Home & Garden

Ice dam caused by vent stack

  • Last Updated:
  • Dec 21st, 2014 7:33 pm
Tags:
None
Deal Guru
User avatar
Oct 24, 2012
11641 posts
2620 upvotes
Montreal

Ice dam caused by vent stack

This is only my 4th winter in my current house so I haven't experienced all types of "weathers" yet so I don't know if something is wrong.
I know that my attic has a vapor barrier sheet and is insulated with R52 fiberglass (Did it myself before the first winter). So unless something got broken up there (no mouse problem), the insulation should be fine.

The main part of the ice dam is really just in front of the vent stack over my main bathroom. Considering the vent stack is copper, I assume that heat from running water transfers much more easily up into the attic and right over the roof area around the stack.

Is this something normal when we have such mild temperatures that dip below 0c at night?

It is a bit embarassing, a lot of neighbors have ice dams, but mine has icicles reaching 4 feet of lenght :razz:
23 replies
Deal Addict
Sep 7, 2010
1338 posts
134 upvotes
Calgary
Is the pipe just transferring heat to the roof and melting the snow? Is the roof metal as well?
Deal Expert
User avatar
Feb 8, 2014
32144 posts
15423 upvotes
Socially Distanced
Pictures would be helpful
In fact in Rand McNally they wear hats on their feet and hamburgers eat people
Deal Fanatic
Dec 9, 2003
5132 posts
981 upvotes
Calgary
Metal vent stack + high humidity inside + long showers (or craps)+low temperature inside = lots of icicles. Not your attic insulation problem - but you could wrap the vent stack with insulation and it would help
Deal Guru
User avatar
Oct 24, 2012
11641 posts
2620 upvotes
Montreal
mangina11 wrote: Is the pipe just transferring heat to the roof and melting the snow? Is the roof metal as well?
No, but the vent stack is exposed in the attic as it goes through the roof, so it radiates heat inside the attic, causing the roof area around the vent stack to be above 0 celsius, causing the snow on top of it to melt and drip.
Here's a great 5 years old's drawing
The red lines are radiating heat :razz:
[IMG]http://i57.tinypic.com/2rcr40o.png[/IMG]
Vent stack is hot, warms up the roof as well as melting the snow around it on the outside part, snow on the roof melts, drips to the gutter, freezes in the gutter, gutter fills up with ice, water flows over the gutter and drips down forming icicles.
Quentin5 wrote: Pictures would be helpful
I broke the ice dam with a broom handle last night and it snowed over the balding spot on my roof, not much to see anymore for now :(
Cough wrote: Metal vent stack + high humidity inside + long showers (or craps)+low temperature inside = lots of icicles. Not your attic insulation problem - but you could wrap the vent stack with insulation and it would help
Yeah I might just do that next time I go in the attic to install a ceiling light.
Deal Expert
User avatar
Feb 8, 2014
32144 posts
15423 upvotes
Socially Distanced
I am amazed here is enough heat to cause that, probably 95+% of the heat in the water goes down the drain, how much hot water are you using?

I would not be surprised if there is a different problem causing this, do you have a second washroom you can use for a few days next time we get snow?
In fact in Rand McNally they wear hats on their feet and hamburgers eat people
Deal Guru
User avatar
Oct 24, 2012
11641 posts
2620 upvotes
Montreal
Quentin5 wrote: I am amazed here is enough heat to cause that, probably 95+% of the heat in the water goes down the drain, how much hot water are you using?

I would not be surprised if there is a different problem causing this, do you have a second washroom you can use for a few days next time we get snow?

The problem is that the outside temperature is only 1-2 degree celsius below 0, it doesn't take much to get the snow to melt.
Also, that side of the roof faces south, so it gets a lot of sun.

Even the south side of the roof over my unheated garage has a bit of an ice dam.

The whole north side of the house has no icicles whatsoever, and that is where the other vent stacks are located.
Deal Expert
User avatar
Feb 8, 2014
32144 posts
15423 upvotes
Socially Distanced
alkizmo wrote: The problem is that the outside temperature is only 1-2 degree celsius below 0, it doesn't take much to get the snow to melt.
Also, that side of the roof faces south, so it gets a lot of sun.

Even the south side of the roof over my unheated garage has a bit of an ice dam.

The whole north side of the house has no icicles whatsoever, and that is where the other vent stacks are located.
You may have solved your issue right here, but if it makes you feel better you could wrap the pipe with some insulation in the attic, since its only a plumbing stack you can use almost anything that will fit, no worries about fire resistance.
I'm still kind of confused though, can you use one of those north side stacks to see if it affects this problem?
In fact in Rand McNally they wear hats on their feet and hamburgers eat people
Deal Guru
User avatar
Oct 24, 2012
11641 posts
2620 upvotes
Montreal
Quentin5 wrote: I'm still kind of confused though, can you use one of those north side stacks to see if it affects this problem?
The north stacks are for different drains. One is for the kitchen which is 2 floors down from the roof, so a long distance before the roof and not much hot water. The other stack is for the laundry room, which again doesn't use much hot water, just warm water.

