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What is the diameter of a 12" mushroom vent?

I know this may sound like a strange question, but I have a contractor who says the the mushroom vents that measure 15" across are called 12" mushroom vents because of how much air flow they actually let in!

Asked by Susan Cockrell
Posted Sat, 02/01/2014 - 12:17


6 Answers

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The maximum diameter of a mushroom vent is larger than the opening that is cut in the roof.

As far as I know, there is no consistent industry-wide standard for describing the size of mushroom vents.

To determine the precise dimensions of any mushroom vent, as well as the manufacturer's description of the product's net free vent area, I suggest that you visit the manufacturer's website.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 02/03/2014 - 09:50

Helpful? 0

Thanks for answering.

Let me be more specific: The contract was written specifying three !2" mushroom vents. This square inch area satisfied the government-subsidized discount insulation program for required attic venting. The installer installed mushroom vents that I measure the diameter of the circle as 15". The installer says that they call these "12" mushroom vents." But it seems to me that they are 30% larger than was required by the contract.

The opening cut in the roof is a square that is big enough to accommodate the 15" circle of the bottom of the vent.

Answered by Susan Cockrell
Posted Mon, 02/03/2014 - 10:53

Helpful? 0

I don't think there will be any problems that will occur, even if the mushroom vents are somewhat larger than those specified.

Moreover, I don't think think there is a standard way of measuring mushroom vents, so it is hard to know whether you have a leg to stand on, even if you don't really like the vents. I can imagine measuring the vents three different ways: by measuring the maximum diameter of the part that sits above the roof; by measuring the size of the hole cut in the roof; or by referring to the product's net free vent area.

If the vents really bother you, I suppose you can negotiate with your builder to see if a different vent can be installed. But here's my advice: don't sweat the small stuff.

For more information on attic venting, see this article: All About Attic Venting.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 02/03/2014 - 11:03
Edited Mon, 02/03/2014 - 11:04.

Helpful? 0

Thank you. That's a good answer. To go a little bit further. I didn't want any vents at all; I just wanted insulation under a subsidized program. My attic has been bone dry for 100 years (I can vouch only for the last 12) and has 3 windows. This insulation program requires attic venting and, they say, windows are not enough or don't count. They installed mushroom vents. Two days later, it snowed. There was snow on the attic floor beneath each of the vents; there was frost on the aluminum loop (bottom of the vent) above my attic ceiling. I'm objecting in any way that I can. What I read is that the idea of venting is to remove moisture from the attic. But my attic had no moisture. I liked how the house breathed. The vents are letting moisture in! So if the installer put in 30% larger holes in my roof than were "required," I'd want to know and they could remove one of the vents completely. But, really, my top choice is to remove all of them. I don't see the necessity of them. Do you?

Answered by Susan Cockrell
Posted Mon, 02/03/2014 - 12:03

Helpful? 0

I agree with you that the mushroom vents are unnecessary and counterproductive. Sometimes building codes (and utility rebate programs) are illogical.

Once you have satisfied the requirements of the rebate program, and all of the inspectors have gone away, you can probably remove the mushroom vents, patch the roof sheathing, and weave in a few asphalt shingles to restore your roof to its previous watertight condition.

I know -- that's a lot of work, and it's an unsatisfying solution to your dilemma. But it's one approach to consider.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 02/03/2014 - 15:53

Helpful? 0

Thank you so much for your sharing your time and expertise!! You are agreeing with one of my options - and likely the one that I'll take. I'm glad to have more insulation in the attic and trust that I can put the roof right again!

Answered by Susan Cockrell
Posted Mon, 02/03/2014 - 16:02

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