Must exhaust vents equal intake vents?


#1

New construction with asphalt shingles. Cold edge of Climate Zone 4. About 3500 sq ft of attic area. There is exactly 1:150 with 1680 sq. in. in 13 soffit vents on the back and one side. For the exhaust, there is 610 sq. in. in 10 square static vents. From using the online calculators, my exhaust should equal the intake. If so, I only have about one-third the required 27 exhaust vents. Is this OK? If not, not sure where we could put that many more vents. A few more might be easy enough. Thanks.


#2

Can you do a ridge vent or is your roof all cut up?
How far apart are you setting the vents?


#3

You have the reverse scenario most people have. Normally people add roof vents but fail to add sofit vents. As patch said upload some photos. It might be possible to use ridge or hip vents instead of box vents.

Is there a reason why you only installed intake vents on 2 sides of the house?. With new construction the time to fix it is now. You don’t want to be back on here in 5 years posting problems.


#4

IMO you want more intake if possible.

The intakes are more labor intensive to install after the fact and the amount of exhaust will only be limited by the amount of intake.


#5

See attached photo of 10 exhaust vents.


#6

You can add a vent in between each of those.


#7

What do the soffit vents look like? I’d like some kind of intake on the front of the house as well.

No vents on the highest ridge? Do the attics of the dormers connect to the main attic?

It would be a good idea to ask the roofer for his number. You may have to add the vents after the house is ‘yours’.


#8

That is a massive house with a massive roof that you don’t want to be replacing prematurely. The consensus seems to be you need more exhaust and intake all the way around.

What did the architect / engineer spec?. This forum is a great place to ask questions and get advice, but on a project of that size and cost, if it was my house I would want the liability of design to fall on a certified professionals.

If it was my house I would install intake vents all the way around on the main house as well as on all of the dormers, then install Ridge vent on all ridges.


#9

Contact the shingle manufacturer, talk to the Tech department and ask them about the warranty and ventilation. If you do not follow it, you will not have a shingle warranty. Problem I have found is some homeowners do not like the vents on the house. If the shingles degrade faster than they should have, the manufacturer can say it was because of poor ventilation. Sorry no warranty. Here is a good link to use to check your numbers again. You numbers I found are not the same as here. http://www.lomanco.com/


#10

All the online roofing calculators are essentially the same. The only variations are 1:150 or 1:300 and whether those apply to exhaust or intake or both together. That’s the biggest confusion. Some say the code means 1:150 total; some say the code means 1:150 intake and 1:150 exhaust.

I have a CertainTeed roof. Their smartphone app calculator basically calls for 1:300 (double if code uses 1:150). In my 1:150 case, the CertainTeed app specifies that means 1680 intake & 1680 exhaust. I do have 1680 intake, but only 620 exhaust. 3500 sq ft attic.

The 2012 IRC (which my city uses) says:

R806.2 Minimum vent area. The minimum net free ventilation area shall be 1/150 of the area of the vented space.

Exception: The minimum net free ventilation area shall be 1/300 of the vented space provided one or more (changed to both must be met in 2018 IRC) of the following conditions are met:

  1. In Climate Zones 6, 7 and 8, a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling.
  2. At least 40 percent and not more than 50 percent of the required ventilating area is provided by ventilators located in the upper portion of the attic or rafter space. Upper ventilators shall be located not more than 3 feet (914 mm) below the ridge or highest point of the space, measured vertically, with the balance of the required ventilation provided by eave or cornice vents. Where the location of wall or roof framing members conflicts with the installation of upper ventilators, installation more than 3 feet (914 mm) below the ridge or highest point of the space shall be permitted.

I’m in Zone 4, so that means Exception #2 can apply for 2012, but not 2018. My total NFVA is 2300 (15.97 sq ft). That ratio is 1:219. Using 1:219, 37% is in the upper portion (1680\620). Not sure how that fits with the code only talking about 1:300 there. The code is not clear to me here on this exception. For 2018 code (not used here), this doesn’t apply at all anyway.

The engineering plan boilerplate says, “NFVA shall not be less than 1:150 of the area of space ventilated, except where the ventilators area located in the upper portion of the space to be ventilated the required area may be reduced to 1:300.” Not sure what that means in plain English?

The CertainTeed 2018 Warranty states:

“Any shingles applied to inadequately ventilated or non-ventilated decks, other than the shingles and deck systems described in the section titled ‘Insulated Decks and Radiant Barriers,’ are subject to a reduced limited warranty period of ten (10) years and do not qualify for SureStart Protection. SureStart™ Protection and the Warranty Period applicable to the shingle are available if CertainTeed determines that the shingle damage was caused exclusively by a manufacturing defect that is unrelated to the inadequate roof system ventilation.”

So my lifetime warranty can be reduced to 10 years with inadequate ventilation. Guess I have to ask CertainTeed which standard apples–their online calculator or the code?

One other disrupter is the IKO website. “The NFA will be calculated differently if your roof slope is greater than average. If the slope of your roof is greater than 6:12 you will need more ventilation to reflect the extra volume of attic space: Calculate 20 per cent more for 7:12 to 10:12 pitches, and 30 per cent more for a pitch that is 11:12 or greater. (Source: Asphalt Roofing and NRCA.)”

I have a mix of 3:12, 6:12 & 12:12. I’d say it averages out to about 6:12. Thanks everybody for the comments so far.


#11

The 13 existing soffits are in red, I think I need some more in the green area. The front is tough, a lot of it is gabled.


#12

You have alot of roof on the front of the house. I am not the biggest fan of smart vent but yours would probably be a case where I would use it.

Ridge or box vents on 3 largest dormers would help add some exhaust.