Replacing roof vent with a smaller vent

2376 Views 15 Replies 4 Participants Last post by brendelac, Jul 19, 2020

brendelac

Discussion Starter · Apr 25, 2020 (Edited)

Replacing roof vent

I'm inexperienced to this stuff so please forgive me if this is a dumb question:

I'm wanting to replace a dead powered attic fan with a new passive vent and I'm pretty sure I've decided on the Ventilation Maximum 301-12 (see link below). The 301-12 requires a 12"x12" square hole in the roof and its flashing has a 19.5" diameter, but the existing hole from the attic fan is an oval shape with a 16.5" diameter at the widest measurement giving me about 3" of overlap. Is that enough or is there a flashing extension that I can add on or something else? Thanks.

https://www.ventilation-maximum.com/en/sloped-roof-ventilators/

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Bud9051

· #2 · Apr 25, 2020

I can't answer for sure on the opening size others will be along. My question would relate to the overall net free area both high and low for passive ventilation. Have you gone through the calculations and what do you have for other vents, both high and low?

Bud

brendelac

Discussion Starter · #3 · Apr 25, 2020 (Edited)

Bud9051 said:
I can't answer for sure on the opening size others will be along. My question would relate to the overall net free area both high and low for passive ventilation. Have you gone through the calculations and what do you have for other vents, both high and low?

Bud

Yes and I think I'm good in that department, but some conflicting information on the NFA of the soffit vents I am using. I have eight Master Flow EAC16x8W soffit vents. The manufacturer's website and every retailer's website that I have checked (except for Home Depot's) say they have 65 sq-in of NFA each. Home Depot's website says 50 sq-in NFA. Even Home Depot's Canada website says 65 sq-in. I posted this question on Home Depot's website and the manufacturer responded saying 50 sq-in is correct. Why would Home Depot's website be correct when every other website (including the manufacturer's) say otherwise?

I've got 1000 sq-ft of attic space. My soffit vents give me 400 or 520 sq-in of NFA (depending on whether they are 50 or 65 NFA each). Following the 1/150 ventilation guideline, I need 480 sq-in of soffit NFA. The 301-12 roof vent doesn't mention NFA at all but is rated for up to 1200 sq-ft.

Bud9051

· #4 · Apr 25, 2020

Ya, I've looked at those vents and like them, but how to fit them into the NFA calculation is a cross between a powered vent and a passive vent. When the wind blows it dominates passive air flow and they are a wind driven vent.

Don't worry about the 50 vs 65 in² rating, basically the same. You used the 150 for your calculation but could have justified something towards using 300. Long story, so as I said they guessed so you get to estimate. The modern item at the top of the list is to air seal to prevent warm moist air from reaching the bottom of the roof.

Bud

PatChap

· #5 · Apr 25, 2020

Get the bigger maxi and it should fit perfect.

brendelac

Discussion Starter · #6 · Apr 25, 2020

PatChap said:
Get the bigger maxi and it should fit perfect.

https://studor.net/en/pageid/products-airadmittancevalves-maxivent

Are you referring to this?

Ventilation Maximum makes larger roof vents as well, but by special order only. I contacted them about it and they suggested I stick with the 301-12 because I don't have enough soffit venting to keep up with the requirements of their larger models.

brendelac

Discussion Starter · #7 · May 9, 2020

Can anyone else comment on my idea of adding a larger piece of flashing over the 301-12 vent? I was thinking of just using a 24"x24" piece of galvanized sheet metal with a 12"x12" square cut-out in the middle to place over the base of the 301-12 vent (it comes in two pieces). Then using roofing cement all around the 12"x12" cut out to keep water from getting between the two layers of flashing.

I can't think of why this wouldn't work, but I'm no roofer and I can't find anything similar to this on youtube or anywhere else for that matter.

Bud9051

· #8 · May 9, 2020

I mentioned there is a long story behind the calculations for attic ventilation and sounds like you may need some of that information to ease your concerns. Questions:

What is your roof pitch?
Estimate the height from attic floor to the top of the Master Flow?
Have you done any air sealing between house and attic?
I'm guessing you get above average wind where you are, is that correct?

The question as to which fan will work for your attic/roof, either one would be fine. Your concern is more of a roofing one, how to cover that hole, extra flashing or larger fan and the answer is again, either one.

Exactly how to add the flashing you mention, I'm not a roofer so I hesitate to guess but I suspect your suggestion would work just fine.

Give me answers to the questions above and I can tie those into my confidence that either fan would work.

Bud

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3onthetree

· #9 · May 11, 2020

Do you have the vent yet? Looking at the drawn details, the throat of the vent is 12" square, so that is why they require the 12" hole. The 19.5" dimension is from the outside of the deflectors, the flashing size is not given, but looks smaller in the real picture. So the outer rim of flashing might not cover your existing 16.5" hole, or even if it does, the weight of the stack might sag into the hole or on the sheetmetal rig you want to make.

I would block under the roof to support sheathing to fill in the hole, then cut the proper size hole for this vent (unless you have it on hand and can verify how big the flashing rim is).

Also, the spec says its good for 144sq in (each fin opening is I guess 1"x12"), don't know how they come up with venting a 1200sf attic. The attic venting questions are valid ones, as what was the reason for the mechanical fan? What is the shape of roof where this lone passive vent down low from the ridge will be adequate?

brendelac

Discussion Starter · #10 · May 11, 2020

Roof pitch is 4:12

I have not done any air sealing between house and attic.

