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| Green Building Bible, Fourth Edition |
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These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment. PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book. |
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Posted By: tonyI have seen plenty of examples of fully filled with no problems -- especially W,S and E facing ones. I used to think -- no or very little air gap -- no moist air as it will all be dry -- so no condensation?I think there is something in this. A number of BBA certificates for warm roof constructions mention not leaving voids, I assume it is for this reason.
Posted By: tonyAh yea but some warm roof constructions also require a 50mm tooThe ventilation gap in a warm roof construction is above the sarking membrane. This performs a subtely different role. It's not so much removing damp air from the loft or roof void as encouraging vapour to pass through the breathable sarking membrane.
how small can the gap be?
Posted By: sinnerboyWell vapour permeable insulations like fibreglass will leave voids to collect condensate - thereby reducing the insulation performance . What happens to that condensate ? Summer temp cause it to disperse harmlessly ? Winter wind penetration ( un intended ventilation ) will disperse it too - along with thermal performance . So why do calculated U Values so often fail to deliver in real life ?If flexible mineral wool is correctly installed then there should be no voids, certainly less than you would get with a rigid insulation board.
Posted By: tonyDid you all just say no gap?I said, I don't see the point of NOT ventilating the batten cavity. For a warm roof I would ventilate the batten cavity above the breathable sarking membrane, 25mm to eaves, 5mm to ridge.
Posted By: fostertomBRE's boffin expert on air permeability of lapped tiles/slates, who said that even after years of silting up the batten space remains completely vapour-open, even with triple lap shingles
Posted By: davidfreeboroughIf flexible mineral wool is correctly installed then there should be no voids, certainly less than you would get with a rigid insulation board.
Posted By: davidfreeboroughDuring the winter the partial vapour pressure gradient is from inside to outside. So vapour will be driven out of the roof to be dissipated by the breathable sarking membrane.
Posted By: davidfreeborough During the summer it will be driven in the other direction towards the inside.
Posted By: davidfreeborough You cannot rely on unplanned air leakage to dissipate moisture,
Posted By: davidfreeborough. Mineral wool insulation needs to be contained in a windtight box with the right vapour permeability properties. The ventilation needs to be planned to make sure that this box doesn't collect moisture long term.
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Posted By: Mike Georgesome kind of above fascia vent is necessary to ventilate the area above and between counter battensbut AFAIC that was discredited - because the slating batten space is well vapour-open to outside even without actual vent gaps.
Posted By: fostertomIf there's liquid condensation somewhere within the sandwich, it will stay as liquid until its temp, and the temp of the air within the sandwich, rises, so evaporation begins. Until evaporation happens, no amount of ventilation will remove it in while liquid state.So I don't think that ventilation helps drying out, even "in the case of a minor roof leak" - temperature rise is the only way that vapourisation can happen, and once there's vapourisation, simple breatheability is is quite sufficient to allow its strong urge to disperse. Ventilation is only necessary is there's not breatheability.
Once evaporated into vapour state, its own partial vapour pressure is perfectly adequate to disperse it away from its own centre of concentration. Air movement is not necessary for that dispersal - it will migrate through still air (or even 'upstream' against an air current).
So ventilation thro air gap above the insulation is completely pointless and irrelevant, and is based on ignorance or misunderstanding of the partial vapour pressure mechanism. It seems 'common sense' that ventilation will remove moisture, but that's fortuitous, in fact frequently counter productive as it can actually import more moisture.
Posted By: Mike Georgerisk of condensation in the timbers if a sufficient surface area is left exposed to a vented 50mm gapis actually increased because the area of rafter surface left naked to actually a convective draught of cold air is tripled - may increase from one-sixteenth to one quarter of the rafter's surface. So the rafter stays much colder, poss below dew point.
Posted By: davidfreeboroughApproach b) probably works well with rigid board insulation well sealed to the raftersI'd say that b) [i] is main reason why insulation performs disappointingly;
Posted By: davidfreeboroughWithout wishing to complicate things further, let me re-state the definitions:
a) forming a ventilated gap between top of insulation and underside of an old-style impermeable slating felt;
b) forming a ventilated gap between top of insulation and underside of a breathable membrane, [i] vapour open insulation between rafters, [ii] non-vapour open insulation between rafters;
c) forming a ventilated gap between top of breathable membrane and underside of slates/tiles;
d) forming a ventilated gap between top of fully supported breathable membrane and underside of slates/tiles, [i] vapour open sarking board, [ii] non-vapour open sarking board.
I don't know why anyone would use a) for a new build roof.
Approach b) probably works well with rigid board insulation well sealed to the rafters. However, it makes sense to use the sarking board/membrane to keep the wind out of the insulation, so I would seek to avoid b).
Approach c) is not required by any breathable membrane BBA certificate I have seen. Counter battens are required if the membrane is pulled tight, but this is to allow liquid water run-off.
Case d) [i] implies a breathing roof with relatively low vapour resistance, for example, constructions with no internal vapour barrier & woodfibre sarking boards. This brings with it relatively high rates of vapour transfer & a need for ventilation of the batten cavity. For example, the BBA certificates for Isolair woodfibre sarking boards requires 25mm minimum counter battens, 25mm ventilators to the eaves & 5mm ventilators to the ridge.
Case d) [ii] implies a non-breathing roof with a relatively high vapour resistance insulation over the rafters. Here the rates of vapour transfer are relatively low & can be readily dissipated through the tiles. So all the BBA certificates I've seen do not require any ventilation & instead place their emphasis on having a continuous vapour control barrier internally. Counter battens are required, but this is to allow liquid water run-off.
David
Posted By: Mike GeorgeIt all hinges though on how often the air in any ventilated gap rises above the temperature within the structure that caused any condensate to occur. If it happens often [and I think it does], then clearly rapid ventilation will cause the rate of evaporation to occur faster than if no ventilation is present yes?I think this is key. On a clear night the roof's outer surface radiates into space & can drop well below the ambient air temperature. So it tends to collect moisture like a car left out overnight.
