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-   -   Ice dam in the making (http://www.diychatroom.com/f9/ice-dam-making-163119/)

burnt03 11-12-2012 05:38 PM

Ice dam in the making
 
Had an early snow here yesterday/last night, here's a pic of my roof:

http://img32.imageshack.us/img32/4497/img3564vl.jpg

Supposed to have above-freezing weather for the next week so it should all melt off, but this is bound to happen again once it gets cold and I'll have a bit chunk of ice hanging off the roof. Downspout is just to the left of the picture, will probably freeze solid too.

What can I do about this?

bbo 11-12-2012 05:49 PM

I'm sure many others will chime in, but I see two approaches

1) treat the symptom - use something to melt the ice dam - either heat or salt.
there are heat wires you can plug in to melt - or those "socks" filled with ice melt.

2) treat the issue
ensure adequate insulation in ceiling inside attic and all vents under the eaves are clear and there is adequate ventilation near the roof peak.

I also noticed what looks like a bath fan appears to be melting the snow. not sure of the proper fix, but i suspect getting it to blow up into the air instead of down the roof would help.

be interested to see when the pros chime in if i am on the right track.

burnt03 11-12-2012 05:52 PM

I was actually just in the attic a couple days ago air sealing things, so I know that the insulation is fine. It looks as though it's just the bathroom fan exhaust that's causing the melt.

gregzoll 11-12-2012 06:35 PM

If you have not had problems the last time it snowed, I would not worry. The melting is just from the heat blowing out of the vent fan on the roof. Overall, as long as the snow stays as it is overall, that would mean that you did a good job in keeping heat from escaping in the living area of the home's envelope, into the attic.

joecaption 11-12-2012 06:38 PM

Looks like your gutters may need a few more hidden gutter hangers.

the roofing god 11-13-2012 12:50 AM

Insulation is key to keeping the snow from melting,,,the bathroom fan vent is fine,,blowing up would allow water to blow in ,,,Issue I don`t like is the dripedge over the gutter,,with frozen gutters,,,ice can creep under the dripedge into your overhang,,defeating your waterproofing system above,,I prefer to use a 3"x3",,or 4"x 4" piece of metal flashing bent to match the roof pitch,,I call it backflashing,,the one side of the backflashing goes down behind the gutter,ultimately with a tripolymer sealant against back lip of gutter where it contacts the flashing--other half of backflashing extends onto shingle roof,,closing gap between fascia /sheathing,,,and Ice and water shield seals to it

Windows on Wash 11-13-2012 08:41 AM

+1

If the melting is as a result of the warm air from the bath fan exhaust, not to worry.

If the melting is from warm air in the attic, more of an issue. Air seal an insulate if that is the case.

the roofing god 11-13-2012 03:10 PM

the melting would be more even,if that was the case,,just saying----3 points to stop ice forming are having the proper amount of attic insulation,,which will prevent wasted heat in attic from melting snow on the roof,,,r-38 is minimal here,,,,then ventilation to remove excess heat..idea being keeping the attic temp as close to the outside as possible to prevent condensation,etc,,and proper waterproofing as briefly outlined above

Windows on Wash 11-13-2012 04:46 PM

I would add to your list of 3 and point out that air leakage is just as important as insulation when it comes to preventing ice dams.

It is tough to get enough R-Value at the outside wall top plate and the air leakage at that location is usually as culpable for the ice dam as anything else.

the roofing god 11-14-2012 07:54 AM

True,,,but that is already on my list under proper insulation,,,all pipes, ducts,and high hats should always be insulated as they move thru floorspaces,and can defeat insulation by allowing air thru ,,,The airspace at the rafter joists ,,It is important to allow for insulation,,but just as important at that area to provide space for air intake (as part of ridge/soffit vent system),venting the wasted heat at that area is very important,,,Today with a new home,,the insulation value can be reached by using spray foam,with much higher r-values,,typically about 6.5 R per inch thickness

Windows on Wash 11-14-2012 08:53 AM

I don't see air sealing on your list under attic insulation. The two are certainly exclusive and plenty of homes have R-38 and still leak like sieves.