The problematic stack is really evident, the roof section around the stack (And over the main bathroom) is barren of snow while the rest of the roof is snow-covered.
Deal Addict
May 23, 2009
3681 posts
2388 upvotes
Mississauga
Mine is wrapped in insulation
Deal Fanatic
Dec 9, 2003
5132 posts
981 upvotes
Calgary
As above
Cough wrote: Metal vent stack + high humidity inside + long showers (or craps)+low temperature inside = lots of icicles. Not your attic insulation problem - but you could wrap the vent stack with insulation and it would help
Member
Jul 14, 2012
338 posts
146 upvotes
Hamilton
IMO, it's very unlikely to be the stack causing the problem. I would be looking at your gutters/downspouts, and the ventilation in your attic.

When was the last time you cleaned the gutters/downspouts?

You mentioned that it's R50 up there, but is there proper ventilation? Are the soffit vents clear of obstruction?
Deal Guru
Feb 9, 2006
13378 posts
8308 upvotes
Brampton
The stack should be insulated regardless to help prevent moisture issues in the attic.
Deal Expert
User avatar
Feb 8, 2014
32144 posts
15423 upvotes
Socially Distanced
alkizmo wrote: The north stacks are for different drains. One is for the kitchen which is 2 floors down from the roof, so a long distance before the roof and not much hot water. The other stack is for the laundry room, which again doesn't use much hot water, just warm water.

The problematic stack is really evident, the roof section around the stack (And over the main bathroom) is barren of snow while the rest of the roof is snow-covered.
I would still like to see it not used for a few problem days and see what happens, but that does not seem feasible.
I suspect you have an air leakage issue, warm air going from the house into the attic where the pipe goes through the ceiling, did you air seal the framing around the stack where it enters the attic, or do you remember if it is airtight? I doubt you would have paid attention when you insulated, but it is the most likely explanation if the problem is being caused by heat from indoors. It is possible to access where the stack enters the attic and have a look? You can seal any openings with a can of spray foam.
tebore wrote: The stack should be insulated regardless to help prevent moisture issues in the attic.
I don't think this will be a problem, if its warmer then ambient that means no condensation is produced on the pipe, and colder then usual means it would condense vapour already present, not bring in new vapour (and how could it be colder, people use warm water or at worst ground temperature water, we dont have massive piped water chilling tanks in our basments, we have water heaters. The OP says his problem is heat anyways.
In fact in Rand McNally they wear hats on their feet and hamburgers eat people
Member
Apr 28, 2014
303 posts
175 upvotes
Waterloo, ON
Quentin5 wrote: I suspect you have an air leakage issue, warm air going from the house into the attic where the pipe goes through the ceiling, did you air seal the framing around the stack where it enters the attic, or do you remember if it is airtight?
This is likely it: air leaking around where the stack enters the attic, and not heat from the stack itself. Like you say, alkizmo, when it's just below zero it doesn't take too much extra heat to melt a bit of snow near the top. But I still wouldn't expect the plumbing stack to be able to do it, without a bit of air leakage.

Or it really could just be the sun if it's been particularly sunny during the day where you are.
Banned
User avatar
May 4, 2006
2914 posts
180 upvotes
Hamilton
Any vent pipe running through unconditioned air space should always be insulated. OP, how many inches/feet (roughly) is the top of the vent pipe from the roof?

Where the vent pipe goes through the ceiling, how is that sealed? Also, is the stack near the bottom of the roof where ice dams tend to form more easily?
Deal Guru
User avatar
Oct 24, 2012
11641 posts
2620 upvotes
Montreal
freeloader1969 wrote: Any vent pipe running through unconditioned air space should always be insulated. OP, how many inches/feet (roughly) is the top of the vent pipe from the roof?

Where the vent pipe goes through the ceiling, how is that sealed? Also, is the stack near the bottom of the roof where ice dams tend to form more easily?
Stack goes about a foot above roof.
The stack goes through a 6mil polyethylene sheet that acts as a vapor barier between the heated space (rooms) and the attic. There is tuck tape and acoustic sealant around the stack.
Yes it is near the bottom of the roof.

Here, I forgot I had taken a picture before breaking the ice.
The tiny window in the middle if the bathroom.
[IMG]http://oi58.tinypic.com/2chory9.jpg[/IMG]
Deal Expert
User avatar
Feb 8, 2014
32144 posts
15423 upvotes
Socially Distanced
That is not caused by heat in the stack, the "sphere of influence" is way too large for it to be that.
Polyethylene is not an air barrier, a vapour barrier can act as an air barrier but the two are very different (i've been reading a lot about this recently). Have you checked if its actually blocking air in that area (a blower door test would be ideal but you would never have had any reason to do so until now)
The insulation you have is also a mixed bag, fiberglass is a poor performer, especially in a vented attic because air goes through it like a colander.
I suspect you have some sort of heated air escape into that area, my air around the stack idea does seem kind of unlikely given the large area affected, but i can't believe this can possibly be caused by heat in the stack, it just does not add up.
If there a washroom ventilation fan, is it vented into the attic by any chance?
In fact in Rand McNally they wear hats on their feet and hamburgers eat people
Sr. Member
User avatar
Jan 3, 2008
978 posts
268 upvotes
W.GTA
I have to agree with Quentin5, heat is sitting there with no place to go. Check with the venting of that area.
Deal Addict
Mar 8, 2012
2720 posts
1462 upvotes
Ottawa
Where does the bathroom fan vent?
prae praesto filum clostrum

Top

Thread Information

There is currently 1 user viewing this thread. (0 members and 1 guest)