How would you insulate the slope of a pitched roof with 400mm deep rafters? I have a possible roof design for my attic room using trusses that in effect create an extra deep rafter (without the weight if it was solid timber). Now I am wondering f this makes the insulation job easier or harder! Looking at an unventilated roof (permeable membrane, battens and counter battens, natural slate on top) so no need for gaps, and over a habitable attic room so need a good VCL in there too.

Plenty of between rafter space to stuff with something.... Could fill it with cheap (0.044 W/mK) loft roll and still get a space heading towards a u value of 0.11 (not quite that because of the truss timber contribution), but how to keep the fluff up there before fitting the VCL and plasterboard? Could stick 400mm of PIR board in there, but it would be nice to save some build costs at this stage by using a cheaper material. Mineral wool bats perhaps? Pumped cellulose or foam?

How can I insulate well and economically given the depth of this between "rafter" space?

I was asking myself the same question about a similar construction: 250 mm OSB I-beams with mineral wool between and 100 mm or so of PIR inboard. How to hold up the mineral wool until the PIR is in place? What I thought was sticks (e.g., short bits of batten material) wedged across the I-beam flanges until sheets are added below. Pull out as many as practical as the PIR is added. Not ideal but not the end of the world if one or two get left behind. Maybe even tie bits of string to the ends of the the sticks to make them possible to pull out. Could that be adapted to your trusses?

Posted By: PeterStarckIcynene or Knauf Supafil Frame would work but not cheap, but then are there any cheap loft rolls any more?

Works out about £3600 for Icynene to be sprayed, compared to £1300 for "Supaloft" (the recycled plastic stuff) plus fitting costs, have yet to do the numbers for 27m^3 of other materials. But the Icynene will be nice and air tight in every nook and cranny, while the Supaloft will be lots of plastic dangling around my head!

If I could get a cheap loft roll, how could I install it between the rafters?

Posted By: Chris P BaconThe resulting void was pumped with loose rockwool, this gives a calculated u-value of 0.082

Yummy u value! Did each gap between rafters have to be pumped separately to avoid air pockets?

Out of interest what was your truss structure? Any pictures?

I always fix the ceiling first, insulate next, then wiring then fix the deck, have lunch and waterproof the deck in the afternoon usually permanently sometimes just felt and battens or poly.

Posted By: tonyI always fix the ceiling first, insulate next, then wiring then fix the deck, have lunch…

And there was me hoping to get the whole house built before elevenses.

Posted By: barneyFailing that, weed membrane is OK

What I used in my loft. Just stapled it in place.

But might settle

But might settle

That's what put me off Warmcel. Also the lack of DIY install (in the UK).

Netlon do a specific product to support batts / quilt:
http://www.tdpltd.com/netlon-products/insulation-support-net.html
Or you could use the green stuff from the garden centre.

I assume you are at 400 or 600mm rafter spacings. Rockwool flexi just stays where you put it. The inner rafter flange helps keep it in. It's fairly resilient stuff. Decent lambda value too, 140mm is 0.035.
I've done this job and it works.

this time with the photos

Thanks tychwarel - that's more or less what I have in mind. I'd hoped the rockwool would mostly wedge in OK; my sticks suggestion above was really as a fallback for if it didn't. Useful to know.

If you've decided on 400 mm depth for structural reasons it seems awfully unlikely that spending much more to get the U value down below 0.11 W/m²·K will make much sense.

It's a bit harder to decide when you've more freedom on the depth. The direct cost of insulation value for the more expensive materials is worse but the saving in structure around them needs to be balanced against that. Not easy when it's difficult to work out what the cost of the materials will be - one of many downsides to the lack of transparency in building pricing.

Wind tightness - I am assuming that the fully tape sealed vapour permeable roof membrane will block the wind, but will have to take care at the eaves. A piece of insulation board glued in there for that perhaps?

I have poked about with some loft roll, it really is a "low density/variable quality mineral wool product" as David says. I now have concerns that it is so soft that stacking 2 x 200mm layers, the weight of top one will compress the bottom, or the whole lot shift down the slope over time leaving a gap at the top hidden behind the plasterboard. Going to take a closer look at product density and compression figures if I can find them.

The "rafter" and "timber frame" variants do seem to offer comparable thermal conductivity for wildly different prices. I get why they are more than loft roll, but do they differ from each other other than the target market?

The 400mm is for structural reasons Ed, but also offers an opportunity to hit better u values. Putting 500mm of (0.04) wool in the ceiling of the non-habitable loft parts, so it gives the space to match it in the attic. The remaining weak design is the flat roof (bit of everything in this build) that I have discussed that in another thread.

I do wish the cost of materials was more available, I will post what I find out in case it is of use to others