Roof detail - how to get a continuous insulation layer?

Price Check

Ive been lurking around here reading about raft foundations poured straight onto and surounded by a structural EPS layer, which I asume could be met with an external insulation layer on the walls - say you used a glue-on and rendered system like rockwool rockshield, the one that doesnt require mechanical fixings, then so far so good, you have a continuous insulation layer under and around the house with no thermal bridging.

As a theoretical excercise, could you continue this layer over/under the roof structure continuously in some way? I thought Id read somewhere that you could fix a solid insulation over the rafters (assuming they were clad in OSB or some such) and then what? battens over that to take the tiles? could the battens be fixed through the insulation? Could you still have an overhang that way? A free standing roof construction maybe as part of a verrandah with the insulation inside the rafters?

Any thoughts anyone, Id love to know if there was a way to totally wrap the house, preferably with no/very few fixings or bridging of any kind...

Slig

Depends greatly on your wall construction.
Your common 300mm wall construction leaves lots of cold bridges at GL and eaves because the insulation is sandwiched between the structure.
Something like ICF with a alrger insualtion outer leaf leaves this alot easier but it leaves a very thermally light structure and because it is EPS it doesnt breathe.
I have been toying with the idea of this ICF system combined with a deep roof construction insulated with a breathable rigid mineral fibre insualtion, and perimeter insulated raft foundation. I reckon if the internal walls are blockwork they should provide enough of a thermal store

topcatcbr

Price Check wrote: »

Ive been lurking around here reading about raft foundations poured straight onto and surounded by a structural EPS layer, which I asume could be met with an external insulation layer on the walls - say you used a glue-on and rendered system like rockwool rockshield, the one that doesnt require mechanical fixings, then so far so good, you have a continuous insulation layer under and around the house with no thermal bridging.

As a theoretical excercise, could you continue this layer over/under the roof structure continuously in some way? I thought Id read somewhere that you could fix a solid insulation over the rafters (assuming they were clad in OSB or some such) and then what? battens over that to take the tiles? could the battens be fixed through the insulation? Could you still have an overhang that way? A free standing roof construction maybe as part of a verrandah with the insulation inside the rafters?

Any thoughts anyone, Id love to know if there was a way to totally wrap the house, preferably with no/very few fixings or bridging of any kind...

Look up warm deck roof construction.

KS have a good diagram of this

Price Check

Slig wrote: »

Depends greatly on your wall construction.
Your common 300mm wall construction leaves lots of cold bridges at GL and eaves because the insulation is sandwiched between the structure.
Something like ICF with a alrger insualtion outer leaf leaves this alot easier but it leaves a very thermally light structure and because it is EPS it doesnt breathe.
I have been toying with the idea of this ICF system combined with a deep roof construction insulated with a breathable rigid mineral fibre insualtion, and perimeter insulated raft foundation. I reckon if the internal walls are blockwork they should provide enough of a thermal store

I was thinking more along the lines of:

EPS under slab - meeting directly with
EPS around edge of slab - meeting directly with
Rockwool rockshield on outside of walls (breathable-ish)

But then can you get this to meet directly with (breathable rockwool) insulation over/under/in the roof structure with no bridging, i.e. a totally continuous coating of insulation around the whole conditioned space of the house?

(p.s. was thinking of using a TF structure with fermacell inside surfaces so there would be moderate thermal mass/decrement delay when you take into account the dense rockwool, slab, and fermacell internal walls)

topcatcbr wrote: »

Look up warm deck roof construction.

KS have a good diagram of this

Thanks, will do, this involves fixings through the insulation right? Can it also be used on a (slightly) pitched roof?

And who are KS?

Cheers!

topcatcbr

Ill PM you as i dont want to run foul of mods. They dont like brand placement.

Price Check

Cheers for that, I see you could go for over rafter or over and between rafter, and could of course bring the external insulation from the wall directly up to meet this, surrounding the rafters as they meet the wall. This still leaves a slight cold bridge through the uninsulated overhanging rafter portion into the wall insulation and along through the rafters...

I suppose you could have the rafters terminate directly inline with the top edge of the wall, i.e. straight from vertical to the pitch of the roof with no overhang whatsoever, just straight from external wall insulation to over rafter insulation. This would result in a complete envelope with zero bridging, but overhangs are handy things.

Could you cantilever the counterbattens over the edge to create an overhang or would this be too flimsy? I suppose it would...

sas

An acquaintance of mine is building a certified passive house. It is being designed directly with the PHI and they are approving every detail.

The house spec is very similar to what you have described. He is importing a PHI certified insulated raft which is being fitted in the next month.
The walls are 4 inch solid blocks on the flat externally insulated with 250mm white EPS, again a PHI certified system which is readily available in Ireland.

The roof will be insulated on the slope using one of those rigid boards which adjusts to small variations between rafters and also underneath the rafters another layer of insulation will be placed. This is satisfying the PHPP for a house over 3000sq ft. A certain amount of bridging (by timber elements) is acceptable even in a passive house.

Some of the ICF companies produce a roofing panel which is effectively a thick tongued and grooved board of EPS (e.g 25cm ) which has steel Z profiles embedded within them to carry the weight. They could be an option also but are very pricey.