Roof Insulation and Air Tightness Question

Out Of Order

Hi all, I'm looking for some opinions from the good people on this forum with regard to insulating and air-tightening the roof of a new build.

I am building a dormer bungalow with 4" blocks on flat and external wall insulation. My roof is a 45 degree A-frame with gables and three velux windows.
My initial spec was for a roof with 9" rafters and standard felt.
Rafters to be filled with 7" of PIR board leaving a 2" gap for ventilation.
Then slabbing underneath with 72.5 mm insulated plasterboard. U-value of this buildup is 0.12 which is good enough for me.
It should be easy to meet the EWI with the insulation board at the eaves so cold bridging is eliminated. My concern is with air-tightness.

Should an air-tight membrane be installed between the rafters and the insulated slabs?

Would it be more economical to use breathable felt and fill the 9" rafter completly?

I see some people here using softboard over a 9" rafter pumped with cellulose, and either an air-tight membrane or taped OSB on the inside.
In some cases a 100mm counter batton inside the rafter to allow for greater thickness of cellulose. Is this easily achieveable on site? Does the softboard make this a far more expensive option, albeit far more sustainable? Are people happy with the result?

I don't understand the need for a service cavity unless downlighters are being put on the slope? Sockets etc will be on the knee wall and lighting cable can be run along the top attic space.


Then theres the major issue of implementing air-tightness at the eaves.
Putting joists on the top course of blockwork as normal would be almost impossible to make air-tight.
I was thinking of adding two courses of blocks, lapping membrane over them, and then installing the joists on hangers. The membrane will hang below the joist and can be fixed to the blockwork with expanded metal, then plastered over. The membrane on top is simply taped to the membrane/OSB on the rafter. Is this possible? Any ideas on how to manage the gable wall?

Any input is appreciated, thanks.

soldsold

Out Of Order wrote: »

Should an air-tight membrane be installed between the rafters and the insulated slabs?

Would it be more economical to use breathable felt and fill the 9" rafter completly?

You will need some airtight membrane/ board inside the rafters in either case.
15mm OSB 3 board 14.50 euro inc vat per 8x4 sheet or 2.88m2.
Airtight membrane 155 euro inc vat for 75m2 (I think this is the correct membrane roll size)

Out Of Order wrote: »

I see some people here using softboard over a 9" rafter pumped with cellulose, and either an air-tight membrane or taped OSB on the inside.
In some cases a 100mm counter batton inside the rafter to allow for greater thickness of cellulose. Is this easily achieveable on site? Does the softboard make this a far more expensive option, albeit far more sustainable? Are people happy with the result?

Softboard 20 euro per m2 for 60mm thick
22mm thick is a cheaper version
60mm thick uses very expensive screws as the screws need to get through battens and counter battens then 60mm softboard and into the rafters a decent depth (40-60 cent per screw so could be 3-4 euro per m2 just for screws)
Cellulose pumping approx 18 euro per m2 for 225mm rafter cash or + vat
15mm OSB 3 inside adds racking strength so may allow 600mm centres as in my build.
100mm counter batten is made from 2 x 50mm battens, 2.35 inc vat per 16 ft length. Easy to put up, can put in cheap glass wool. Only necessary for downlighters but no harm to add anyway as the screws are 4x70mm and are cheap. I used a paslode nailgun to tack the OSB and the service cavity battens into position, then screwed to hold tight. Did all this before electricians come on site. Warn your electricians from the start not to touch your OSB/ membrane/ tape/ hollowcore wrap, they will anyway, for example when they put in window contact alarm cable, and to run chase walls conduit inside the hollowcore wrapping membrane, etc.
Borrow or buy a cross cut saw with a track to give neat, fast cuts on the OSB boards, rather than tracing a line and trying to follow it. (eg festool track saw).
Softboard is expensive (20 euro per m2) but for a dormer I think its well worth it. My attic has not completed airtightness as the eaves are all still open, and has not been pumped with cellulose yet but during the really hot spell we had heating on in the evenings while everyone else was finding it hard to sleep, the softboard really does seem to be a barrier to heat getting into the house, but I will report when fully done how it works. Once the insulation and airtightness is done Im convinced it will be a brilliant way to keep the heat out, Im sold on dense insulation such as cellulose and softboard vs EPS and PIR boards.
Windtightness is also added for free when using softboard, less condensation risk, removes thermal bridge from roof rafters, some extra racking strength, makes house watertight early in the build. But the price is hard to swallow, so maybe 22mm is the sweet spot. Personally if I had (hypothetically) the choice to switch from 60mm to 22 mm now and take the savings on softboard and long screws, Id definitely still stick with the 60mm boards. But I wouldnt go over 60mm.

