Our house (under construction slowly by me) has attic trusses so that we can use the roof space. The warrant (we're in Scotland) specifies that we will have insulation between the rafters and further insulation under the rafters. The intention is that it will all be super airtight.

The house will have an MHRV system in it with the ducts running in the eaves space and in the smallish space above the attic ceilings. My intention is to take the insulation and vapour barrier down to the eaves and up to he ridge. As a result these duct spaces will be insulated but not ventilated.

Does anybody foresee condensation problems in these spaces? My guess would be that they will be slightly cooler than the living space in winter and warmer in summer.

aa44,

your construction detailing sounds very like my own (Perthshire), but instead of continuing the VCL up to the ridges, I came horiz across at the flat ceiling level, at the top of the coombs.
I was wondering why you chose to continue to the ridge. Genuinely interested to learn. I didn't for the following reasons;

- the VCL does not need to be lapped around the ceiling ties
- you can use cheaper glass wool insulation on top of the flat ceiling part, rather than a much larger m2 of expensive rigid stuff between/under rafters

to get around being able to store up in the loft area, I got the joiners to put in a flase floor, just above the glass wool (say 450mm above the ceiling), with left over timbers and chip board flooring - great to get back to the few ducts/cables up there.

GreenPaddy

There's a thread on here somewhere about air tight attic hatches.

FWIW I will be taking the exact same approach as GreenPaddy.

GreenPaddy, you could market it along with the patent cat extruder device (Passivhaus cat-flap) discussed elsewhere!

Thanks for the replies.

@GreenPaddy, do you run any of the MHRV ducts above the ceiling?

aa44,

I tend to run most of the ducts in the first floor void, and only have the "must have" extract ducts above the 1st floor ceiling. The supply ducts are more critical to protect from winter cooling and summer heating, as that's what gets dumped directly to the room. Of course losing any heat from the extract ducts will reduce the heat exchange performance.

So I try to run the least amout of extract ducts between the rafters, so you get max. insulation cover with the glass wool. It is VITAL that attention is paid to these ducts being very carefully insulated.

Davidfreeborough - I suspect that taking the membrane around the ceiling tie part of the tusses will be more work than a few duct/lighting cable penetrations. By the time you've wrapped around the truss tails and the truss-hanging posts, you absolutely will not want to start again at the ceiling ties - been there, got the rope burns on my neck to prove it.

It's why I've boarded the Vikinghouse/Fostertom express to OSB heaven, a land with no membrane type internal air barriers (unless the client specifies a build type that necessitates it).

But each to their own, as I firmly believe there is not one right solution to designing/building a house. Listen to the options, understand them and the consequences fully, and choose the one that suits your particular case.

Not quite sure how my roof fits in to that. I am using 22 x 150 traditional softwood sarking boards with a tiny gap between each board to provide the roof ventilation. There is then a gap below the sarking boards to the top of the insulation that sits between the rafters.