any smart ideas on simplest way to insulate my roof

[Big Jimbo]

Morning guys and girls. having a bit of a mare, working out my insulation. Floor, no problem, External walls, no problem, but then i come to my roof. Shallow pitch on all sides leading upto a flat top. Pitch aprox 25 to 30 degrees. Trusses at 600 centres. As im going to all the effort of doing everything i can to make my walls airtight, with a membrane, and a service gap, i'd like to try and make my ceiling on the first floor as good if possible. leaving the roof space cold. That way i can run a vcl at ceiling level, tying it into the external walls, and creating my airtightish bubble. The total height of the roof space will only be about 1400mm high, and being triangulated trusses, there is no way i'm going to be able to get up there to insulate after , or no chance of getting a membrane around the inside of the trusses, due to all the triangulation timbers.

So i am looking at insulating at ceiling level. and though about some sort of fluffy stuff between the rafters, and continuous layer of PIR under. Then battens to provide a service gap for wires, plasterboard, and skim. The problem is the u values i am getting are pants....Any smart ideas ?

[SteamyTea]

Make up some breathable 'sausages' from garden weed control fabric, fill with mineral wool insulation, then staple to rafters.

 

[gc100]

1 hour ago, Big Jimbo said:

Morning guys and girls. having a bit of a mare, working out my insulation. Floor, no problem, External walls, no problem, but then i come to my roof. Shallow pitch on all sides leading upto a flat top. Pitch aprox 25 to 30 degrees. Trusses at 600 centres. As im going to all the effort of doing everything i can to make my walls airtight, with a membrane, and a service gap, i'd like to try and make my ceiling on the first floor as good if possible. leaving the roof space cold. That way i can run a vcl at ceiling level, tying it into the external walls, and creating my airtightish bubble. The total height of the roof space will only be about 1400mm high, and being triangulated trusses, there is no way i'm going to be able to get up there to insulate after , or no chance of getting a membrane around the inside of the trusses, due to all the triangulation timbers.

So i am looking at insulating at ceiling level. and though about some sort of fluffy stuff between the rafters, and continuous layer of PIR under. Then battens to provide a service gap for wires, plasterboard, and skim. The problem is the u values i am getting are pants....Any smart ideas ?

 

My roof is 19 degree pitch with zero roof space (ie its vaulted). We ended up with PIR inbetween the rafters, AVC, batterns for services/lights and then plaster. We had an Ok u-value and the house if anything is too hot so works well. Why not put PIR in between the rafters rather than 'fluffy' stuff?

 

[Russell griffiths]

I have 200mm of batts between rafters and 75 mm pir underneath all joined together with squirty foam and joins taped to form my air tight layer, foamed to walls and taped between walls and pir. 

 

Not as as good as some with u value, but airtight no gaps in insulation detailing absolutely spot on as I did it. 

So some may have a better u value but badly executed by the builder. 

 

[A_L]

1 hour ago, Big Jimbo said:

The problem is the u values i am getting are pants....Any smart ideas ?

 

A metallised breather membrane above the battens could add 0.6m²K/W (about 15mm of PIR or 25mm of loft insulation) and taped will provide air tightness. A liquid applied air barrier with a vapour permeability appropriate to its position applied to continuous sheet layer could provide air tightness.

[Big Jimbo]

Just a thought. What about if i put a vcl layer under my ceiling layer (rafters) a service batten, (for wires etc) then plasterboard and skim. Then insulates my sloping hipped sides. with like 150mm in between, and 50mm over ? would this make my roof a "Warm" roof ? Would having my vcl layer just above my ceiling be a problem? I think i read somewhere that for a warm roof whatever insulation is outside the envelope has to be the same as whats inbetween. (ie; 50mm outside, 50mm between etc ???)

Thanks for your help.

[A_L]

57 minutes ago, Big Jimbo said:

Just a thought. What about if i put a vcl layer under my ceiling layer (rafters) a service batten, (for wires etc) then plasterboard and skim. Then insulates my sloping hipped sides. with like 150mm in between, and 50mm over ? would this make my roof a "Warm" roof ? Would having my vcl layer just above my ceiling be a problem?

 

1) No this is where a VCL should be.

2) The hipped areas would have warm roof characteristics but since you have other areas not really

 

1 hour ago, Big Jimbo said:

i read somewhere that for a warm roof whatever insulation is outside the envelope has to be the same as whats inbetween. (ie; 50mm outside, 50mm between etc ???)

I cannot see why, but it should be insulating enough to prevent interstitial condensation in the internal layer, but it should not be less vapour permeable unless very carefully designed

[Big Jimbo]

I have worked out that Kingspan K7 100mm above rafter, and 100mm inbetween would give me 0.1. However, it would be a right pain in the butt to do. An alternative will be leaving the roof cold, and putting 150mm of warmcell between the ceiling rafters, and 300mm of warmcell on top of that (total 450mm) with a vcl under the ceiling rafters, a 50mm service batten, 12.5mm plasterboard and skim would give me a "u" value of 0.09. I would just have to make sure that i left the eaves open (Not to hard) to allow the roof to breath. (ie; cold above the insulation.

It will be interesting to see the price difference, but given the price of K7 and the hassle, i think it might be the way to go.