Insulation Strategy to Prevent Ice Dams
We are doing a whole house gut remodel of a 1954 home in Minneapolis, MN (Climate Zone 6A). It is a single level house with a full basement that has a history of bad ice dams. We want to figure out the best insulation/air sealing/ventilation strategy to prevent future ice dams now that we have everything opened up to the framing. The exterior of the house including the roofing are not being touched. The main insulation challenge is the minimal heel height of the 2×4 rafters – about 4″. The existing insulation (since removed) was nothing in the walls and blown-in cellulose in the attic with ventilation chutes bringing air in from the soffit vents to about 5 box vents near the peak of the roof.
The current insulation plan for the remodel is 3″ closed cell spray foam in the walls and blown-in fiberglass in the attic with new ventilation chutes. The problem is that a 4″ space between the top of the exterior wall and the bottom of the roof sheathing leaves only about 2.5″ of insulation on top of a 1.5″ deep chute.
I would love to get advice on the following questions:
1. Would it be better to forget about ventilation, do a hot roof and spray foam the 4″ space full and spray foam under the roof sheathing? (this would be very expensive)
Sticking with a ventilated attic strategy…
2. Would it be better to spray foam the 4″ full, eliminate the chutes, and install a Edge Vent above the blow-in height? We’ve never used these and I don’t know how well they work if covered in snow and the homeowner doesn’t like the way they look.
3. Would it be worth the expense to do 1-2″ spray foam over the ceiling from above (“sealing the lid”) and then do blow-in on top of that? (in this case the insulators would not install the usual poly on the ceiling from below.)
4. Would it be a good idea to install “whirly bird” vents to help suck air out of the attic?
5. A further factor effecting air leakage into the attic is that there will be HVAC supply ducts running into the attic supply heat to the kitchen and den. These will be insulated to code and sealed from inside with Aeroseal, but would it be a good idea to encase them in spray foam?
Any advice is much appreciated.
Dave
Waterstone Construction
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Your question did push a few of my buttons and I may not have any answers for you but let’s talk about your choices.
1 Spray foam filled walls? I am guessing 2x4 16 on center. 3 inches of CCSF is R21 total wall R = 11.62 If you filled the walls with damp sprayed cellulose for R12 total wall = 9.62 for 1/3 the cost and a much greener choice. My guess it the ROI on the R 2 difference is over a thousand years
2 HVAC in a vented attic? Really it is total impossible to get heat there without going thru the attic? Please find a way to say no to stupid. Consider a concealed ducted mini split in a dropped ceiling.
3 Aeroseal? I think if you look at their pricing model you will find with some team training and care you can achieve their low priced target for almost no cost. And the higher priced targets are priced unaffordable.
As much as I dislike spray foam it does do something no other insulation can do and that is put the most R value in the smallest space. Consider putting your vent chutes in place and 2 feet of ceiling drywall then filling the gap with CCSF.
Walta
Walta, thanks for you advice. Can you tell me (or point to a resource) how you calculated the total wall R values? I'm new to this and it surprises me that R21 goes to 11.62 while R12 goes to 9.62. Thanks.
+1 on everything Walta said. If you can get demp spray cellulose or dense pack cellulose, it is the way to go. It will seal up the place just as well and have none of the potential issues with SPF gone bad.
One thing you can do to get a bit more height is use 1" baffles over the walls. This lets you put just a bit more insulation there and won't effect the airflow enough to matter.
The most important part in stopping ice dams is air sealing the wall top plates, unfortunately SPF is really the simplest way to do that. The fact that you get a bit more R value is a bonus.
If you do want to go SPF free, one option is to use a thick layer of rigid polyiso for the vent baffle by the soffits (if you have a 4" gap use 3" polyiso). You can seal this in place with canned foam. The rest of the attic insulation can be standard loose fill.
https://constructioninstruction.com/tool/r-value-calculator/
Just a few comments, in addition the advise already given. You did mention ice dams were an issue:
"3. Would it be worth the expense to do 1-2″ spray foam over the ceiling from above (“sealing the lid”) and then do blow-in on top of that? (in this case the insulators would not install the usual poly on the ceiling from below.)"
There are likely all kinds of air leaks into the attic, and many will be from interior walls where they meet the ceiling. Applying 2" of closed cell to the attic from above (with baffles at the eaves) resolves this issue and will help at the eaves where there will only be a few inches of insulation. If that roof deck warms at all, then you will get ice dams. Then you can blow in cellulose. I've used this approach here in zone 7A with excellent results, including eliminination of ice dams. If you are doing a full gut inside, spraying the top plate area of exterior walls to full depth spray foam will ensure a good seal/r value there, and then you do the dense pack for the rest. This is a nice balance of minimizing spray foam use, while ensure air sealing. Then you can do poly on the exterior walls, sealing to the top plate.
Again, spray foaming over attic ducts, and burying them in cellulose has worked well for us here in zone 7A. Obviously you want to avoid this if you can. We've also done ducts in a vented attic (commerical project) where a low ceiling made it difficult to incorporate the size of ducts required inside the envelope. These were as large as 18", but were carefully taped/wrapped, and then buried in cellulose. In that case the larger metal/spiral ducts were special ordered with acoustic (fibreglass) lining to deal with noise issues, so they had some R value at the duct interior as well. The upper floor of this commercial project was approx 3500 square feet and 100% of the HVAC ducting was in the vented attic, but again, with precautions to address air leaks and insulation, before the cellulose (about R80) was blown in. This building (approx. 9000 square feet in total) required less to heat (over a February where average temp was -20C ) than my home, at 1800 square feet. Like your project, it was an interior gut. Spray foam was only used at the 1st floor rim joist and a few 2nd floor skylight wells.
I talked to a fellow builder in Golden Valley, MN who did just what Dennis has suggested. He cleared the attic of existing insulation and then sprayed a couple of inches of closed cell foam on the drywall lid covering all attic bypasses. To this he added proper venting and then blown insulation to code. He said the energy saving and comfort increase was very dramatic so that he chose more conventional new windows over high performance. I am not a fan of spray foam insulation but this system should put it outside of the building environment.
Doug
A truss roof would not have the percentage of framing from that calculator.
Less than ten percent I would think
Not sure I like their math anyway....for a 13x40 truss roof section on 24 centers I get R16 and R10.58
40 sq ft of [email protected]
480 sq ft R 12 or 21
maybe my math is wrong....