Perennial cathedral ceiling ice dam issue meets hot roof twist
Martin Holladay, I can imagine you grinding your teeth at yet another tongue and groove cathedral ceiling issue! I thought I could be smarter than the problem. #fail
Background: of our design https://www.greenbuildingadvisor.com/blogs/dept/qa-spotlight/designing-hvac-system-cold-climate
Problem: double-walled stovepipe penetrating cathedral ceiling / hot roof appears to be source of heat leak, spurring ice dam and interior condensation on cathedral room walls. (see photos)
Setting: 1,100 sq foot construction. About 28,000 btu heat load. We close off unused bedroom and mudroom. Cathedral roof (4/12 pitch) is insulated with closed cell foam ~R56). Raised heel trusses. Bedrooms either side of great room are cold roofs with spray foam on sheetrock lids, then blown-in cellulose. Meticulous building envelope with .8 blower door test. Inside humidity throughout the winter has been low 20s. High teens when I fire up the wood burning stove.
Recessed lights. Ah, yes, the ol’ pot lights problem. However, the lights with LED 200W equivalent bulbs are located in conditioned space with at least 16″ clearance between pan and hot roof insulation. Given these conditions,I can’t see hot the pot lights can be contributing to the ice dam problem.
HRV air exchanger. Fujitsu mini splits. Jotul stove is about 23,000 max btu. We used stove maybe a dozen or so times, primarily when night temps were below -15 degrees. Passive solar heat is primary daytime heat source, when there is sun (scarce this winter).
Builder’s Proposed Solution Remove inside stovepipe collar where it penetrates cathedral ceiling. Insulate stovepipe with rockwool in space between ceiling and roof deck. Cut interior vents at low end and high end of t&g ceiling. Thinking is that the vents could create a convection loop to vent moist air.
Warm air rising in the winter to the peak of the cathedral would seem to be a hindrance to the convection loop idea. Perhaps our ceiling fan could help ventilate the space if we did go with vents. However, my wife and I are not keen on the cathedral ceiling venting idea. We live in the country alongside a lake with periodic hatches of flying bugs. We don’t want to create a space for bugs, spiders, etc.
Question 1 Comments on likelihood of success with proposed solution? Any variations?
Question 2 Assuming rockwool approach, is there any need to hold the rockwool back from stovepipe given its fire resistance? Would building a chase to contain the rockwool and provide opportunities for caulking add value to containing the stovepipe heat?
Question 3Assuming that the builder has correctly diagnosed the problem, do folks have any other proposed fixes to the condensation on the walls, heat loss through roof, and subsequent ice dam?
Final comment: we are delighted with how well our triple pane, high solar heat gain Alpen windows helped heat our home during the winter. Typically, we reached 79 to 81 degrees in our great room, no matter how cold outside. Concrete floors reached similar temps later in the day. I wish though that we had put more exterior insulation on the slab on grade perimeter (currently have 2 inches). Floor perimeter is significantly cooler. Love our clerestory windows and look forward to using them to “night flush” heat out of the house in the summer. Unexpected bonus of clerestory windows is being able to see the moon and stars at night.
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