So the architect has described the wall and roof construction of our proposed new build. This what is suggested, any thoughts:-

The roof un-ventilated, using a breather membrane to let moisture out, doing away with continuously venting at the eaves.
- Slates over
- 25 x 50mm battens over,
- Breather membrane, ‘klober’ or similar, over,
- 125m deep rafters.
- 100mm Celotex Insulation between the rafters, set flush with the bottom of the rafters, void between top of
insulation and underside of felt.
- 90mm Celotex insulation under joists, joints staggered and taped
- batterns over the insulation
- foil faced plasterboard with skimmed finish
expect to achieve 0.11 W/m2K

The wall construction : -

- Rain screen - weather board or render over mesh and 25mm battens, to give a ventilation space behind.
- Framesheild breathable membrane over external Panel line or Panel vent, products designed to be breathing
boards, compared with the standard ply / OSB.
- Timber frame, 200mm thick, either timber stud or I beam construction.
- 150mm Celotex insulation fitted flush with the inner side of the stud.
- 69.5mm EPS insulated plaster board, joints staggered and tape sealed.
achieving a U-value of 0.12W/m2k

In both the roof and the wall construction the taped joints combine with the foil face of the insulation to form the Vapour Control Layer, (VCL).

I'm left scratching my head at some of the detail so please let me have your comments / experience / questions. All gratefully received as I only get one chance at getting this right !!

Edit - U vales added

I dont like the vapour control, how about a thin sheet of poly? this could then be joined to air barrier in other surfaces to do both ab and vb.

I like 0.12 but there is some designed in cold bridging at the studa and this could be bad round openings and at corners, is there any mitigation proposed at say window openings?

what about studs at the openings?

Posted By: CWattersThat's the normal reason (drape between the rafters). Otherwise insulation traps the membrane against the underside of the tile batten and there is a risk of water pooling above the batten (especially when you get a bit of dirt there). By allowing the membrane to drape the water can run down the membrane to the gutter.

Counter battens are an alternative to draping the sarking membrane between rafters. This is a better approach when using breather membrane, especially in a hybrid warm roof, because it puts the gap where you want it, i.e. above the breather membrane.

Posted By: CWattersEdit: Some membrane manufacturers discourage you from putting the membrane in contact with the insulation. Not sure why, perhaps because it somehow causes water to wick through the membrame (remember how a canvas tent behaves in the rain - if you touch it with your head it lets the water through).

This depends on the type of breather membrane. "Non-tenting" types can be used in direct contact with insulation & I wouldn't use anything else. Kingspan Nilvent & Tyvek Supro are non-tenting. Check the BBA certificate of your preferred breather membrane. If the certificate allows use in direct contact with sarking boards/insulation then it is non-tenting.

David

Posted By: tonywhat about studs at the openings?

They also get covered by the final (inner most) layer of insulation.

Posted By: hairydudeMy final consideration would be on insulated plasterboard - a cheaper option would be a continous IWI layer of EPS (or other rigid slab insulation), polythene sheet VCL, 50x50 battens then plasterboard. This allows a service gap for cabling and pipes and the gap can be largely filled with 50mm glasswool if required fir additional insulation. Ask your electrician how much he likes running cables in insulated plasterboard!

Exactly!

Paul in Montreal.

Posted By: collonachTake a look at Govici's Model D house
http://www.ads.org.uk/sust/case-studies/model-d-house

Thanks for the link. I have a lot of respect for Gokay Deveci's talents (I approached him about designing my house but the distance meant it didn't make much sense). I like the look of this house but there's a few features I don't understand from that article:

- it says there's a double stud construction (so good so far) with a layer of OSB between the studs for airtightness. Surely that breaks up the insulation, making it more difficult to install? Why not put the airtightness (and presumably racking strength) towards either the inside or outside of the build up?

- it goes on to say there's a layer of polyethylene for airtightness. It would be interesting to know why that was used in addition to the OSB.

- it says it was designed to Passivhaus and that the gross floor area is 155 m² (i.e. PH TFA is almost certainly noticeably less than 155 m²) but then it says the primary heating system is a 5 kW wood burner. That's over 32 W/m², which is triple what you might expect for a passivhaus. So I'd be interested to know what the story is there too.

- the plan in the report that is linked to shows a ramp extending out from the front door, presumably for accessibility, but I can't see the ramp in the photos and the ground appears to slope away. So I'm interested in how their accessibility solution works, since I have a similar problem to resolve.

I'm very impressed by the build cost though. That is really exceptional.

Does anybody know any more?

Posted By: djh- it says there's a double stud construction (so good so far) with a layer of OSB between the studs for airtightness. Surely that breaks up the insulation, making it more difficult to install? Why not put the airtightness (and presumably racking strength) towards either the inside or outside of the build up?

Putting it in the middle protects it from electricians, mice and other vermin.

If you're putting 95 mm solid insulation (EPS? - they don't say what insulation they're using) between the studs then having OSB in the middle makes some sense. You do one side of studs, and the OSB then work along with studs and insulation sheets on the other side squeezing them up tight with a solid structure to work against. Probably best to do that on the outside as that needs most protection against thermal bypass by wind washing.

Posted By: djh- it goes on to say there's a layer of polyethylene for airtightness. It would be interesting to know why that was used in addition to the OSB.

Yes, it would be interesting. But it could be that it's more as a vapour barrier.

Posted By: djhThat's over 32 W/m², which is triple what you might expect for a passivhaus. So I'd be interested to know what the story is there too.

But if you only burn the stove for 8 hours a day it averages out? OK, 10 W/m² is supposed to be a peak demand but putting it in the ventilation is sort of making the assumption that it's continuous. Dunno.

Posted By: djh
- it says it was designed to Passivhaus and that the gross floor area is 155 m² (i.e. PH TFA is almost certainly noticeably less than 155 m²) but then it says the primary heating system is a 5 kW wood burner. That's over 32 W/m², which is triple what you might expect for a passivhaus. So I'd be interested to know what the story is there too.

Just because the wood burner has a 5kW output doesn't mean that the heat load is 5kW. The PH criterion for heat load says that it must be less than or equal to 10W/m2, so in this case it's probably about 1.5kW, but you'll not find many stoves rated with that output. The problem they will probably have is that to run the stove lazily to avoid overheating won't give a clean burn, tarring up the flue and making lots of nasty particulates.