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Moggy 12-06-2009 09:43 AM

Question about attic venting
Hey everyone. Since I bought my house I've had ice dam problems. Insulation was poor so I had a bunch blown in. Helped with the melting but not eliminated. I have a continuous ridge vent, a gable vent at each end, and now soffit vents. Until this fall I did not have soffit vents. I rented a boom lift from home depot, pulled the vinyl soffit covers off, hole sawed 3.5" holes in each rafter bay and replaced the vinyl with perforated vinyl. I also at the same time routed my 2 bathroom fans to the soffit. (previous builder/owner had them just hanging out in the attic)

I still have melting that I fear will cause ice dams. I did not yet put in the styrofoam channels in the rafters to let the air travel up the sheathing easier and some of my blown in insulation is touching the roof surface, so I know I need to fix that. My main question is about the gable vent. If I want the air to come in mainly through the soffits, could the gable vent be hurting that airflow? Should I block the gable vent since I have so much soffit and ridge venting. Seems to me that blocking it would make for a better exchange of air even if it does mean less surface area of venting. Any input is appreciated.

kok328 12-06-2009 12:46 PM

Yes, block off the gable vents, they are short circuiting (so to speak) the airflow that you need to create/direct.

Moggy 12-06-2009 12:50 PM

Thats exactly what I figured. I googled like heck trying to find someone to say it, but I couldnt find it. I'm going to give it a shot and see what that gets me. Maybe I'll just staple some cardboard over it for now and see what happens. Thanks for the reply!

Hube 12-06-2009 02:09 PM

moggy; proper attic ventilation has to have air entering at the LOWEST point (soffits) and exiting at the HIGHEST point on the foof (either by a continuous ridge vent or near the peak mushroom type vents)

Also, the total sq ft area of these vents have to be aT LEASTenough to ventilate the attic's floor area. Not enough air going in or out can cause problems. An attic should have at least 1 sq ft(144 sq inches) of ventilation for every 300 sq ft of attic floor area. This venting total is split 50/50 between the incoming and the outgoing.

Example; An attic floor area of 1200 sq ft would then require at least 4 sq ft of total venting; 2 sq ft at the soffits and 2 sq ft at or near the ridge.
A sq ft is 144 sq inches but more is needed when there is an mesh /screen on these vents to stop entry of insects, birds,etc. A 1/8" wire mesh can restrict approximately up to 25% of air flow. That is why we use the words "at least".
The more venting area the better.
Also make sure your insulation is not blocking the entry of air. And installing the styrfoam rafter channels is a very good idea too.

Moggy 12-06-2009 05:08 PM

Well, my attic is 900 square feet. According to the formula I should have 3 square feet of venting. I estimate that I have 3.2 square feet of soffit (9.62sq" vent times 48 vents) and 9 square feet of ridgevent. (36'x3" vent total) So I think I have the ventilatoin covered. I plugged the gable vent and we shall see if this improves my melting situation. I should be able to eliminate melting altogether shouldn't I ? That's what I'm shooting for. It would really help with the "death stair" situation on the back deck. :)

Hube 12-07-2009 07:35 AM

moggy . It would seem that you have adequate venting for the attic.
If by chance you do get some major ice damming, then you should look at the insulation to see if it is covering all the surface of the attic floor and also it should be an R value of at least R28. A major cause of ice damming is lack of insulation. Check it out.
Note; the temperature in an attic in the winter season should be VERY CLOSE to whatever the OUTSIDE temperature is. In the summer months that will not hold true of course because of the intensity of the Sun on the shingles and other parts of the roof.

Moggy 12-07-2009 07:52 AM

well I have about 6" of cellulose blown in past the 2x6s so that's approx 12" or so of blown in insulation. At an r value of 3.7 per inch i should be at R44.4 ( or so) I wonder how much heat I'm losing through the hatches to the attic. All they are is a piece of Mdf with 1.5" insulating board stuck to the back. I did a weather strip last year but it's kinda beat up. when I did the vents for the fans this year I foamed the crap out of the bathroom fixture. before, there were some serious gaps around it. Plugging the gable vent made a significant difference. There is still some minor melting off the roof, but you have to wait and listen whereas before it was quite obvious. Thanks for all your help, folks.

GBR 12-07-2009 09:02 PM

The total attic venting of 1/300 is if you have a vapor barrier on the ceiling. If no v.b. present use 1/100. You are figuring the ridge vent numbers wrong. A good, high end ridge vent with baffles gives 18 per ft. x 36' = 648/144 = 4.5sq.ft. So with 1/2 intake at soffit, and 1/2 exhaust at ridge, you are just right. As said block the gables.

Ice dams are forming because you are losing heat from the ceiling or just from the fan. I would route it out the roof above rather than to the soffit where it is recirculated into the attic by the intake vents. Did you seal all pipes and wires at the plate line walls?
The styro baffles have 25% less air flow than the plastic from the other box store. And the plastic ones protect the ends of the insulation pile from the cold soffit air there: Berger Building Products And you won't pay extra for the special wall separator: Windblock
Energy Savers: Attic Access Insulation and Air Sealing Stay warm....

Be safe, Gary

St-rider 12-23-2009 10:51 AM

do you have hvac duct work in your attic?
i added a forced air system to my house with the ducts in the attic space and have had ice dam problems ever since. i'm now planning on covering the duct work with more insulation.

Moggy 12-23-2009 07:35 PM

That's a good thought, but I don't have any ductwork up there. I think I've got it pretty good though. I only get meltiing now if it's above 30ish degrees. If its in the 20s it stays cold enough not to melt. But, at 30 it does melt, drip and freeze on the back stairs. Makes for an interesting trek to the car in the morning, hence the term "death stairs." Oh well. I guess it is what it is. Not sure if I should expect it to be any better than it is.

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