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icedams · 02-19-2009, 10:51 AM

I've read a few threads on this site concerning roof ventilation systems. Many of the posts suggest a ridge/soffit vent system, which is what I have. The past couple of winters I've had ice dams. I've learned that ice dams are caused by heat loss and/or poor ventilation. Having a finished attic and a heating system in the attic, I can see where the heat source may be. However, I don't seem to have the ice dams when the ridge vents are clear of snow. They seem to occur when the vents are blocked. This leads me to believe that ventilation may be the key issue. A contractor suggested that I install a couple of small gable vents to the sides to alleviate the ventilation issue when the ridge vents get blocked with snow. From many of the posts I've read it seems like that might not be a good idea. Any thoughts?

**Ed the Roofer** · 02-19-2009, 12:09 PM

What brand of Ridge Vent do you have currently?

The reason I ask, is that some perform MUCH better than others.

I use the Shingle Vent II nearly exclusively.

Last winter, I took photos of several roofs that I installed it on and after a 14" snowfall and also a 6" snowfall, the heat from the escaping flowage had melted the trough along the External Baffle sufficiently enough to be able to see that the vent was clear and unimpeded and exhausting as predicted.

If you don't know the brand name, then describe what it looks like.

Ed

icedams · 02-19-2009, 01:23 PM

Hi Ed,

Thanks for your replying.

I don't know what kind of ridge vent I have. We moved into the house last year. The house was built in 2000. I did take a couple of pictures. 2 are of the roof over the main house and 1 is of the roof over the family room. Not sure if you can gather enough info from these pictures.

http://picasaweb.google.com/mlclimbe...eat=directlink

It seems to me the ridge vents get blocked after 6 inches of snowfall and can remain that way for several weeks. It's also not uncommon to get more than a foot of snow on the roof. I've been told by roofing contractors that this isn't unusual for our area. We live in the Boston. Another interesting note is that the family room has a recessed ceiling, and we've never had any ice dams or ice forming on this roof.

dmc@RCR · 02-19-2009, 05:44 PM

icedams,

I have the same problem.
I live in the snow belt off Lake Erie. We get snow from Thanksgiving to Easter and my roof and ridge vents are covered for long periods of time.
I have installed large gable vents on my house to complement my ridge and soffit vents and have had no problems.

Take a look at this link for more information;http://roofingcontractorreview.com/R...cuit-Myth.html

icedams · 02-19-2009, 09:14 PM

Excellent! Does that mean you haven't had ice dams since you added the gable vents, or you haven't had issues with backdrafts ... snow and rain coming into the attic due to pressure differentials.

jaros bros. · 02-19-2009, 10:21 PM

Any extra ventilation can only help dissipate the heat loss you are experiencing and keep the ice dams from forming. Chances are that you have a lot less insulation in the attic than is required or there is little soffit ventilation. Due to the fact that you are thinking about installing gable vents I am making the assumption that there is a space above the ceiling. Are you sure you have venting from the soffit up into this area? How are the kneewalls, if you have them, insulated? This is sometimes a huge heat loss area. You should have continuous insulation from the top of the wall plates up through. How many inches of insulation do you have and what kind?

icedams · 02-19-2009, 10:49 PM

There are soffit vents from one side of the house to the other ... back and front.

There is space above the ceiling, but not much. The ceiling width of the finished attic is about 6 feet, so if my geometry calculation is correct there's about 3 feet of space from the ceiling to the peak. Of course you have to take the rafters into consideration, so it would have to be a small gable vent.

The attic floor is R-38 to the soffits. Baffles are in place in the rafters. The knee walls are insulated with R-19. From the top of the knee walls and on top of the ceiling is R-38.

I believe any potential heat loss is probably from the heating system. Maybe from the room. The heating system and ductwork are not in the "conditioned" space. I've thought about insulating pulling the insulation from knee walls and insulating the rafters, but that's a pretty sizable job. Not much room to work in with knee walls that are 2 1/2 feet high with 2 access doors.

jaros bros. · 02-20-2009, 07:38 AM

That very well may be your problem. Heat loss from the ducting is a common problem. Also, are your penetrations from wiring and mechanicals leading from the conditioned to unconditioned space sealed with spray foam or fire caulking?
Gable vents will help but studies show they only ventilate a few feet in from the end. Anything helps though if you are having problems.

dmc@RCR · 02-20-2009, 03:26 PM

icedams,

I have had no problems, either ice dams or draft/ventilation issues.

PerpetuallyRepairing · 02-25-2009, 08:57 AM

I got the same problem. Completely ineffective ridge vents when it snows. I would agree that combining ridge & gable vents should not be an issue but I would put a pan underneath the gable vents in case rain gets blown in from winds.

One thing I have read that does make sense: The combined area of the ridge & gable vents should not exceed the soffit vents. This would negatively pressurize your attic & cause more heat to be sucked out of the living spaces. Probably not as important if the house is well sealed.