I'm weighing up the options on the type of insulation to use in a warm roof construction. The roof will be made from 350mm I-joists and my original plan of using Earthwool (@0.032w/mK), full-fill, works out very expensive. I have been thinking about alternatives and am not keen on rigid boards as it will be hard to get a good air-tight fit around the periphery. How about sealing the cavities between the joists, all around, and then getting them blow-filled with EPS beads as per walls?

I can't see anything in the BBA cert about it but I shall ask the local firm that quoted for my 300mm wall fill.

Any thought welcome?

I would have thought that quilt would work out cheapest

<blockquote><cite>Posted By: ward32</cite>I'm weighing up the options on the type of insulation to use in a warm roof construction. The roof will be made from 350mm I-joists and my original plan of using Earthwool (@0.032w/mK), full-fill, works out very expensive. I have been thinking about alternatives and am not keen on rigid boards as it will be hard to get a good air-tight fit around the periphery. How about sealing the cavities between the joists, all around, and then getting them blow-filled with EPS beads as per walls?

I can't see anything in the BBA cert about it but I shall ask the local firm that quoted for my 300mm wall fill.

Any thought welcome?</blockquote>


Flat or pitched roof?

The advantage of the deep roof joist is to allow for the less efficient and cheaper stone or mineral wood. 1/3 of my roof is a deep sissor truss and I used mineral wool. The other 2/3 is a room in roof truss and I used PIR 150mm between joist and 50mm under.

I used a couple of different wools deliberately for the hands on experience. The cheapest mineral wool was crap, it compressed too easily, and was a nightmare laying the fibres got everywhere. The earthwool was not bad, could fit it without gloves etc and more or less kept its shape. The best where both Rockwool and isover Metec both of these semi rigid could compress between the josts and stayed there.

Blown bead would be interesting but I would think it would be very expensive, and might bulge at lower level, and lose through eaves.

I also had the issue on my sissor trusses that I suspect the electrican moved the insulation when he was fitting the lights, unfortunely I only have picked this up with a thermal camera, and I've no access, in time I with under sheet with 50mm of PIR.

I would use good quality mineral wood and create a service void below For all electrics.

I would also investigate the sprayed insulation, I've never used it myself but I've heard good reports, although if it is sprayed hot you might need a rigid sarking to protect your breather felt.

To clarify, it will be a pitched roof (38Deg) and the insulation will go in-between the I-Joists. I like the idea of some of the blown insulation but am not sure about sagging over time. I must get some prices for the blown insulation mentioned above.

There will be about 150mSq to insulate to a depth of at least 300mm, so using Knauf earthwool @0.032 will cost around £5400. Using 0.040 roll only costs£1050. NB. these are all just prices off the net and I would expect to getter better when I shop around. Thinking about it, I still get a U value of 0.114 using the 0.040 roll so that may be the way to go.

Although I am at a very early stage of my design process (in my head) I want a roof similar to yours with I beams and am thinking to line the inside with OSB, bubble glued joins etc to form a vapour barrier (ala Fostertom) then install batts from above (also using batts in the wall cavity so bulk buying) then either OSB or membrane, battens, tiles. Only problem I can see is getting the insulation wet if it rains before getting the outside membrane on!!!

Hi Joe, regarding your wet insulation I wouldn't worry too much if you are planning high density rockwool, it will have no problems with a bit of weather while you roof. I've had a few proper high density batts outside for a long time and they do not really seem to soak up the water or change too much at all. Wouldn't like to say how lower density ones would fare, but high density I would be happy with.

Yes CWatters, not truly a warm roof construction but a good one as far as I can see. The only addition I could see was to use 300mm Ibeams and add a additional inner timber spaced from the Ibeam to fix the inner OSB to stopping a lot of the thermal bridge and giving 400mm for insulation. (hope that makes sense without a diagram?)

I am homing in on the same construction layers: 360mm I-Joists, woodfibre sarking (breathable), battens, counter battens and tiles. In between beams: 300mm earthwool 40 roll, 50mm PU board cut to hold earthwool roll in place, 12mm OSB lining, VCL, 38mm battens (services void) and finally plasterboard. Overall U = 0.095 (excluding cold bridges).
NB. I have to use 360mm I-joists because of the span, but then it helps to be able to use cheaper insulation.

I also plan to insulate from below once the roof is complete, a bit more difficult but I intend to do most of it myself and time is not too much of an issue.

Did the same on my BCI floor joist ends using EPS to make filling in between joists easier.

i managed to fill the voids with 25mm eps that i had (it fit exactly) - took no time at all running them through a table saw and squeezing them into place.

Pic...every pic i post ends up the wrong way round..even if i rotate it before posting.

This is my BCI floor joists, but same principle if used for roof.
Black stuff on end is just end treatment to protect timber, came like it from supplier.
EPS stuck with illbruck PU 010 adhesive foam.