Spray foam on attic floors as an insulation is a big waste of money in my opinion. Spray foam should be used as a strategic air sealant in those cases loose blown cellulose works great and is pennies on the dollar compared to foam.

Air sealing the outer top plate is critical and if the spacing between the baffle and the top plate is tight, you can apply additional foam there in the CC SPF variety to get that R-6'ish per inch and get as much R-Value at that location.

Air loss is invariably the bigger issue and while R-38 is great (I actually recommend R-50 because insulation is cheap once you are up there) the heat loss after R-25 gets statistically insignificant.

burnt03 11-14-2012 01:19 PM

Quote:

Originally Posted by Windows on Wash (Post 1051909)
I don't see air sealing on your list under attic insulation. The two are certainly exclusive and plenty of homes have R-38 and still leak like sieves.

Spray foam on attic floors as an insulation is a big waste of money in my opinion. Spray foam should be used as a strategic air sealant in those cases loose blown cellulose works great and is pennies on the dollar compared to foam.

Air sealing the outer top plate is critical and if the spacing between the baffle and the top plate is tight, you can apply additional foam there in the CC SPF variety to get that R-6'ish per inch and get as much R-Value at that location.

Air loss is invariably the bigger issue and while R-38 is great (I actually recommend R-50 because insulation is cheap once you are up there) the heat loss after R-25 gets statistically insignificant.

How can I air seal the outer top plate? With the pitch of my roof, I can't get that close to the outside edge... unless I pull off the soffits?

Is the sealing the same as how I sealed the interior wall top plates? If so, I'm going to have to be doing it with a mirror and my finger :(

Windows on Wash 11-14-2012 02:33 PM

Quote:

Originally Posted by burnt03

How can I air seal the outer top plate? With the pitch of my roof, I can't get that close to the outside edge... unless I pull off the soffits?

Is the sealing the same as how I sealed the interior wall top plates? If so, I'm going to have to be doing it with a mirror and my finger :(

Most of the 2-part kits will spray a good distance and we have previously rigged up an extension trigger before. It can done as can rigid foam be slid to the outside edge.

jagans 11-14-2012 05:14 PM

Ice Damming
 
The key to avoiding ice damming is just as another responder said. You need to keep heat from within the structure from escaping and melting the snow above the eaves. Ice damming is much more prevalent in truss construction where a severe pinch condition exists at the eaves due to using 2x4 dimensional lumber. It is exacerbated by minimal slope like 4/12 also.

Trusses made of 2 x4's restrict airflow through the soffit and up to the ridge. You want to maintain the temperature on the underside of your roof deck as close as possible to the exterior temperature.

Were I you, I would elevate the fan outlet so that it isnt shooting down right onto the surface of the roof. Maybe you could move it to a slope that is not in the sight line, or run it out a gable end? Not sure if you have a bi-hip or not.

Back when everything was stick framed, Ice damming was not even an issue, because we used at least 2 x 6 framing lumber for rafters.

If you use trusses at 4/12 you had better be installing SBS peel and stick Ice dams material at the eaves, to at least 2 feet inside the interior wall line. I spec 3 feet. (Im a Registered Roof Consultant)

jagans 11-14-2012 05:30 PM

No, no, no
 
You do not want to seal off air flow at the eaves. you want air at outdoor temperature to flow up through the soffit under the roof deck and out the ridge or gable end vents. Please read my other post. It is important to understand the concept, and physics. The eaves hang out over the exterior wall, and are therefore cooled from below. Think of the signs you see on the highway "Bridge Freezes before roadway" Thats because air is flowing under the bridge, as well as over. If you stop outside air from flowing up under the roof deck and out the ridge, interior heat will eventually melt the snow above, where it will flow down and freeze to the shingles above the soffit, (Bridge Condition) at the eaves. That is what is known as ice damming.


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