vaulted ceiling insulation options

domscan

looking for opinions on best insulation options for a vaulted ceiling about 120m2 area

Roof construction is 225mm rafters and a vapour membrane - no counter battening

Builder thinks spray foam is the way to go as it’s a fairly exposed site

Not sure what depth i’d need to spray or the corresponding u values (I’ve heard its not the best insulator and BER preliminarily report requires U.15 target . Builder even suggested if budget was tight (which it is) I could just spray 50mm to stop draughts and then use other insulation underneath.

Also wondering if Metac or a similar mineral wool insulation would be a good alternative and do the same job...and maybe cheaper..with an airtight membrane or vapour barrier on room side of roof.

Unfortunately using insulated plasterboard won't really work due to a gable window frame being too close to underside of rafter.

Im also unsure on whether I should leave leave a 50mm gap or not between the insulating material (spray foam or mineral wool) and current vapour barrier.

On a related point, Builder thinks soffit vents are a bad idea due to strong local wind conditions (site looks straight at the Fastnet rock, so he may have a point). He thinks air and wind will find its way through to rafters through the tiny gaps in slates so vents are unnecessary and undesirable. He is fairly old school but I guess his 40 years local experience and good reputation as a roofer counts for something.

Any views on either of these issues appreciated...

BryanF

Leave a 50mm gap between outer felt and start of insulation. Foam isn’t cheap but if your roof is on, it’s tough .... now. Metal/mineral wool won’t give you the insulation thickness. Put a intelligent vapour barrier(also air-tightness membrane) in the inside.

Your builder doesn’t seem to be paying any attention to the building regulations.

Get an arch tech or engineer on board.

domscan

thanks for your advice BryanF.
So I think you're saying 50mm gap plus 175mm of foam then a vapour barrier and plasterboard would be the best I could do here.

I do have an architect on board but he's only signing off for mortgage purposes as I'm self certifying. A structural engineer has also been involved for the main decisions so we haven't left it all up to the builder by any means

BryanF

Not sure that will meet u-value. What the arch spec?

domscan

there wasn't one on this as unfortunately his remit didn't include that level of detail..just planning and basic working drawings, structural calcs and mortgage sign off.

I guess it was for me as a self certifier to sort this out of which I obviously didn't do a very good job. So I guess I'll just have to do the best I can with what I'm left with and live with that.

Is there any point adding an insulated slab to the equation even I could only squeeze in 25-30mm?

air

Use the trays with spray foam to achieve the 50mm gap for ventilation or else fit our/pur between the rafters. This is slow and tedious but will give a good result especially if spray foamed into place. Take the hit and sort out the window so you can put at least a 50mm layer of PIR (separate ideally so you can foil tape it) inside the rafters. You need to eliminate the cold bridge through the rafters.

Skimping on the professional fees is the worst possible place to do it, especially when you don't have the requisite knowledge yourself. It'll always end up costing you more for an inferior result.
I guess you see this now.

The fact that the ceiling is vaulted makes it all the worse here as you have a much greater area of ceiling per m2 (and thus heat loss) of floor area and you'll have to maintain a higher air temperature than normal for the same comfort level as heat will rise to the apex.

At a minimum get a professional to advise on the insulation, air tightness (very important in your location), suitable vapour barrier and roof ventilation.
If your guy with local knowledge was up to speed you wouldn't be in this situation.

domscan

thanks air. mistake was the window positioning and also lack of counter battening. too much steel involved for any easy window changes..and not really in the mood for ordering a new one, although I get your point about taking the hit on this..Ill mull it over

In fairness my architect tells me he did mention leaving room for insulated slab ( don't recall mind!) so can only blame myself being swayed by builder and window fitter who thought 9 inch rafters gave enough room for insulation. Twas a bit of a snap decision at the time..

if theres any consolation for me..its that I'm used to living in fairly cold and draughty houses so not really not that hung up about comfort levels as such. It is also a relatively warm if windy part of the country in winter. Yep the heating bills will reflect the mistakes but that's the rub. Undoubtedly the wrong place to say this in a BER forum..but there we are..

Anyway lesson learned and all that..but I think ill just have to do the best I can with the space I have and live with the higher u values here. So by the sounds of it fitting PIR between the rafters would give a better result than spray foam or mineral wool in this case?

Metric Tensor

How is the architect going to sign a cert for the bank at the end saying it complies with the building regulations when it doesn't?

The 7 inches you have left after leaving your 50mm gap isn't even close to compliant. You need an insulated board. There's usually ways of doing it without having to change the actual structure of the window ope.

I hate being a "told you so type" but it will probably cost more if you fix this right than it would have to have paid a consultant to spec the whole job for you from the start! - Not getting at you OP - this is to help any future readers of the thread!

Any chance of throwing up a picture?

What's your air-tightness target?

domscan

substantial compliance Metric Tensor gives a bit of leeway I guess, at least down West Cork way thankfully
I can get a 50mm insulated board in there so will go with that I reckon
and Air tightness let's not even go there..I've done enough upsetting here:)

Metric Tensor

Haha. That's why I asked about it!

Calahonda52

Maybe think outside the box here and consider adding additional insulation over the rafters: maybe a wood fibre T&G setup which would be wind tight.

some ideas here http://www.ecologicalbuildingsystems.com/Ireland

Are you going for concrete barges?