Attic conversion rafter insulation detail

air

Hi,
I'm just looking for some advice with regard to attic insulation.
I have 170mm rafters and am currently installing 100mm rigid PU foam boards between them, cut to size and glued in place with PU foam adhesive.
I'm doing this right from the eaves to the apex of the roof.
At the eaves I'm installing a short length of PU foam board between the joists and will seal this to the foam board between the rafter with an off cut of PU board and expanding foam.

I have a 70mm void between the felt and insulation which is greater than the recommended 50mm, however I'm wondering if there are any issues caused by running this insulation right to the ridge? Effectively the air can flow up behind the insulation but as there is no exit path the circulation would be reduced a lot vs if I left the insulation open at the ridge.

Thanks for reading.

muffler

air wrote: »

Effectively the air can flow up behind the insulation but as there is no exit path the circulation would be reduced a lot vs if I left the insulation open at the ridge.

You need to use a ridge ventilation system something like this
or alternatively fit individual vents on both faces of the roof just below the ridge.

air

Cheers, that's what I was thinking.
However, I was hoping to avoid having to do that and it shoudn't be strictly necessary as the internal ceiling won't be vaulted.
I would just like to keep the attic area as a semi warm space to improve the overall ceiling insulation value and keep the cold water tank up there protected.
Would it be acceptable to create a sort of micro attic below the ridge beam as illustrated below in order to allow ventilation from side to side?

DOCARCH

air wrote: »

Would it be acceptable to create a sort of micro attic below the ridge beam as illustrated below in order to allow ventilation from side to side?

Not really from a building regulation point of veiw. In the set up you have drawn, building regs require equivalent of 5mm continuous strip ventilation at the ridge. The micro climate you suggest could be a damp one!

Is your roofing felt breathable? If it is, then (theoretically) you do not have to ventilate (but personally I would not rely on that) - where possible, I would always try and provide the recommended ventilation.

air

Hi Docarch, thanks for the reply.
My roofing felt is the old style bituminous non breathable variety unfortunately.

Do the regs mandate ridge ventilation in all cases then?
I guess what I'm getting at is, is there a point at which the attic space is considered big enough that it no longer needs ventillation?
IE if I stopped the rafter insulation just above the lower horizontal insulation layer shown above, would I still need ridge vents?

Thanks again.

DOCARCH

air wrote: »

Hi Docarch, thanks for the reply.
I guess what I'm getting at is, is there a point at which the attic space is considered big enough that it no longer needs ventillation?

Not really. See page 28 of this: http://www.environ.ie/en/Publications/DevelopmentandHousing/BuildingStandards/FileDownLoad,1647,en.pdf Very similar to your proposed set up.

air

Thanks for that. I guess I'd argue that by stopping the rafter insulation below the ridge as shown, the upper void becomes an uninsulated pitch roof as shown in diagram A of Fig 11 on the doc.

I think I'll go for what I've proposed and inspect it at regular intervals until I'm happy that it's adequately ventillated.
I will add a wall vent in the gable to help matters further.
The length of roof concerned is only 4m in length.
If the worst comes to the worst and there are signs of condensation I'll add ridge vents.. It won't be any more difficult doing it after the fact anyway I suspect.

DOCARCH

If you've got a gable, that's half the problem solved! As you suggest, put a vent in the gable. At the other end of the roof space, simply add a proprietary vent tile/slate (to replace an existing tile/slate) either side of the ridge or add a proprietary vent tile to the ridge (at that end) and that should be the issue resolved.

muffler

DOCARCH wrote: »

If you've got a gable, that's half the problem solved! As you suggest, put a vent in the gable. At the other end of the roof space, simply add a proprietary vent tile/slate (to replace an existing tile/slate) either side of the ridge or add a proprietary vent tile to the ridge (at that end) and that should be the issue resolved.

+1 on the above

air

Thanks guys, that's mighty stuff, really appreciate the help.
Glad I asked before I went any further with the insulation install.