Construction Design-Fixing Ice Dams from the Interior, Part 2

This past winter has been one of the snowiest and coldest I can remember in my 50 years living in Minnesota.  I wasn’t completely sure what to expect from the home I purchased late last fall.  I know it has issues, especially with the roof air sealing and insulation. There have been ice dams, but thankfully no back-up of water has ended up in the home.  (YET!)  I have had some minor dripping in the basement from a wall containing plumbing that extends to the upper level of the home.  I’m almost positive this is the result of frost forming in either a shed roof dormer side wall or roof area.  Something that will be fixed with the upper level renovation.

My 1952 Cape.

The best way to fix an ice dam issue is from the exterior.  Adding insulation above an existing roof works very well if some rules are followed.  In Northern Minnesota, climate zone 7, 60% of the insulation needs to be above the existing roof deck, air sealing needs to be completed below the new insulation and should be connected to all wall air sealing materials. It would be best if there is an air space to allow for ventilation above the new insulation.  The roof on my home is almost brand new and has a 12/12 pitch along with 3 dormers.  I’m not going in that direction.

The next best option is to increase insulation levels and improve air sealing on the interior.  Unfortunately this will involve a major renovation of the upper level, which I am intending on completing.  Before I tell you how I plan on accomplishing this goal, I will discuss what I have found during an inspection and some tests that I have conducted on my home.

These first two pics show some warm areas on the roof at the rear of my home.  I was surprised to find much more heat loss at the side wall of the shed roof dormer than the roof itself.

There is also a warm area right at the eave of the shed roof dormer (in the photos above).  I believe this is due to a pinched R-value because of the compression of the fiberglass insulation at the rafter heal.  It could also be an air leak from poor air sealing.

Now for the front of the home. The dark orange areas are heat escaping the roof, mostly in the sloped ceiling area of the roof.  There is also some heat escaping high in the roof, most likely from air leaks in the ceiling.  Notice the orange glow at the ground level.  Heat leaking out the concrete basement walls.

I conducted a blower door test on the home before I purchased it.  The test results were 12.34 air changes per hour at 50 Pascals, a total of 2976 cubic feet per minute of air moving across the fan.  (Not Good!)  The air leaks alone are adding over $500 per heat season to by natural gas bill.  I am hoping when all renovations are complete to have the air leakage rate to under 3 air changes per hour at 50 Pascals.  Now you know a little about the problems I am aware of, next up, how to fix them.

The entire roof is framed using 2 x 6 lumber.  Right now there is substantially less than R-20 worth the insulation in the roof.  This low R-value and the lack of air sealing in both the roof and walls are my issues.  My thought is to add to the existing roof framing to gain more space for insulation while also providing an area for ventilation, which there is currently none in the sloped ceiling or knee wall areas.  In adding depth to the roof framing, I would also like to provide a thermal break.  There are a few ways I could accomplish this, but I am leaning towards the product below.

This product is called the Tstud.  It contains a wood webbing similar to a bar joist connecting a pair of 2 x 3 studs and a closed cell spray foam thermal break, total depth is 5 1/2 inches.  This stud has a listed R-value of 20.  I plan on attaching the Tstud to the bottom of the existing roof framing, giving me a cavity of 11 inches.  This roof framing extension will run from the second level floor to the flat attic portion of the roof and will require removing and replacing the existing attic knee wall framing.  I will be losing some living space (sloped ceiling height) within each of the two upper level bedrooms.  A small price to pay to eliminate the ice dams.

After I install the Tstud, I plan on adding a ventilation channel from the roof eave to the flat portion of the ceiling. The flat portion of the roof is ventilated by a gable end vent and ridge vent.  I want the channel durable because I plan on over stuffing fiberglass insulation into the ceiling joist which is the reason I’m choosing plywood for the channel.  My plan is to take a 2 x 2 board and nail it to either side of the original rafter.  I will then nail a 1/4 inch plywood to the underside of the 2 x 2’s.  The idea is to draw air through this channel keep the exterior of the roof cool so that the snow does not melt during the winter due to heat loss through the roof.  Even after the additional insulation is added, I still won’t have the recommended R-value for a roofing assembly in my climate.  

The point where the ventilation channel exits the roof eave is hard to air seal.  Using closed cell spray foam at this point will increase the R-value of this narrowed area and also provide some additional air sealing.

As I mentioned, I plan on overstuffing the insulation into the rafter bays.  I’m hoping to use 11 inches of fiberglass which will give me R-38.  (After adding the ventilation channel, I will only have 9 inches of space to fit 11 inches of insulation.  Because the insulation is compressed, there will be a reduction in the R-value of the insulation, but the R-value will still be higher than installing only 9 inches of fiberglass, which is around R-32.)  I am choosing fiberglass because of it’s cost.  Closed cell spray foam would be a better, but a much more expensive choice.

I need to air seal the new roofing assembly, which will also provide the needed vapor control layer or barrier.  Remember, I live in northern Minnesota.  Code requires a class I or II vapor barrier on the warm in winter side of all walls and ceilings.  My plan is to use a class II smart vapor control layer, either Pro Clima’s Intello or Siga’s Majrex.  Read more about these products on a previous blog posting here.  The vapor control layer will be air sealed using appropriate tapes and acoustical caulking.  The space will be finished with drywall.

That’s my plan!  It is subject to change as I continue to think about the design.  I will definitely be blogging and posting to Instagram about the work as it progresses.  Stay tuned.



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