Issue: My house has 12 soffit vents with net free area of 780. While replacing my shingle roof (hail damage) the roofer installed ridge venting replacing 10 or 11 roof vents. He installed at least 71 lft providing 1278 NFA. (His measurements...mine show even more ridge venting installed.) Worst case, that's more than 60% more ridge ventilation than provided by the soffit vents.

I understand from on-line research this is not correct for proper attic ventilation. The numbers should be even or if not the soffits should provide more ventilation space than the ridge.

The roofer suggested installing another 8 soffit vents.

Here are my questions:
1. If the house was originally built with 12 soffit vents. Is it wise to install 8 more?
2. Will this make my house that much colder in the winter? I live close to 7000' near the Rocky Mountains.
3. Rather than cut another 8 holes in the eaves, is there a way to reduce the ridge ventilation by installing flashing under the ridge vent shingles?
4. Who should pay for these modifications?

Really appreciate your help. I paid for the top-of-the-line roofing system and warranty from a national manufacturer and did not expect to this problem.
Thanks!

 

IMO, this is not a big deal.

You provided no information on the size of the house, which would have been helpful. Ventilation of an attic is not an exact science and having a little more vent at the top than at the soffit is IMO not a problem. Your total net free area is around 2000 sq in, or 13.9 sq ft. For an average house, that is about right.

Depending on which ratio you subscribe to (there are varying opinions about how much is enough) the ratio could be either 1:150 or 1:300 or anywhere in between. So if your house is anywhere close to 2085 sq ft, i think there is no problem here and no additional ventilation is needed. I have never been a firm believer that the intake and exhaust needs to be divided up 50:50. I have always been a proponent of more ridge ventilation than soffit. The CFM of the existing soffit intake will increase slightly according to the amount of makeup air needed.

The amount of attic ventilation is a non-issue as it relates to the house being cold in the winter. If it is, you probably don't have near enough insulation in your attic.

 

As X said, this is not exact science. The original 50/50 split was merely an assumption that the vent areas needed to be equal to achieve the targeted air flow. Only recently have they (one expert in particular) begun to recommend 40% high and 60% low. Their reasoning was an attempt to slightly reduce the pressure across the attic to ceiling boundary which would in turn slightly reduce the related winter air leakage. If you have done some air sealing to that attic floor, then the additional pressure will amount to a couple of bucks a year. If not, maybe $20 a year. Purely a wild guess to put the importance into perspective.

As for performance as it is, 62% high and 38% low, that works fine. You don't get into limiting the total air flow until you pass the 75/25 range, be that high/low or low/high.

If your ridge vent has a fiber filter to keep bugs out, then their 18 in² per linear foot is not correct as that is the full 3/4" opening on each side. In other words, they have not reduced the area to a real NFA. But, I don't know what brand ridge vent you have.

As for covering (from below) some of the ridge vent vs adding more soffit vents, that depends upon the calculation explained by X and that is where this decision should start.

Bud

 

Thank you. My house (attic) covers about 2350 sq ft.

I realized late in the game that I'm more familiar with buying news cars than buying a new roof, so most of this is new to me.

I called the manufacturer tech rep and asked what would happen if there was more ridge ventilation than soffit/eaves ventilation. She said "twice as much ridge ventilation as soffit ventilation is bad; twice as much soffit ventilation as ridge ventilation is good". And BTW, they would not honor their 25 year warranty if there was an issue caused by too much ridge ventilation. When I heard this, I started to get serious about making sure my attic ventilation is correct and found this forum.
Thanks again for your help!

 

Just for the record, that tech rep statement is incorrect as high and low ventilation can be reversed and yield the same air flow. The real reason is as explained, to reduce air leakage into the attic, which has nothing to do with shingle warranty. If you speak to her again, ask for that warranty exclusion in writing.

I'm assuming your house is not brand new so will use the 1/150 calculation. With an attic floor of 2,350 ft² divided by 150 you need a total vent area of 15.7 ft². You currently have 14.3 ft². Conclusion, add the new soffit vents and it will meet the targeted vent area for the 1/150 and move you closer to the 50/50 split. If you have not air sealed that attic floor, that should also be done plus be sure you have well above the minimum insulation level. I say above because minimum requirements are there to offer the cheapest option, not the best option.

Bud

 

And adding more venting has nothing to do with making the house colder.
In fact an unconditioned attic should be close to the outside temp. so there's no fungus or ice dams.