"8x16" vent is 0.89 sqft"

Sounds wrong.

You need 'net free area', not outside dimensions.

Typical widow screen is only 50% net free area.
Unless that is just a slot of those dimensions and completely unimpeded (and with 16 inch OC rafters the opening is only 14.5 inches long at most) you need more area.

The roof vent should have a specified 'net free area' per fot of vent.

That is the number you use.

Building Science Corp.'s Dr. Lstiburek notes that the 1:300 ratio is the most common recommendation. He calls for continuous soffit venting and more soffit venting vs. ridge venting to maintain a positive pressurization.

Here is a link that might be useful: Understanding Attic Ventilation

This post was edited by worthy on Sat, May 11, 13 at 18:51

Gotchya......soffit vents have 65 sq inches of net free area. So plugging that into the above calcs I get 13 soffit vents needed. That is at least more inline with my expectations. I can evenly space out 18 so I think I should be safe.....thank you!

@worthy.....thank you for the link. While a continuous soffit vent would be ideal I'm just looking for an easy enough project that will have some benefit....even if not ideal. I figure zero soffit vents to 18 will be an improvement.

The purpose of attic venting is to allow excessive moisture to escape from the attic space so when the vents are arranged high and low the free vent area can be much smaller.

As Worthy mentioned, you want the soffit vents to be smaller in area than the ridge vents to prevent too much air from leaving the attic, causing negative pressure and drawing conditioned air from the space below into the attic.

For that same reason you must close the gable vents and use the whole house fan in between the heating and cooling seasons if ever.

Code requirement:

R806.1 Ventilation required. Enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain or snow. Ventilation openings shall have a least dimension of 1/16 inch (1.6 mm) minimum and 1/4 inch (6.4 mm) maximum. Ventilation openings having a least dimension larger than 1/4 inch (6.4 mm) shall be provided with corrosion-resistant wire cloth screening, hardware cloth, or similar material with openings having a least dimension of 1/16 inch (1.6 mm) minimum and 1/4 inch (6.4 mm) maximum. Openings in roof framing members shall conform to the requirements of Section R802.7.

R806.2 Minimum area. The total net free ventilating area shall not be less than 1/150 of the area of the space ventilated except that reduction of the total area to 1/300 is permitted provided that at least 50 percent and not more than 80 percent of the required ventilating area is provided by ventilators located in the upper portion of the space to be ventilated at least 3 feet (914 mm) above the eave or cornice vents with the balance of the required ventilation provided by eave or cornice vents. As an alternative, the net free cross-ventilation area may be reduced to 1/300 when a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling.

Generally for the soffit vents to work properly they should either continuous as was already mentioned of one per bay.

Now I'm confused. If I ran a continuous soffit vent or added one between every rafter then the soffit net free area would greatly exceed the ridge vent area. The net free area for the ridge vent is only 8 sqft.

Any votes for this attic hasn't had soffit vents in 60 years and the house is still standing without any obvious negative consequences so skip them? What would happen if I just blew in fiberglass insulation without soffit vents? Eves are 18" (not 12") and have no insulation in them. I have no obvious moisture issues.

Well, idmd, this is just my opinion...I grew up in a one-storey ranch house built in 1958, the roof was hipped but fairly shallow and my dad put insulation batts in the attic, but I'm pretty sure it wasn't up to modern code. This was back in '61 when we moved in, and lived there until '87. We never had moisture problems and had no ridge vent and I don't recall any soffit vents either, although the crawlspace had sliding louvers every six feet or so.

My current house is a 1908 two and a half storey frame house with a steep pitched hipped roof with three dormers and two chimneys. It has box gutters, so no soffit vents, and no ridge vent either. There is not one sign of moisture damage in the attic, which is a finished single room which takes up all the territory except for about 5 feet per side for storage under the rafters. The roof is skip-sheathed which meant original roof was slate, but is now asphalt shingles.