OSB is easy to put up if the walls are straight and you plan ahead, time consuming otherwise. I'll cover this in more detail below.

Out Of Order wrote: »

I don't understand the need for a service cavity unless downlighters are being put on the slope? Sockets etc will be on the knee wall and lighting cable can be run along the top attic space.

Exactly my thinking, dormer rooms with slopes have 2 x 50mm battens making a 100mm service cavity, but only on the slope except where no other services such as HRV pipework or central vacuum system pipes need to run.

Sockets on knee wall and downlighters and other wiring go at the top as you suggest.

I would still put in the service cavity though on all sloping parts of the roof if you can allow the space, of course not on flat ceiling parts as you can just roll out more glass wool insulation on top. I can spare the space in some areas such as under the eaves but not others, the extra thermal break inside the rafters would be worth the small service cavity cost, however bear in mind Im doing the OSB, service cavity and airtightness myself over a few weeks with help (and free loan of power tools) from a friend/ chippy so the cost is fairly low. At a wild guess, allow for airtightness, OSB and service battens around 20 minutes per m2 plus half an hour per window for airtightness taping for a dormer roof.

Maybe OSB 3 for open areas with big sheets, and intelligent membrane for tricky corner areas is the best bang for the buck?

How I taped the house:

Moy vario multitape on any clean, neat OSB joints as its cheaper and easy to use, doesnt stick to your fingers as much as the SIGA/ Pro Clima tapes - maybe this is a measure of its long term life but Im happy enough it will last, I rolled it hard with an ink roller until I could see the pattern of the OSB coming through (like how you see the trace of a coin showing through paper as you fill in with a pencil)

Siga Rissan for OSB joints wider than 5mm, I just trust the SIGA tape more as its much stickier and no signs of it coming off (the vario tape has come away from some non planed rafter timber) but it is slower to apply.

Pro Clima tescon Profile for windows/ Velux, the backing tape splits in two so easy to apply and seems very good.

Contiga pro clima tape for joints between OSB and block walls, you just plaster over it. Or in some cases a strip of solitex plus, taped to the OSB and 4 inch expanded metal lath nailed to the blockwork for plastering over.

To make the eaves airtight on the old part of my house that has timber ceiling joists:

SIGA primur, tescon profile and siga double sided tape (cant remember the name) at eaves, see a bit about this below for details.

Note: Cut the OSB boards with the rough side facing up so when you turn it around it has a clean edge on the smooth side for taping, otherwise you have dust and sharp edges to clean off.

Use a nail gun to tack the boards up, much faster than screwing them up, you can tape over the nail holes if you are concerned about air leaks or else finish the board with screws rather than nails.

Before putting up any stud walls at external perimeter try to get OSB boards in place over the studs, or an airtightness membrane, then put the studs up. Saves a lot of taping afterwards. In my case I rolled out a strip of solitex plus on top of a couple of stud walls, easy to tape to the OSB afterwards.

Its easier to get a good rolled joint on OSB compared to membranes, but membranes can be cut with a knife, you dont need a crosscut saw!. Also holding a membrane overhead is less work than the same job with an 8x4 OSB sheet.

I felt like exploding a couple of times, luckily I didnt as my pockets were full of nails and screws and I would have torn the whole road to shreds If I exploded.

