Ventilating attic - too much?


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Old 10-09-14, 10:22 AM
O
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Ventilating attic - too much?

Hi All,

I had some insulation done professionally recently, i'd done a a lot of prep on the attic including opening up 10 soffit vents to a previously unvented attic. I'd asked the insulation crew to add styro channels to the soffit vents. Now that the work is complete they only added styro channels to 5 of the soffits.

I want them to complete the work but they say that for my 300 sq.ft attic you only need 1 soffit vent and i have at least 5. Hence they don't want to come back to finish the job which will be hard given the attic is insulated.

My attic had signs of moisture which is why i over ventilated it. And the soffits are only on one side of the attic (opposite the roof vent) so i though it better to over ventillate as if the wind is travelling the wrong direction i assume the soffits won't work (as well).

Would you push the insulation guys to complete the job or would you let it slide? I have the option of getting a discount.

cheers
oman
 

Last edited by stickshift; 10-09-14 at 12:17 PM. Reason: Removed unnecessary links
  #2  
Old 10-09-14, 12:06 PM
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I would push to get the job done right and making them do it will also include NOT messing up the insulation in the process.

The calculation is 1 ft² of NFA (net free area) of venting for every 150 ft² of attic floor. Since you already had a moisture issue I would not take the 1/300 allowance used for new construction with a vapor barrier, lots of insulation, and well air sealed attic floor.

Whatever that 1/150 number comes out to be it is then divided in half, half for high and half for low.

If you have 1,000 ft² of attic floor you would need a total of 6 2/3 ft² of NFA venting. Your soffit vents would then need to be 3 1/3 ft². A typical reduction of 50% for screening on your vents brings that number back to 6 plus ft² for soffits alone.

Where you are venting only one side, more is definitely better than less, however putting it all on one side may not help the other side.

Vent chutes are simple and inexpensive and it was a mistake on their part to not install them, for which they are grasping for excuses to avoid coming back.

Your building codes may actually dictate the calculations I posted.

Bud
 
  #3  
Old 10-13-14, 09:36 AM
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thanks Bud,

I did get a 2" spray foam layer in the attic to combat moisture leaking up into the attic so perhaps the ventilation isn't critical.

However i looked at the Ontario building codes and noted point 9.25.2.4.a) and b) state

(6) Where soffit venting is used, measures shall be taken,

(a) to prevent loose-fill insulation from blocking the soffit vents and to maintain an open path for circulation of air from the vents into the attic or roof space, and
(b) to minimize air flow into the loose-fill insulation near the soffit vents to maintain the thermal performance of the material.

Hence perhaps a bigger problem is that the soffit vents i have will blow air into the insulation and ruin its insulating rating.

I think i'll get them to finish the job.

cheers
oman
 
 

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