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Joe F 02-11-2010 07:26 AM

Damn Ice dam -damn!
It seems that once every 5 years or so, we have an ice dam issue. My attic is pretty well insulated, so I believe that my ice dams are caused by a combination of temp (30-35) and sun warming near the peak of the roof. Melting starts, and the water runs. The north side of my house gets little to no direct sun in the winter which causes refreezing down near the gutter. Does this theory make sense? What can I do? Gutters are full of ice.

Big N8 02-11-2010 07:43 AM

We have the very same issues here in MN. The refreeze is just a part of life. Some use a heated cord to keep the ice from forming on the eves.
That link is good stuff to use.

handy man88 02-11-2010 08:23 AM

As well insulated as an attic can be, it won't be as the outside temperature because a lot of newer homes have furnaces in the attic and gas flues which radiate heat.

Grumpy 02-11-2010 09:02 AM

Insulation is only half the equation. Ventilation is also key. When you say you believe it's well insulated, what do you mean? To today's standards you'd need approximately 12" regardless of insulation type to be considered well ventilated IMO.

As for ventilation, there is a science to the attic ventilation. What kinds of vents do you have and how many? Do you have an intake vent at the overhang? You might possibly need to add more vents.

Ice damning is primarily caused by excessive heat build up within the attic, which melts the snow from the bottom side, and then the snow refreezes, turns into a damn, more melting, more freezing. You've got a problem. The attic space should always be as close to the exterior temprature as possible. While that's somewhat unrealistic, it's still the goal. When it's 15 degrees outside your attic will also be 15 degrees? No, but you'd ideally want the attic also below freezing. Keeping the heat inside the living space is important and is accomplished with insulation. Allowing what heat does collect inside the attic is accomplished with a properly designed and well balanced attic ventilation system.

If you have a heater or furnace within the attic, then the ventilation must always be increased to allow for this heat to escape as much as possible. I understand why the furnaces are installed inside the attic spaces, to save living space, but it's just a plain bad idea that usually causes alot of problems IMO. It can be dealt with though, like I said when designing the attic ventilation system it must be designed with the furnace in mind.

I refer you to a link on my website which will explain in a little more detail as well as a manufacturer of attic ventilation which will explain more about how an attic's ventilation system works.

Even with proper insulation and ventilation, ice damning is a fact of life. However with insulation and ventilation the ice damn should be minimized. When your roof is/was replaced it should have an ice and water shield installed no less than at the gutter lines. This should extend from the gutter to at least 24" past the exterior wall. If you have an ice and water shield on the roof, if an ice damn does form it will prevent that ice damn from getting into the attic space. It forms a barrier that the ice can not penetrate.

Gutter and roof heating cables are also an idea which work with mixed results. I install one or two of these jobs per year. What I tell my customers is don't expect there to be no snow or ice on your roof, just expect that yours will melt off sooner than your neighbor's. The heat produced by these cables is very gentle maybe 50 degrees or so. HOWEVER you can sometimes cause more harm than good when installing these cables on the roof if you install them improperly. You can also void the manufacturer and or roofer's warranty so you may want to check with them before installing these heating cables.

jpsmith 02-11-2010 09:19 AM

Joe F,

I think your theory holds water. Being a DIYer, I can't offer an answer as thorough as Grumpy the Roofer, but I can say that I, too am having ice dam issues for the first time in the seven years that I've owned my house. In my case, the water is getting into the soffits. My attic space is below freezing. I'm pretty sure this time around it's a perfect combination of 2' of snow on the roof, temps around 30F and sunny during the day, and 10-15F at night. My roof doesn't have any drip-edge or ice shield. I'll be re-shingling this year I think, and I'll definitely fix the problem when I replace the roofing.

toeey1 02-11-2010 09:47 AM

Im having major problems with ice dams as stated in anopther thread. I came from a 60 year old bungalow that had little to no insulation in it and just a bare minimum of roof venting. I got ice dams pretty bad but never had any water seepage into the house.

I just movbed to a 24 year old home with insulation however because of a poorly insulated and bventilated vaulted ceiling, im getting major ice damming again only this time im getting major water leakage into my garage attic. Insulation companies couldnt help me, they all recommended I hire a roofer.

MJW 02-11-2010 10:10 AM

Great post Grumpy.

Just to add to the occuring scenario with this years winter weather........The heat that is present in the attic can possibly be above freezing, either from warming outside temps coming in the soffits or heat loss from the house. This creates a problem especially when we have deep snow and your venting on the roof is covered. The heat will come out the vent and start to melt under the insulation of the snow. This will then run down under the snow and create ice dams and other problems. This makes it possible to have ice dams almost anywhere on your roof.
When an area gets too muc snow and not enough melt, you must open up the venting. I recommend hiring a professional with proper insurance to do the roof "winter maintenance" for you.