Out Of Order wrote: »

Then theres the major issue of implementing air-tightness at the eaves.
Putting joists on the top course of blockwork as normal would be almost impossible to make air-tight.
I was thinking of adding two courses of blocks, lapping membrane over them, and then installing the joists on hangers. The membrane will hang below the joist and can be fixed to the blockwork with expanded metal, then plastered over. The membrane on top is simply taped to the membrane/OSB on the rafter. Is this possible? Any ideas on how to manage the gable wall?

This seems a good plan, anything you can do to get a membrane in place before adding timber will help make life easier. The denby dale passive house blog (google building.co.uk, denby dale, bill butcher) has details of hanging rafters off a horizontal timber. probably a 7x2 or so, worth a read.

Solid concrete slab with wrapped solitex plus membrane around it is the only real answer, and the prices are very low at the moment, Ive heard quotes of 21 euro per m2 plus vat for 120mm thick solid slab.

My dormer renovation has ground floor ceiling joists unfortunately so Im doing the following (see my 3.5 glasses of wine sketch attached):

1) Let OSB boards drop down as far as floor joists so they lie flat on top of floor joists
2) Cut rectangles of OSB board to fit between the joists to close the gap, about 450mm long vertically and about 5-10mm narrower than the space they have to fill (eg 370mm space between joists needs a 360mm wide x 450mm vertical OSB rectangle)
3) Cut an angled cut on the bottom of this piece where it touches the ground floor ceiling wallplate/ plasterboard, to match the angle of your roof (in my case 35 degrees). This gives a bigger surface area to work with and the bottom of the rectangle is flat with the wallplate/ plasterboard. Try to use a skillsaw on a track to get a 100% perfect angle
4) My plan (over weekend) is to apply SIGA Primur roll (6mm thick gel about 8mm wide) to the bottom of the angled cut (horizontal) and this will bond to the wallplate/ plasterboards. I'll attach tescon profile tape to the OSB rectangles at each side (ie tape runs vertically with the inside 10mm stuck to the sheet and rolled tight, rest still has backing tape on it. Then I'll apply (horizontally) double sided SIGA tape to the OSB rectangle higher up just where it overlaps the main OSB sheet, so when I screw the OSB rectangle onto the main OSB sheet its automatically airtight between them. I'll leave the backing tape on this double sided tape too. So now I have Siga double sided tape horizontally aboout 2/3 way up, tescon profile on both sides with the flaps still having backing tape, SIGA primur mastic roll at the very bottom. I'll slide it down into place so the primur roll grips, then angle the rectangular board forward and remove the double sided tape's backing tape and screw the rectangle to the main OSB board, finally remove the backing tape from the tescon profile and tape the sides tight.

My way sounds complex but the tapes are applied before getting into the eaves so a lot easier than working in tight spaces and can be secured properly, it should be a fast job and give a good seal. Bob's your uncle and Fanny is your aunt.

Any easier method anyone? Can SIGA primur stick to plasterboard at the wallplate or do I need a primer/ another plan?

Steve

soldsold

To answer my own question, primur does stick to plasterboard.

Any comments on my plan for air tightening the eaves? I've the strips cut and tape ready, hoping to do the job this afternoon.

S

Out Of Order

Thank you soldsold for that excellent reply.
That is exactly the kind of information I was looking for.

You have me seriously considering the softboard now.
I think the 22mm board at about €10 per square meter is about all my budget will allow for though. That and 225 mm of cellulose should give a u-value of 0.14 with either membrane or OSB beneath.

Re:the eaves, I've looked at the Denby Dale house which used joist hangers for the first floor junction, but it is a two storey house with a truss roof so the detail is different.


I have attached a drawing with three options for this detail that I would like some feedback on:

Option 1: The joist sits on the wall and is cut a 45 degrees. The rafter is fixed to the wall plate and is nailed at an angle to the joist.

