Rafter insulation

[TripMeUp]

Can anyone answer this question regarding insulation between rafters on the “old” methodology with felt versus the new breathable membrane

The scenario is this….Dormer house that will have an uninhabited attic space. The plan for the insulation is to insulate rafters on the slopes to where they meet the collar ties (flat attic) and then to insulate between and across the ties/joists. There will also be an air tight membrane put on the underside of the slopes and flat attic space.

The breathable roofing underlay membrane I used states amongst other things:
- Cold and warm roof application
- No ventilation required.

My question is whether you still need to leave this often quoted “25mm or 50mm air gap” between the top of the rafter and the insulation?
The fact that the membrane states “No ventilation required” presumably means that you do not have to? And therefore, can fully fill the depth of the rafter with insulation if you wish??

Obviously, there is ventilation on the roof to a degree anyways with approx 6 vent slates on the roof and whatever else will get in along the vented PVC etc..

 

[onq]

Hi Trip,

Breathable membranes are a case of, if it works, its fine, but you may find you cannot show prima fascie compliance in relation to ventilation if it doesn't work if you haven't left the 50mm gap. This can result in remedial works and problems at sell on.

If you're determined to use a breathable membrane, you will need to check if it has an agrement certificate and ensure the works comply with the specification thereon - for example the installation of a vapour check or variable vapour membrane. If it still fails I don't know what you can rely on to finance a remedy unless you get a guarantee that's enforceable.

Incidentally I've seen several websites dealing with attic conversions and roofing insulation packing the rafters the gills with rockwool or quilt. They have no clue what they are at. Insulation works by trapping air. Compressing it doesn't trap MORE air, but less, since you now have more insulation fibres in a given volum than normal.

ONQ

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[TripMeUp]

Hi ONQ,

thanks for the reply...

I suppose it's a bit frustrating that these all claim these things such as "No ventilation required" etc but as you say, how do you fully prove or stand over that....
The product I used is definitely BBA certified anyways but that's just off the top of my head from the wrapping..it probably has other certification too....

Maybe best to leave the 50mm gap

I know what you mean about the insulation, alot of people are panicking after the cold spell I think...and maybe not thinking thru what they are doing....

 

[onq]

I have seen the breather membrane used. I think in a large attic with a proper vapour check on the cold side of the insulation and a decent plenum it should work well, but with inadequately sealed visqueen sheets, possibly perforated with downlighters and using materials which absorb water vapour like fibreglass improperly compressed, I'd be a bit concerned. All the "breather" property does is reduce the need for ventilation, nothing else AFAIK. The issue is one of correct detailing of the insulation and packed insulation doesn't work properly. If its supposed to be quilted, it shouldn't be compressed. Worse, if its supposd to be quilted on the flat, its difficult to lay it pitched. You end up with compromises with bits if wire holding it in position and it STILL sags. Reinforced rockwool doesn't hold water and it doesn't sag, but it may not be as efficient at insulation as say a closed cell foil backed warmboard - I'd check its effectiveness against the quilted fibreglass and see how it fares. Last house I did with Rockwoll - completed in 2008 - with all the other bits and bobs of heat exchangers and the like achieved an A3 rating.

I'm not an insulation expert BTW, just giving you the benefit of my experience.



ONQ.

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