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How much insulation and roof vents are needed in attic


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03-23-14, 02:13 PM   #1  
How much insulation and roof vents are needed in attic

just bought a home and was wondering how much insulation i need in the attic, and how many exhaust/intake vents there should be. i was getting some condensation thats why i am concerned. thx

 
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03-23-14, 08:23 PM   #2  
We would want more details, but the general rule for venting is 1 ft of net free vent area for every 150 ft of attic floor. Half of that vent area goes high and half low. Additional steps involve sealing leaks from house to attic and some form of vapor barrier at the drywall level.

Be sure bath and kitchen fans are vented to the outside and not into the attic.

More details if needed.

Bud

 
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12-08-14, 08:36 AM   #3  
ok i measure the interior space of that level and it is 22ft x 20ft. so please advice how many vents i should have thx

 
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12-08-14, 09:52 AM   #4  
Air sealing is number one and I added a link below that covers all air sealing in a house.

The general rule is 1 ft of vent area for every 150 ft of attic floor. 22 x 20 is 440 ft which would suggest 3 ft total net free vent area. Then, since this is net free area, doubled that number, that gives you 6 ft of vent area to divide half high and half down low. A perfectly air sealed ceiling with a vapor barrier in place next to the drywall would reduce that number to half, but difficult to reach perfect.

The "half down low" should be open to every rafter bay. What do yo plan to use for high venting, ridge vent, gable vent, or roof vents.

The 440ft doesn't sound like the entire attic, although it could be. Is this a cape or just the area over part of the house?

Bud
http://www.efficiencyvermont.com/ste...ide_062507.pdf

 
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12-08-14, 10:04 AM   #5  
That's there area of one of the attics it's a multi level home. So 6fsquared. That's approx 3 vents in/low and 3 high

 
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12-08-14, 10:17 AM   #6  
If you are talking roof vents, check the NFA (net free area) for those. I allowed for 50% and yours could be less restrictive. The air sealing is what will reduce the moisture the most, Venting just exhausts what gets past.

In most attic moisture issues I see I find a bath fan exhausting into the attic or some other direct path, like a disconnected duct. Even a bath fan vented out into a soffit or just below a soffit can be the source. The location of the condensation is usually close to the source.

The vertical height plays a role in how effective natural venting will be and roof vents usually don't start at the bottom like soffit vents nor end at the top like ridge vents. But roof and ridge vents are also subject to snow depth where soffit or gable vents are not.

As for the amount of insulation, Canada will have required minimums for different areas, but r-50 is probably the least you would see. Are you blowing the insulation into place or using batts?

Bud

 
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03-17-15, 03:38 PM   #7  
I'm going to be blowing in r50. So currently I have 6 vents at the soffit s and I believe 2 at the roof. Is this enough?? Why are all the newer homes have the entire soffit channels open/perferated

 
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03-17-15, 06:40 PM   #8  
Post #4 explains the 1/150 rule.
Baffles in every rafter bay to keep the new insulation from touching the roof and to maintain an air path.

Bud

 
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03-17-15, 07:16 PM   #9  
My eaves trough does not have vents on all of them. If there is a 1/150 rule why do u need baffles on all of them

 
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03-18-15, 06:39 AM   #10  
You want to look at worst case scenarios to really get an understanding.

Stagnant warm air can lead to moisture and moisture can lead to mold.

If your attic ceiling is the same temperature as the exterior roof, there is really nowhere for the warm air to condense and create moisture. In a perfect world, you would not want any insulation at all so that the ceiling is always the same temperature as the outside.

But, we humans don't care for things to be that cold in the winter and we really hate having to pay a lot for heat. In THAT perfect world, the attic would be filled with insulation. But all that insulation allows the warm air to stay still against the cold ceiling/roof and condense; leading to mold.

Putting baffles in every rafter bay so that the insulation does not touch the ceiling/roof lets you have all the insulation you wan to stay warm while keeping the ceiling/roof cold and allows the air to flow (and any condensation that does form to drip away) so that the risk of mold is reduced.

Due to the above, the best practice is baffles in every rafter, entry vents leading into every rafter cavity, and exit vents as high up in the attic as possible so that air can flow through each cavity and reduce the chance of mold.

EDIT: Check pages 16-17 in the .pdf that Bud9051 provided (thank you for that, Bud)

 
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