Option 2 is what I was describing in my original post. I've have found pictures which sum it up (attached). I think this would only work on a truss however as the rafter does not actually join to the joist, which is required for triangulation on a dormer roof to prevent the rafters from spreading.

Option 3 is what my roofer suggested which uses two wall plates. One connects the wall to the joists and the other connects the joists to the rafters.


The gable should be relatively easy afterall. Just tape the OSB/membrane where it meets the block from the inside before plastering.

sydthebeat

i dont get the joist wall detail...~??

is the membrane punctured at all the joist hanger locations???

soldsold

It looks like the hangers are just using the weight of the blocks above them to hold them in position with the membrane folded over a horizontal bar at the top of the hangers? Maybe nails are used but if not is this a structurally sound thing to do?

sydthebeat

i thionk i can make out whats happening... the membrane looks as if its actually folded into the cavity to follow the extreme dimensions of the hanger ie folded the probable 50mm down the inner face of the cavity and 100mm in the block course.

interesting solution, and way better than trying to tape joist ends....

soldsold

@ out of order:

Will this joist detail suit your build? it looks like it is for a two storey house?

Out Of Order

sydthebeat wrote: »

i dont get the joist wall detail...~??

is the membrane punctured at all the joist hanger locations???

That's what I'm trying to avoid, while also maintaining structural integrity.

There is actually no cavity, it's block on flat.

My idea is to wrap a strip of membrane, maybe 700 wide, over the top of the wall. Then place the joist hangers over the membrane and nail from the top through to the block. Then hang the joists into the joist hangers and nail from the side. The wall plate is placed beside or above the joist hanger and the rafter is birdsmouthed and installed as usual. The membrane continues along the side of the wall plate and up the rafter where it can be taped to the mebrance/OSB on the slope.

@ soldsold:
I'm not sure
The pdf above shows the only three options that I can think of.
I will be meeting my engineer this evening to discuss them but he has not done this before so structural integrity is his main concern.

I've spoken to an airtightness membrane supplier but their solution was to run membrane underneath the joists and up the knee wall. I don't know how they propose to bring it up through the joist and this would also mean puncturing the membrane for every cable, pipe, etc.
I don't think this is feasible.


I was hoping that someone might have tackled this before and maybe came up with the optimal solution?

Out Of Order

OK, I think I have found a good solution on page 17 of this document here.

The roof structure is built as normal.
A line of bridging pieces are inserted between the joists, just out enough from the eaves to make access easy.
Air tight membrane is taped all along the underside of the bridging, is stapled into the corner, and hangs 25 mm down the wall, which is then plastered.
Mastic joins the sides of the bridging to the joists.
Air-tight membrane/vapour barrier is taped to the top of the bridging, pushed loosely into the eaves and stapled up along the rafters as normal.

This is done as soon as the rafters are put up.

Then insulate between the rafters from above with 225 mm of loftroll 32 giving a u-value of 0.18 (allowing for 10% thermal bridging with rafters at 400 centers).
Breatheable membrane over this with counter battons for ventilation and then tiling battens.

A 50 mm service cavity can be put underneath the rafters which would reduce the u-value to around 0.14 if insulated with rocksilk batts.

I like the use of mineral wool as it's environmentally sound and it remains effective despite any movement/shrinkage that may occur with the timber.
Fitting the insulation all along the rafters makes my small attic spaces warm, so the water tank is in no danger of freezing and my HRV pipework doesn't need to be insulated.
Fitting the insulation from the top also makes it easy to install it correctly into every corner.

Any thoughts on this?


This picture shows a similar detail but the additional 50 mm of insulation is placed over the rafters.
Can anyone tell me what the pros and cons of insulating over vs under the rafters are?
Installing it over would mean a little more internal space but you would lose the service cavity and there may be a danger that the extra wide fascia would require a joint?

L driver

Out of order,
If you use the softboard roof make up, this being air tight, why not continue this line down about the fascia and soffit to meet the block on flat. Then give a light coat of the insulation adhesive to the blocks. Job is air tight and the trades won't drive you mad with puncture holes.