Loft insulation in timber frame property

[HXSparky]

We now have a new timber frame property erected, and trying to work out how best to install loft insulation in the (cold) roof space.

It's a 1.5 storey building, and the roof trusses have been constructed as "Howe trusses" (per attached example image, please ignore the measurements). There's obviously room to fit loft insulation between each joist on each truss, but trying to lay a 90 degree layer above that to get the insulation to 270-300mm will obviously be problematic.

Any advice would be very welcome!

 

[John D v2.0]

You're not aiming for a depth you're aiming for a u value. What do the plans say? Presumably approved by building control?
I would assume you put the full depth between the joists and then either a further layer on top or counter batten the underside and insulate underneath before plasterboard. But someone would need to calculate which way works and meets your needs too.
I assume that diagram is just an example as it seems to give the dimensions in inches rather than mm.

 

[HXSparky]

The plans say 0.16 W/m2/K with 300mm of rockwool roll, and 270mm would give a u-value of 0.16 according to Rockwool's online calculator. The architect plans state 300mm. To achieve 0.16 above and below, I'd need either 80mm boards between the rafters and 60mm below, or 100mm between and 50mm below.

What I forgot to say was that we're planning to drop the ceiling (it's 2.6m at the moment) down to standard height so we can fit downlights and run other services under the insulation (and vapour control layer). What I can't get my head round is how I'd securely fix the frame to the rafters above (through the insulation) without compromising the vapour control layer?

And yes, the attached diagram was just an example. I'm visiting the site today, so I'll take some pictures and post them later.

 

[John D v2.0]

300mm of rockwool where though? It's the designer's job to specify something that can be built, they are taking the risk on the design so if you go off piste they don't have any liability. And i don't have any liability for obvious reasons!
You can use celotex if you're rich, but wool is easier to fit in irregular spaces anyway. If you're dropping the ceiling then that's no problem, I'm not a builder but i did a drop ceiling in our ensuite and i would counter batten anyway, add wool from above and staple the vcl underneath. The vcl doesn't have to be perfect that's taken into account with the condensation risk analysis but a few screws and staples through it won't let anything through anyway.
Then you can add the hangers or whatever for your suspended ceiling.
However the designer should either use standard details or do a condensation risk analysis on the proposed design to show which elements are needed, rather than guessing like i am doing.

 

[^woody^]

Lay insulation at least the depth of the truss chord between the trusses, and whatever it is above to make up 300mm. What's difficult with that?

 

[HXSparky]

Maybe I'm over-complicating things by trying to be "neat" with wool insulation. The attached photo shows the trusses, which are 600mm apart with a 3500mm total span. The truss beams are 75mm high, so I guess I can lay 75mm between the trusses (on 400mm cross battens, or do I need to go closer than that?), and then 200mm at right angles (or even 100mm and 150mm to achieve the target 300mm)? 3 layers might make it easier to work around the truss junctions?

In terms of the design, we've parted company with our architect, but he specified the 300mm (as two 150mm rolls at right angles) before he'd seen the detail that the timber frame company had produced.

 

[John D v2.0]

If they are only 75mm then do a layer of 100 and a layer of 200, they are both readily available. You can use stapled mesh to support the insulation and then staple your vcl against the rafters directly. Then you can make whatever holes you like in the plasterboard as it's all inside the heated envelope.

 

[^woody^]

Just throw 300mm in or more if you like. 100 between, 200 over. It does not need to be at right angles to the first layer.