Foil Insulation for attic

[Jake]

Completely under leaves the whole depth of the rafter just as ventilated as it is with insulation over the ceiling joists.

Replying to myself to expand what I mean - if the roofing has been done correctly, there is air gap maintained at the eaves under the 'felt' to blow into the attic. That's still there, it's just now channelled through the gap between the rafters. In a 'between and below' scheme, that air gap gets narrowed to the 50mm or so left above the 'between' layer to allow that air to flow. In a 'below' only scheme the air flow ventilates the full depth of the rafter as well as the cold face of the insulation.

Thinking back on what I did in my own attic, I cross battened before insulating below to make sure the air would have cross flow to avoid trapped air around jack rafters etc. Made it even more internal space inefficient (which is obviously the driver of 'between and below') but its only intended as a storage space.

 

[Jake]

Applying that to the OP scheme, the foil insulation will trap the roof ventilation air above and between the rafters so the attic will no longer be ventilated. The difference is that the main insulation layer will still be at ceiling joist level, and the cold side of that will not now be ventilated. Might get away with that but it is a risk.

 

[Agent_zed]

My (uneducated) take on this -

If you want to just stop heat loss from the house below, you need to insulate between the floor beams (and over the top of them), ideally ~300mm. Glass fibre is cheap and relatively env friendly as it's recycled glass and won't be a problem in landfill etc. If you fit celotex etc it will be about 2x the insulation of glass fibre so 100mm will be about 200mm of glass fibre. But celotex is not nice stuff environmentally speaking.

The multifoil seems to be similar values to celotex but requires being sealed well. I was looking at using multifoil breather membrane which would not have the damp problem as it can allow vapour to move through. Environmentally it is probably better than celotex but worse than glass fibre.

If you put insulation on/in the rafters then the attic space will still be taking heat from the downstairs until it reaches an equalibrium. I.e the attic space will be warmer as heat will still be coming from below.

To reach building code from the multifoil website it seems you need 3 layers for 100mm rafters. One over the outside under the battons, one in the middle and then one across the inside. You could use celotex in the middle instead. Although I think fitting celotex into old roof work is probably quite hard.

It's very expensive but sheeps wool insultion is meant to be very good and much nicer to work with, and there are lots of fibre insulation coming into the market from recycled materials which are interesting.

 

[Jake]

Correct but it really depends if this matters. Leaving it unheated would mean incurring unnecessary heat loss from rooms below but this would still be a lot less than through the current cold roof.

I missed this reply before my second post to you - yes, agreed, the difference is you'll end up paying for a not cold attic space.