Leave gap between rafter vents
yioulin
| Posted in General Questions on
Do I need to leave a 1-2 inch gap between rafter vents? I am using Provent from ADO. ADO and Corning installation guides both say to leave a 1 -2 inch gap between each rafter vent to allow for moisture to escape when installing them for cathedral ceilings. However, some GBA and Fine home building articles tell me otherwise. I have installed the vents without a gap. If a gap is needed, can I simply cut some holes on the rafter vents? I reside in Massachusetts. Zone 5.
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Replies
yioulin,
That's the downside of using impermeable baffles. The main function of baffles is to provide an vent space to allow the sheathing above to dry, but if the baffles are made of something permeable then any moist air that makes it's way into the roof from the house below can diffuse up though them, rather than relying the moisture diffusing through the framing on each side of the baffles.
One way around that lessened drying capacity is what ADO and Corning suggest: providing gaps for the moisture to access the vent space. The downside of that is you now have some wind-washing of the insulation below.
If the ceiling below is well air-sealed, there shouldn't be enough moisture making it's way up to the baffles to worry about. On the other hand, the amount of R-value lost to small gaps in the baffles should be pretty small. All this is a long-winded way of saying I don't think in most circumstances it matters much whether you provide gaps or not. If it were me I would keep them all tight, except maybe for the one at the ridge, which is where moist interior air is most likely to accumulate.
Thank you! I'm inclined to keep them air tight too.
yioulin,
This article, although talking about site built baffles, sums things up pretty well:
https://www.greenbuildingadvisor.com/article/site-built-ventilation-baffles-for-roofs
This would fall into the 'finicky' category, but you could leave the gaps, then tape them with vapor open tape (like tescon vana or similar). This is probably most realistic if you were already using said tapes and had them on hand. Not saying it's necessarily worth the extra effort, just a thought.
An alternative if you didn't have vapor open tape (and even more finicky) would be to use housewrap taped with whatever tape was on hand. At this point, the question may arise: should I just use site built permeable baffles.
maine_tyler,
Also in the finicky category of solutions: I wonder if using tight baffles up to the peak and then covering the top foot or so with a permeable house wrap as a sort of under-roof vapour diffusion port.