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Where is soffit venting necessary?

The December issue of Journal of Light Construction has an article featuring building techniques of four high performance builders. Three of the buildings pictured have truss roofs; one of those clearly has a ventilation channel from the soffit to the space above the insulation; one is unclear and one, a certified Passive House, clearly does not. There, the cellulose insulation is against the inside of the roof sheathing for what looks like 2'-3'. So my question is, in a vented attic over a very tight house, when is venting to the soffit necessary?

Asked by Bob Irving
Posted Sat, 12/21/2013 - 12:19


5 Answers

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Q. "In a vented attic over a very tight house, when is venting to the soffit necessary?"

A. According to the building code, always. According to building scientists, the soffit venting in this type of house may be optional. See this article: All About Attic Venting.

However, cellulose insulation should never touch the roof sheathing, unless there is a significant amount of rigid foam installed above the roof sheathing.

Answered by Martin Holladay, GBA Advisor
Posted Sat, 12/21/2013 - 16:14
Edited Sat, 12/21/2013 - 16:16.

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Martin, why should cellulose not touch the roof deck? I have seen this mentioned before but I can't find an explanation of why, other than ventilation. (Assume ventilation is accomplished elsewhere than the soffits, or is unnecessary.)

Answered by Nick Welch
Posted Mon, 12/23/2013 - 02:48

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It's unlikely that any problems would be caused by an installation with only a small percentage of the roof insulation touching the roof sheathing -- say, for an area extending 1 or 2 feet near the eaves. But when builders try to pack the rafter bays of cathedral ceilings with cellulose that touches the roof sheathing, they are risking sheathing rot.

It's true that most of the moisture in these failure cases originates from ceiling leaks, so (in theory) very careful air sealing lowers the risk. But this type of installation is a code violation, and I don't think it's worth the risk.

For more information on when a vent channel is required by the building code, see How to Build an Insulated Cathedral Ceiling.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 12/23/2013 - 06:44

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National Fiber's recommendation is that if the roof has soffit vents, than install proper vents and wind baffles before installing the cellulose. In roofs without soffit vents, cellulose can safely be installed right up to the roof deck. Cellulose can touch the roof deck without issue since its hygroscopic properties will prevent moisture accumulation. In unvented roof assemblies filled with cellulose, the installed density is critical, so that gaps do not occur between the cellulose and the roof sheathing. The hygroscopic properties of the cellulose can only protect those surfaces in which it is in direct contact with.

Answered by Bill Hulstrunk
Posted Fri, 12/27/2013 - 12:18

Helpful? 0

Thanks for your comments. You and I have disagreed on this point in print before.

Here is a message to GBA readers: Bill Hulstrunk's suggested method of dense-packing unvented rafter bays is a violation of most building codes.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 12/27/2013 - 12:41

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