Estimated height from attic floor to top of vent - 5.5-feet

I would say the wind in my area is average - can be strong, but not often

The powered vent was there when I bought the house and has been dead for at least that long as well. I suspect it was put in because the attic insulation was almost completely blocking the soffits (but I have corrected that).

I also have two gable vents and two small passive roof vents that I plan on blocking once the 301-12 is installed.

I have not yet purchased the 301-12. I doubt it is very heavy - probably a lot less than the fan it will be replacing since it does not have a motor. In the diagram, I thought the flashing width looks to be the same at the deflector width. I mentioned the 19.5" flashing width in an email to the manufacturer and they did not correct me.

It uses wind like turbines do so the 144 sq-in is a bit misleading. The more wind there is, the more effective it will be. They say it only needs a 4 mile-per-hour wind to be effective.

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3onthetree

· #11 · May 12, 2020

brendelac said:
I doubt it is very heavy

Just talking about enough weight or bounce (with wind) to push your jerry-rigged sheetmetal plate down enough where it will start canning, and water may get under it. A wind uplift might pull it right out of the sheetmetal as well. My advice is have it attached to wood, wherever those fastening points land. You also have to nail the shingles around it, so need some wood if the existing hole measures too big.

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brendelac

Discussion Starter · #12 · May 12, 2020

3onthetree said:
Just talking about enough weight or bounce (with wind) to push your jerry-rigged sheetmetal plate down enough where it will start canning, and water may get under it. A wind uplift might pull it right out of the sheetmetal as well. My advice is have it attached to wood, wherever those fastening points land. You also have to nail the shingles around it, so need some wood if the existing hole measures too big.

Good points.

For the board that I would use under the roof sheathing, would the proper way to fasten it be to the trusses? I was thinking that I should probably cut the oval hole into a square (17x17) and then use a second 17x17 board to fill the hole and then cut the 12x12 hole for the roof vent between both layers of board. Does that sound right?

3onthetree

· #13 · May 12, 2020

Sounds like a plan with 5/8 sheathing, but you can put two pieces of 2x blocking straddling the new hole size rather than cutting through one wide block.

Now back to the actual venting . . . I'll restart it and Bud will be chomping at the bit to jump in again.

The best situation, when you can make it happen, is have each joist space vented at the top. If you are going to have only this one tower box vent down from the ridge, the air movement in many corners of the attic (and at ridge) will be diminished.

And even though wind can accelerate the exhaust of air, 144sqin and I don't see any other mechanical apparatus in the tower to cause additional air movement (like a turbine vent creates). So it might be marketing speak, as 144sqin vents a 600sf attic. Yes a 4mph-5mph wind is what is needed for a turbine vent to work, but there are equations for wind effects on attic venting beyond my knowledge base that, even if the 301-12 used these calcs, would be splitting hairs beyond the big picture of good attic venting practice.

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brendelac

Discussion Starter · #14 · May 12, 2020

3onthetree said:
Sounds like a plan with 5/8 sheathing, but you can put two pieces of 2x blocking straddling the new hole size rather than cutting through one wide block.

Now back to the actual venting . . . I'll restart it and Bud will be chomping at the bit to jump in again.

The best situation, when you can make it happen, is have each joist space vented at the top. If you are going to have only this one tower box vent down from the ridge, the air movement in many corners of the attic (and at ridge) will be diminished.

And even though wind can accelerate the exhaust of air, 144sqin and I don't see any other mechanical apparatus in the tower to cause additional air movement (like a turbine vent creates). So it might be marketing speak, as 144sqin vents a 600sf attic. Yes a 4mph-5mph wind is what is needed for a turbine vent to work, but there are equations for wind effects on attic venting beyond my knowledge base that, even if the 301-12 used these calcs, would be splitting hairs beyond the big picture of good attic venting practice.

Ugh, now you've got me thinking I should be getting quotes on a ridge vent.

brendelac

Discussion Starter · #15 · May 31, 2020 (Edited)

I got a quote for a ridge vent and also the the Ventilation Maximum. They are both right around the same cost, although now I am looking at two 303-12's (700-800sq-ft each) rather than a single 301-12 (1000-1200sq-ft). I know the ridge vent is generally considered the best choice, but I can have a foot of snow on my roof in the winter and the Ventilation Maximum just seems so much better designed at handling the weather than the ridge vent. Also, since the ridge vent is made of a type of plastic, I'm skeptical of how long it will last before it gets brittle whereas Ventilation Maximum uses galvanized metal.

brendelac

Discussion Starter · #16 · Jul 19, 2020

I have removed the powered vent and patched over the hole. I have added two Ventilation Maximum 303-12 roof vents (rated for 700-800 sq-ft each). For a few days, I left the existing roof vents and gable vents open, but this morning, I blocked them off with poly from inside the attic. My attic is noticeably hotter now. Does this mean I do not have enough intake ventilation? I have eight 16x8 vents in my plywood soffits (four front, four back). They are each rated for 65 sq-in NFA. The total attic sq-ft is around 1000. Each rafter in my attic has a 12" wide baffle for air to get into the attic.

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