes. Avoiding having to rush things seems to me to be a good strategy for getting them at least approximately right.

Update 2013-12-13: I'm interested to read this Green Building Advisor post on the effect of low temperatures increasing the conductivity of PUR (polyiso) which I'd vaguely heard of before but hadn't realised was so significant. That could apply to only one or a few brands, Scotland is not really a cold climate by the standards of the US mid-west and it may well be there's a fairly sharp cut-off temperature at which this effect kicks in (when the blowing agent starts condensing) but still it seems to support the idea of keeping the PUR on the inside part of the insulation where it'll work most effectively.