Joe F 02-11-2010 03:09 PM

Grumpy, thanks for the great tutorial on ice dams. I guess I'm trying to determine if the current weather conditions are responsible for the ice dams independent of the condition of my attic insulation and ventilation. Using your 12" insulation guideline, I probably fall a little short, but not too bad. On the ventilation side, my 50 year old ranch style house has 2 large gable vents (30" x 40", 1800 sf attic). Is that enough? The thing that has me thinking, is that I only get ice dams on the shady side of the house when we have above freezing days and below freezing nights.

FYI, I'm due for a new roof either this year or next. What can I do during the re-roof process to help address the problem?

concretemasonry 02-11-2010 03:29 PM

To ventilate properly, you need both roof or gable vents AND then enough soffit vents.

The air temp is just an average and some areas are different.

If practical, a snow rake to pull off snow for 2' or more and let Mother Natures sun do the job even on the shady side since ice and snow transpirate. It does a good job even on difficult roof layouts.


drohead 02-12-2010 01:32 PM

Solve Ice Damm Problems for good
While I can agree with your observation that many homes are poorly insulated and have areas were heat loss from the home causes problems and should be focused on to remedy, there are many factors as to whether a COLD ROOF will stop ice dams. With so many architectural challenges by poor designs, dormers, gables, cathedral ceilings, getting to a cold roof via ventilation can be difficult and extremely costly. Your article does not address a basic fact of Mother Nature… THE SUN. What do you do about radiant heat from the sun everyday? Most homes will see some sun exposure during the day. That heat melts snow and creates moisture on the roof, typically following the slope to the eaves and valleys. Then the temperature drops and the melt refreeze. If this happens over a couple of days... ICE DAM! Even on a well vented cold roofs like you have described. Keeping valleys, eaves and gutters clear can only be achieved by keeping the refreeze from occurring by keeping the melt liquefied and allowing it to get off the roof and then too the ground. There are products out on the market that can prevent ice dams and keep them from forming.

BamBamm5144 02-12-2010 04:51 PM

Joe - It is hard to know if it is enough without seeing the actual attic or knowing the exact square feet of the attic. There is an actual formula to determine if you have the proper amount of ventilation. You could have 10 of those gable vents, but without any soffit vents, they would be useless. I hear a lot of how important it is to have a balanced attic intake and exhaust although I am not too sure how important is along as they are similar.

In the meantime, I dont suggest taking any snow off of the roof. I think this is more of a sales gimmick than anything else. With a properly vented roof, it should all melt off evenly. I also dont think there is anything you can do until you have the roof properly replaced and when having it replaced, your ice damming issue will only be solved if the intake (soffits) are also done while doing the roof. I believe the best system to know be a ridgevent, however, you must close up the gable vents to make this possible. Closing them could be as easy as putting plastic over them from the inside if secure enough.

As usual, Grumpy gave you the best information and I suggest you follow his advice.

handy man88 02-12-2010 06:53 PM

I'm considering having having my installation topped off. I already have cardboard (I think) baffles in place between the rafters.

One option offered by the installer was for styro vents which would be stapled in place. They cost $3 each, and to me, it sounds very labor intensive and probably worth it, but not sure if I need to replace the existing cardboard baffles.

A direct benefit is that if the installer is told to install these new vents/baffles, the tech will be able to check and ensure that the soffit is not blocked.

So, what's the benefit of having styrofoam vents?

Grumpy 02-13-2010 04:49 PM

Styrofoam, cardboard, plastic, it doesn't matter what they are made of so long as they provide no less than 1" preferrably 1 1/2" of positive air flow into the attic. It's very ez to block intake with blown in insulation. By installing the baffles first you can actually be clumsy when blowing in the insulation and be sure you're not blocking intake.

handy man88 02-13-2010 07:10 PM


Originally Posted by Grumpy (Post 399363)
Styrofoam, cardboard, plastic, it doesn't matter what they are made of so long as they provide no less than 1" preferrably 1 1/2" of positive air flow into the attic. It's very ez to block intake with blown in insulation. By installing the baffles first you can actually be clumsy when blowing in the insulation and be sure you're not blocking intake.

I heard from an installer that the styrovent baffles are installed in addition to the existing baffles if the existing baffles do not offer the requisite height required to achieve the desired R value. They are actually stapled onto the roof's sheathing.

One installer told me they cost $3 each, while another installer said that they are included in the price.

Bob Mariani 02-13-2010 09:08 PM

I only scanned the other posts, but I see alot of mentions of insulation and venting. According to all science and what is required by BPI which is the only accredited authority on this subject to get government rebates to fix this problem the real solution is air sealing. When I have more time I will try to post a blog explaining how we fix this. And it is fixable whether your roof is vented or not. We test where the leaks are with a blower door test. But a poor mans test to find air leaks is to look at your roof a few days after a good snow. Houses where all the snow is still there are sealed correctly. houses with no snow have not insulation. But the majority of the houses will have areas of melted snow.... these areas are exactly where the air sealing is not done correctly.

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