My guess is that if it has lasted this long without vents, it will last another 100+ years. Old houses, even ones built in the 50s were not so hermetically sealed that moisture was a problem. The biggest problem with moisture from our ranch house was in winter, when ice formed on the aluminum window frames and the marble sills. We tended to put towels on them to catch the water as it melted--this was in the bedrooms, at the far end of the house from the furnace. :)

Thank you for opining columbusguy1.

Kinda what I'm thinking too. I'be had multiple professionals look at roof attic including our inspector and roofer and while both made note of the lack of soffit vents both said they didn't see any moisture issues. The previous roof lasted 24 years and was still functioning fine - we changed it for esthetic reasons and because it was made with a batch of defective shingles that prematurely split.

Three winters ago we did get ice dams but so did 80% of everyone in my town. No sign it had ever happened before and hasn't happened since.

Would blowing insulation into the eves even without soffit vents help protect against this? Since that winter we have had the roof replaced and ridge vent added per code.

But now your adding insulation where before there was not any (if I am understanding correctly), I believe you will be creating a moisture issue without adding the soffit vents.

There is a really good episode on "Ask This Old House" that goes over attic ventilation. I would really suggest watching it.

One thing you say you have is gable vents and a whole house fan. One of the things ATOH talks about is that gable vents actually causes problems for the soffit/ridge vents.

They don't discuss whole house fans but that might be an issue as well.

As for the Ice dams, adding insulation to the eves will not help. Ice dams are created when your attic has excess heat in it that escapes from the house. The heat causes the snow to melt over the house. The melted snow then runs down the shingles to over the eves. Since there is no heat coming up from below the eves the roof area above the eves is colder causing the water to freeze causing the ice dam.

My understanding of having soffit vents in each bay is so that there is air flow up each bay from the soffit to the ridge vent.

Here is a link that might be useful: Ask this Old House

Hang on a minute, rgress...your last statement that soffit vents in each bay allow air flow through each one to the ridge only makes sense if the entire interior of the attic rafters is covered up with plaster or drywall! With most open attics, there is absolutely nothing to prevent air mixing throughout the area once it gets past those little baffles holding back the insulation...and the same thing could be accomplished by just holding back the insulation a couple inches from the very exterior of the soffit as the baffles would do.

Why wouldn't this work: rather than the baffles in each bay, tack up a piece of hardboard onto the rafters and slide the insulation up against that? Since 95% of the bays are open, the air will mix into the ones without the vent holes.

This seems to be one of the if it ain't broke don't fix it. In my experience changing the mechanical parameters in a older home can cause more problems than solving. With the tendency to make older homes " air tight " and providing energy saving fixes the medicine does more harm than the cure in some, not all cases. If your only issue is icedams occasionally I might just provide heat tracing as we have done here. The utility bill is a heck of a lot less expensive than sagging trusses I have seen post venting and insulation improvements.

Well what I really want to do is add blown-in insulation. Living in a ranch with a huge attic space I can really see this as a money saver. Preventing ice-dams is kind of secondary since like I said 80% of houses in my area had dams which had never happened before or since.

So the house as is has 18" overhangs with eves made from 1" solid wood. Presumably there is zero air movement here. If I added blown-in into the eves without soffit vents don't we now just have insulated eves with no air movement (as we had before) but now an additional 12-18" of insulation minimizing the chance that heat from the house could melt snow on roof and cause dams?

So now we've gone from venting eaves to sealing them and stuffing in insulation!

The reason your roof is now working without ice damming is likely the combination of gable vents and ridge vents. Or maybe you're in a moderate climate where ice doesn't form.
Discussions where the o.p.'s climate is secret are not that useful.

Dr. Lstiburek's rules for vented attics are simple: "No powered attic ventilation; more vents down low than up high; wash the entire underside of the roof deck. But all of that is secondary to having the ceiling plane airtight." *(emphasis added)

This post was edited by worthy on Sun, May 12, 13 at 12:36

I'm in southern CT along the shore. Winters range from very mild to below freezing high temps for 7-10 days at a time.