celotex/kingspan , is it breathable?

[robgil]

i will have a warm roof , 150x50 rafters with breathable felt ontop, counter battened , battened then cedar shingled.
its over a kitchen and bathroom so moisture and condensation may be a problem , i hope to have a lime plastered reed board ceiling.
for insulating i was thinking 100mm celotex but i am not sure if it would let moisture evaporate through the roof or if condensation would build up underneath it.
what do you guys think? is there an alternative that is breathable and has a good insulating value in a tight (150mm , 110 after the 40mm space is left between felt and insulating) space?

 

[Lime]

What you need to be aware of is that it isn't high permeability of the insulation that is important in the loft.
If the insulation is totally impermeable it doesn't really matter.

What needs to be avoided is any situation where cold surfaces (of any sort) are exposed to warmer moist air.
Then condensation will occur.
Depending on the circumstance, this condensate (water) will either accumulate in situ, evaporate or move elsewhere.

In general two different circumstances can create this water problem.
1. Moist warm air from the living area coming in contact with cold surfaces or even cold air in the loft.
2. Warm moist air from outside the house entering the loft and condensing on the cooler fabric of the house.
(leaks of any sort (pipes/tank/roof) can create a similar problem with water too)
Then the permeability of the insulation may be important because it can prevent evaporation in some situations.

However I don't see how water can accumulate between the warm ceiling and the underside (warm side) of the insulation as long as the warm moist air is prevented from gaining access to cold surfaces.
The temperature of the underside of the insulation will be much the same as that of the ceiling.

If the loft is well ventilated I don't see a problem.
also, it can be argued that using permeable insulation in the situation you describe could allow more moisture to enter the loft than otherwise might be the case.

 

[robgil]

i wont have a loft lime , just a ceiling below the rafters , insulating in-between the rafters and felt over the top. but i do hear what you are saying , in this case stopping any warm moist air from getting to the felt will stop condensation gathering there , so the more impermeable the insulation the better?

 

[christianb]

Hi Robgil,

Foil-board insulation is totally impermeable - and that's how it should be. Underlining your rafters with it - as well as inbetween - will prevent vapour getting through in the first place, and having the breather felt will allow any residual to escape.

C.

 

[Feltwell]

Robgil,

If you're counter-battening then you don't need to leave a gap if you use the right membrane - i.e. you can have 150mm of insulation.

See the 3rd diagram down on this page for Proctor Roofshield:-

http://www.proctorgroup.com/index.asp?lm=268

The gap is only there for you to drape the membrane into - if you're not counter-battening, never put roof membrane on so it's drawn tight, it should always be left to sag slightly in between the rafters so there's no possibility of water getting trapped behind the battens. Thus, if you're not counter-battening then you can't fill the rafter depth completely with insulation.

If you are counter-battening, the thickness of the counterbattens is enough to make sure the membrane cannot trap water behind the battens - so you can fill the full rafter depth with insulation.