roof ventilation and air wicking in passive houses

dfader

Hello all and happy new year. Looking at detailing for a cold roof for a new build to be near passive. I am concerned about air wicking heat through the quilted insulation in the ventilated roof space. I have read the technical guidance doc on ventilation and am aware that a continuous ventilation 10mm strip is required at eaves. I would prefer to rely on the breather membrane alone as was done on the denby dale passive house;
http://www.greenbuildingstore.co.uk/page--denby-dale-passivhaus-technical-film.html
any opinions?
Thanks
dfader

Poor Uncle Tom

I am working on a detail at the minute where I'm looking at placing a 1m wide breathable felt strip along on top of the wall plate. Flush with the inside edge of the wall plate along the entire eaves length of the plate, then securing all the rafters in place and then lapping the felt strip around the profile of the rafter end and up along the outside rafter edge. The roof felt would lap down over this separably.

Is this the type of detail you are referring to?

dfader

Yes Uncle Tom that is what i am talking about. Are you not conerned with venting condensation and complying with building regs that require a ventilated roof space. The advantage of breather felt should be that you do not have to ventilate but it seems that most builds do both. If i have to ventilate id prefer to use vent slates higher up the roof to reduce air wick a bit. how are you venting the roof?

Poor Uncle Tom

I still aim to have soffit vents, counter batten over the felt and provide ridge vent outlets.

dfader

Poor Uncle Tom wrote: »

I still aim to have soffit vents, counter batten over the felt and provide ridge vent outlets.

What purpose will the soffit vents serve if you do not puncture the felt membrane to continue ventilation up between rafters into the roof space or are you continuing ventilation up behind facia an over membrane. will the ridge ventilation go through the breather felt. The designers of the denby dale passive house say that there should be no air movement in the roof space to prevent wicking heat out through the insulation.

Poor Uncle Tom

I am planning on allowing air movement behind the fascia by putting a 2 x 1 on the rafter end outside the breather felt and applying the fascia to that or a backing board outside the 2 x 1.

I am not planning on allowing the ridge ventilation penetrate the membrane. I am planning on having the MHRV system fitted above the collars and I'll be talking to the suppliers this week about ventilating the space the system is fitted into. The remainder of the upstairs would be internal room space.

esox28

I've done slate vents at the ridge at 1.5 centres Batting and counter on breathable felt solitex, with 10,000 mm over fasica vents running the lenght of eves. The cold roof will be wind tight but not air tight. The vents only vent the roof and not the attic, I presumed this would b suitable set up didn't think the Attic space would need further venting!