Ice dams and Underlayment

[Ben J. Gromicko]

Where ice dams may form along the eave because snow continually freezes and thaws or frozen slush backs up in the gutters, the underlayment application in the area of the eaves is modified to prevent ice dams from forcing water under the roofing, which could damage ceilings, walls and insulation.

Two layers of underlayment should be cemented together with asphalt cement from the lowest edge of the roof and continue up the roof to a point that is at least 24 inches inside the interior wall line of the building as show in the illustration.

The environment within the envelope of the building provides adequate warmth to prevent ice dams from forming above the heated space; therefore, the two layers of cemented underlayment are permitted to terminate 24 inches inside the interior wall line of the building. There are exceptions to the rule.

[Brian A. MacNeish]

Originally Posted by bgromicko

Where ice dams may form along the eave because snow continually freezes and thaws or frozen slush backs up in the gutters, the underlayment application in the area of the eaves is modified to prevent ice dams from forcing water under the roofing, which could damage ceilings, walls and insulation.

Two layers of underlayment should be cemented together with asphalt cement from the lowest edge of the roof and continue up the roof to a point that is at least 24 inches inside the interior wall line of the building as show in the illustration.

The environment within the envelope of the building provides adequate warmth to prevent ice dams from forming above the heated space; therefore, the two layers of cemented underlayment are permitted to terminate 24 inches inside the interior wall line of the building. There are exceptions to the rule.

Ben:

You're not understanding ice dam formation!!! It's exactly that heat which you claim "prevents ice dams from forming above the heated space" (wrong concept) that produces the melt water that freezes at the cold eaves.

[Michael Roberson]

Thank you Ben, I think that is a good illustration, sums it up nicely.

[Jeffrey R. Jonas]

Brian, you are only partly correct. We've had this conversation approx 5 months ago. I suggest you go back and read the entire thread. I won't bother re-hashing the subject again... it's too dang hot out! Ironic, isn't it?!

[Brian A. MacNeish]

Originally Posted by jjonas

Brian, you are only partly correct. We've had this conversation approx 5 months ago. I suggest you go back and read the entire thread. I won't bother re-hashing the subject again... it's too dang hot out! Ironic, isn't it?!

The statement has no place in building science of a truly efficient structure. It essentially says "Your heat loss from the poorly insulated/airsealed ceiling is not a problem since it prevents ice damming over the heated parts of your house". It doesn't say "This heat loss is costing you heating $$$$$ and is probably the main cause of the ice dams/leakage at your eaves/exterior walls."

Heat loss at snow covered skylights on lower slope roofs has caused ice dams on the roofs below the skylight curb......not at the eave but within a foot or two or three of the lower edge of the curb, some times 10-20 feet up from the cold eave.

BTW......How many roofers have taken the time to cement together 2 layers of tarpaper? We never did in our 10 years of roofing. The time saving better solution is using "eave protection" such as WR Grace "Ice and Water Shield" and similar.

[Ben J. Gromicko]

Originally Posted by Brian A. MacNeish

Ben:

You're not understanding ice dam formation!!! It's exactly that heat which you claim "prevents ice dams from forming above the heated space" (wrong concept) that produces the melt water that freezes at the cold eaves.

That statement is basically sharing a quote from the IRC commentary.
To have an ice dam, you need BOTH melted water and solid ice (the dam). IRC believes that once you're past 24 inches, you will not have both. Beyond the 24 inch mark, the warm of the building will prevent both from simultaneously occurring and creating an ice dam.

[Brian A. MacNeish]

Originally Posted by bgromicko

That statement is basically sharing a quote from the IRC commentary.

It's a bassackward quote if they are talking about ceiling heat loss saving the upper levels of the roof from ice damming!! Except for cases of winter solar attic heating, ceiling heat loss is the driving force of the ice melting to begin with......why comment on it as if it's a positive???

BTW.... If IRC refers to Canada's group, I have had a few little "discussions" with the odd one of them about being secluded in the research field.......get out and see what the home owner/buyer/builder wants......don't do research in the absence of the real world and consumer demand!

I had one refuse to answer my question in an open forum at an energy conference about 10 years ago saying he didn't have time!!! The question put him in a position of having to defend energy savings versus costs of an energy conservation improvement he had researched and was recommending.

[James H. Bushart]

What Ben is leaving out is that code is a "basic minimum" protective requirement. The IRC does not mention the intent behind the 24 inch "minimum" requirement and would not object to double underlayment going back to the peak of the roof if the roofer wished to exceed these requirements...nor is there any claim that the 24 inch "minimum" requirement will ensure against damage from ice damming.

Perhaps the recommendation was originally 60" and a political compromise was made with the NAHB.

The wording of a code does not necessarily reflect building science.

[Bert B. de Haan]

In my humble (or not so humble) opinion, Brian is correct. The reason ice dams happen at the eave is because where the wall and the roof meet is a weak point in insulation so that is the place where there is a concentrated heat loss. This heat loss is what melts the snow on the roof (which is only 8 or so inches away.) Heat loss doesn't prevent ice dams, it causes them. The ice and water shield doesn't prevent ice dams but it keeps the water that creeps under the shingles because of the ice dam, from reaching the sheathing. If you have trusses with a high heel, you would be able to do a better job of insulating the wall to ceiling joint + the heat loss would have a better chance to get vented away before it would melt the snow on the roof.
(I think that the cementing together of two layers of roofing felt is a recommendation of the time before ice and water shield or similar products.)

[Jeffrey R. Jonas]

So what Bert and Brian are both agreeing on, is that if the home were to be 100% efficient and 100% heat loss free, then the home would have zero ice dams???

Give me a friggin break!!!

[Marcel R. Cyr]

I don't care how well and efficient an attic can be on heat loss and leakage, around here, you will have ice dams on most low emmisivity roof coverings.
Ice and water shield on the roof is your best second line of defense against damage.
It also protects the roof sheathing from the elements while pending the final roof coverings.
I have never heard of asphalting to layers of felt before nor seen it in 40 years.
Too many other products available that are more economical and quicker to use.

[Peter C. Russell]

http://www.statefarm.com/learning/lo...on_icedams.asp

[Michael Roberson]

Around here, ice dams are common and not always caused my heat loss. (although it is the #1 reason) Mostly in older homes with little to no insulation and/or venting. Its only smart to go at least 36" with Ice and Water shield, if not 72". Occasionally, the entire roof is covered with it, not just the eaves and valleys. I hope I dont ever have to take that stuff off, I hear sometimes its easier to replace the sheeting. (thats another discussion)

[Marcel R. Cyr]

Originally Posted by prussell

http://www.statefarm.com/learning/lo...on_icedams.asp

Good article Peter. Nice to here from you.
Hope you are keeping busy down there.

[Bert B. de Haan]

Originally Posted by jjonas

So what Bert and Brian are both agreeing on, is that if the home were to be 100% efficient and 100% heat loss free, then the home would have zero ice dams???

Give me a friggin break!!!

I'll give you a friggin break
Now that that is out of the way; have you ever seen ice dams on unheated buildings?