We are building a t-f garden room for a customer, and the design has evolved so much from the original dwgs that some details remain undecided. I should be particularly grateful for views on detailing of insulation in the floor (suspended timber - 225 x 50 joists with 25 x 38 battens fixed at the bottom between each pair, and 18mm ply slotted in and mastic sealed to take insulation) and the roof (shallow pitch - about 15 - 20 degrees, 225 x 50 rafters, proposed built-up felt covering, and heavy green roof on top, with up to 150mm of growing medium).

For the floor I propose to run tyvek over and between each joist so that there is a tyvek-lined joist/ply box to fill with insulation. Having taken up about 56mm with battens and ply, we have about 170 left for insulation/air gap. I think I should probably leave a gap of at least 25mm (I often say 50 but am pushed to get sufficient insulation in) below the floorboards. I hardly ever use a vapour barrier above u/fl ins, and have never had a prob. I could, however, use another layer of tyvek, so we end up with a tyvek/warmcel sandwich.

For the roof, it will be underdrawn with an insulation board (pl'bd and extr poly) of appr 50mm, plywood baffles at appr 1m centres between the rafters to prevent settlement to the eaves and help to maintain an air gap and, I reckon, a layer of tyvek over the top, appr 25mm below the top of the rafters to maintain an air gap. This should then be vented both sides at the eaves.

The walls are of 300mm spaced studs between 150 posts and these will be fully filled with warmcel.

Comments and suggestions gratefully received.

I am awful at photos, but hope to post some on the syec web-site soon.

Nick

Tony said:

''Sounds to me like you will have thermal bridging through the studs, plates, posts and noggins.''

Yes, but only partial in that, for example, the posts are 150 of a 300 insulated structure.

''Floor, if it was me I would fix the ply to the underside of the floor joists''

Can't get to them!

''and totally fill the gap with insulation. I cant work out why/where you want a 50mm gap??''

I think I agree with you but had a nagging wonder about condensation on the underside of the floor (though can't see why!) and wet warmcel. Reason for Tyvek underneath floor ins is belt and braces, again re poss of damp in warmcel.

I would use a vapour barrier on top of the floor insulation but cannot see the point of wasting money on breathable membranes in the floor.

''Roof again I cant see how you will use breather felt? ''

Visualise it stretched across between rafters inch up from the bottom of 9 inch rafters, thus leaving an air gap under the decking.


''in general detail seems ok though why not go for a warm roof construction? ''

Surely this is only poss with solid foam ins, and the customer wants the greenest poss materials.

Appreciate your input.

Nick

James said: ''Share your concerns re the soffit of the suspended-timber-floor-with-no-craw-space-detail. Warmcel is a bit prone to moisture, also airtightness and elimination of cold bridging are tricky things to solve without making the detail over fussy and too reliant on good workmanship (workmanship of others naturally sir, your own handy work is of course not a concern), thoughts at the as follows...

- Air gap: not needed the ventilated underfloor void suffices
- Tyvek: seems a good belt and braces detail, lay it nice and loose to get into all the nooks and crannies to avoid any uninsulated voids but...
- Plywood: robust soffit and pretty airtight, but doesn't it negate the breathability? or am I misunderstanding the detail?''

No, you're not misunderstanding. It is not a breathing structure as such, largely because I am relying on ply sheathing for anti-racking strength, but I have used breather membrane as 'belt and braces' in case of any leakage - better something a bit breathable than a sweaty plastic bag, was my view.

''What about...

- Suspended floor: do you need it, groundbearing slab maybe limecrete or earthen..?''

Already got sus floor, and I prefer them...


-'' If your worried about the warmcel, what about a layer of thermafleece with its renowned ability to deal with moisture?''

Cost! I reckon with the tyvek the warmcel is adequately protected, but I take your point.

-'' Instead of ply, bitumen impregnated fibre board''

See above

- Airtighness: warmside membrane ProClima etc..?
- Thermal continuity: batten (ie at 90 to joists) the topside of the floor joists or floating floor over continuous woodfibre board layer

Do you mean for avoidance of cold bridging?

''On the latter point, I had the NBT chap in the office the other week to look at my house an other bits and bobs, left lots of diffu/pavatherm samples. Its good stuff and could on your roof also to provide a thermal continuity layer. BUT... I understand it costs a fortune! However, 'can work out ok for rendered walls over the timber frame as you can render straight over the diffutherm.''

Yes, but I had some prices for the poss ext insulation of the gable wall at the Energy Centre, and it was alarming.

''How are you installing the Warmcel, blown, sprayed or DIY bags...? ''

DIY bags.

''Generally though I'd call up Warmcel and have a chat especially re the roof as they're dead helpful and will sort you out with a full U-value and condensation check calc (needed for Building regs, as no BBA cert available for between rafter in roof application), ''

Thanks. Will do.

You must come and have a look sometime, James.

Thanks all, for continued comments.

Nick