Replies

1. 
   csnow | Sep 28, 2004 07:37pm | #1

   Seems like you would want to stop the heat at the source first, meaning insulating the duct work itself.  Also, check for leaks in ductwork.  If you see duct tape for sealing joints, odds are it has failed by now.  Use mastic or foil tape.

   Also look for other sources of air leakage through the attic plane, such as leaky hatches, electrical/mechanical penetrations, open stud bays, etc...

2. 
   AndyEngel | Sep 28, 2004 09:27pm | #2

   Since the HVAC system is in the attic, it's likely not just heat radiating from the ducts, but hot air being forced from leaky ducts that's the problem. I'd look into insulating the bottom of the roof deck and the gable walls with closed cell foam, closing the vents, and making the attic conditioned space, which approach is approved by the 2003 IRC. Any FG she puts up there won't help at all with the duct leaks, so even though it's cheaper, it isn't likely to actually do much good.

   After that, she probably wants to do some moisture proofing in the basement, and seal off any open framing cavities. See http://www.buildingscience.com for more info.

   Andy

   Arguing with a Breaktimer is like mud-wrestling a pig -- Sooner or later you find out the pig loves it.

1. 
   TommH | Sep 28, 2004 10:41pm | #3

   Andy, thanks for the reply. I'm not sure I understand your point about fiberglass not doing any good. Isn't the point to keep the heat away from the roof deck? Why wouldn't fiberglass batts between the rafters accomplish this, although maybe not as well as the foam? Also, if the objective is limited to the ice problem on the north side (and not improving the overall effectiveness of the heating system), is it really necessary to button up the entire attic? If it were foamed up as you suggest, are there any issues with moisture buildup in the attic?

1. 
   AndyEngel | Sep 29, 2004 12:23am | #4

   FG works OK in a situation where it's encapsulated. Put it somewhere in the open air where there's a high difference in temperature, such as between a 110 F. hot air duct and a roof that's at 0 F., and convective currents develop within the FG that reduce its real R-value to something like 0 per inch. It becomes little more than an air filter. That doesn't happen with foam.

   Because warmer fluids (air) rise, a house with open roof vents and pathways between the lower levels and these vents (attic hatches, open framing cavities, etc.) acts as a chimney. This is the stack effect. The stack effect can suck huge amounts of both heat and moisture into an attic, causing moisture problems and ice dams. The suction created by stack effect can cause combustion appliances on lower levels to back vent, and suck radon and moisture into the house through the foundation and gaps in framing. By foaming the entire roof deck and gable walls, and closing off the vents, you in effect close the top of the chimney, and dramtically reduce the stack effect, and its effects. It's still a good idea to close off and weather strip the aformentioned passages, but closing off the attic substantially reduces the horsepower of the stack effect's engine.

   I'm just spouting what I've learned from others. Go to the oracles, http://www.buildingscience.com and http://www.affordablecomfort.org.

   Andy Arguing with a Breaktimer is like mud-wrestling a pig -- Sooner or later you find out the pig loves it.

3. 
   DANL | Sep 29, 2004 01:05am | #5

   I'm not so sure an overhang would be a help as much as it might make things worse. An overhang would allow all that thawed ice and snow to refreeze (since the overhang and soffit would be cool) and then you really would have an ice dam that would cause the remaining water to back up further up the roof, where it would eventually find a way in and wet the walls or ceiling inside and really make a mess, not to mention soaking the insulation and making it worthless and possibly rotting out studs or rafters.

1. 
   TommH | Sep 29, 2004 06:26pm | #6

   You are probably right and this may explain why there hasn't been any interior water damage. The reason I mentioned it was that the absence of soffits make it difficult to vent the roof with soffit vents to help keep the roof cold.

1. 
   DANL | Sep 30, 2004 12:13am | #7

   Right--no soffits will make it hard to vent--all the more reason to go with a "hot" roof as was suggested by someone else.

2. 
   DavidThomas | Sep 30, 2004 01:21am | #8

   No soffit means you can't drill holes in the soffit, cover with screen and have a bottom vent to a cold roof.

   But it doesn't mean you can't have a cold roof.  I'm thinking 4" ABS that daylights on the gable ends.  With an 4x4x1.5" vent tee for a lateral to each bay. 

   Then place the foam / cardboard spacers in each baby and spray an inch or two or foam to seal it all up.  (having installed a ridge vent for the air to exit.

   You could even get more aggressive about the venting (later if you choose) and put an in-line fan in the 4" to make it a powered vent.  Run the fan through a freeze-protect snap-switch such that it comes on below 35F.

   This would be fair bit of work and probably only worth it if the attic has some value as conditioned or semi-conditioned space.

   Otherwise, I'd insulate the duct, insulate the ceiling, and vent the whole attic

   David Thomas   Overlooking Cook Inlet in Kenai, Alaska

4. 
   jesica123 | Feb 27, 2020 03:56am | #9

   How did you solve your problem?
   I think you can consider installing a heating cable on the roof.
   I used a self-regulating heating cable which can save me much money. Like this:http://heatitcable.com/product/heatit-jhsf1-self-regulating-pre-assembled-heating-cable