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Bifman
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Discussion Starter · #1 ·
Hi all,

I live in an older townhouse in Toronto. After noticing strange temperatures and humidity levels in the unfinished attic, I discovered that the soffits were blocked by plywood, allowing no air inflow. I hired some workers to cut holes through the soffits and then though the plywood to allow air access to the attic. The workers installed 8 vents (8” by 4”) over the soffit holes they had drilled. The workers also added additional 8” by 8” roof vents bringing the total to 4 roof vents. which appears balanced with flow at 256 sq inches for both inflow and outflow. (Our roof size is 1025 sq. feet.)

The quote included insect prevention mesh to be placed on the new soffit vents (but made no mention of mesh for the roof vents which I did not notice at the time.

Half way through the job, I noticed they had not installed mesh on the new soffit vents. The forman told me that that mesh was not necessary and not a good idea since it could get clogged.

I spoke to the company afterwards and they told me I could go with the foreman’s opinion and they would give me 12% discount on the job OR they could send the crew back for free to add the missing mesh. What should I do?
Is insect prevention mesh important/common?
How much does mesh impact air flow?

I am wondering if the calculation for net free area included the impact of mesh? It appears no. If that's the case then wouldn’t adding mesh to the soffit vents only mess up the balance? My understanding is that if there is an imbalance then you want the inflow to be stronger not the outflow.
If there is no mesh on roof vents, is there any point in adding mesh to soffits?
If I do mesh should it be on both roof vents and soffit vents?
Since the job was done I have seen an improvement in air flow and I am worried about creating an imbalance by making the wrong choice.
 

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Trying to get 50/50 on the ventilation is a minor part of the ventilation, it works well over a wide variation. The technical reasoning is based upon avoiding some of the house to attic pressure that affects air leakage through any ceiling leaks. A little effort to seal up those leaks (a good idea anyway) and the balance concerns go away.

Note, the inflow and outflow will ALWAYS be equal. The pressures moving the air automatically adjust to reach that balance.

But your concern about NFA is correct, the mesh or bug screening will reduce that number (estimate 50% but varies with the mesh) and their advice to skip it falls into the stupid category. Without bug screening wasps and other insects will enjoy their new home.

The roof vents also need screening and all I'm familiar with come with it.

Just my opinion but I would add a wire mesh with about 1/8" openings maybe slightly larger, just small enough to keep wasps, bees, and in my case ladybugs. That size opening would have less affect on the NFA.

What is the area of your attic floor?

Bud
 

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Bifman
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Discussion Starter · #3 ·
Thanks Bud for your input,


Attic floor 875 sq feet. According to company including soffits is 1025 sq feet



btw - I've attached a picture of one of the the vents installed on the soffit. I can't check right now but the the air gap looks slightly bigger than 1/8".
 

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While I calculate your NFA requirements how big are those hole under the soffit vents we are seeing and how big are the next holes going into the attic?

Bud
 

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Use just the 875 as soffit area is not above living space.
Two choices to calculate needed NFA, 1 ft² per 150 ft² of attic floor or per 300 ft². When the ceiling is well air sealed and a vapor retarder in place the allow the 1/300.

Using the 1/150 we divide the 875 by 150 = 5.8 ft² or 2.9 ft² each for low and high.
Multiply that by 144.. 144 x 2.9 = 418 in², again each high and low.

You have 8 soffit vents at 32 in² each for a total of 256 in² before any derating for NFA.

If you were to consider your ceiling to be well air sealed and use the 1/300 the result would half or 209 in².

As for derating the NFA your actual opening is smaller than the vent cover we are seeing so adding a screen directly above the vent cover would have less affect than applying it directly to the soffit hole.

I'll stop here but I suggest they add a 1/8th in grid mesh to both soffit vent cover and to the roof vents if there is nothing up there.

Bud
 
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retired framer
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I think the most important detail is how many rafter bays have some air flow from the soffet area.

I am not so concerned with the numbers. There is a weather system in the attic, when you add heat it causes pressure, pressure cause wind. the smaller the hole the greater the wind. Always better to have more intake than exhaust. You never want a neg. pressure that will suck air from the house.
 

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@Nealtw you are correct that the soffit area needs to feed all rafter cavities and the holes they cut may not have accomplished that.

But your comments on attic pressure are not correct.
"There is a weather system in the attic, when you add heat it causes pressure, pressure cause wind. the smaller the hole the greater the wind. Always better to have more intake than exhaust. You never want a neg. pressure that will suck air from the house."

Not sure I want to drift on the op's thread but willing to discuss on another thread if you want.

Bud
 
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