Venting Options for a Hip Over-Roof
I have spent the last week or so planning an insulation upgrade for the vented attic in my 1984 house. Because the trusses are low heel, there’s no way to get a decent amount of r-value over the top plates (only about 3″ – 4″ of height after vent baffles installed). I’m now toying with the idea instead of waiting until the shingles need to be replaced (probably about 8-10 years from now) and installing foam on top of the roof to bring the attic into the conditioned space. Because we get a lot of snow in the winter, we would also need a vented over-roof, as advocated in this BSC article: https://www.buildingscience.com/documents/insights/bsi-046-dam-ice-dam The problem is that our roof is a hip roof (4-sided pyramid). So venting isn’t straightforward. There were two ideas I was considering to help with this: 1. Hunter panels and GAF make vented nailbase panels that allow lateral airflow: https://www.hunterpanels.com/product-over/product-listing/cool-vent https://www.gaf.com/en-us/products/thermacal-1 2. Battens and counter battens installed on the roof deck (with metal roofing panels) as described in this pro-clima article: https://proclima.co.nz/wp-content/uploads/2021/02/Above-Sheathing-Ventilation-ASV-Part-1-Rise-of-the-Cavity.pdf In our area, it is common for roofs to be vented with high-rise vents that rise above the snow pack: https://ventilation-maximum.com/en/products/sloped-roof-ventilators/vmax-301/ Either of the two solutions above combined with a couple of the high-rise vents at the peak seems like it should be sufficient to keep a cold roof deck and prevent ice dams. Grateful for any thoughts folks might have about this design or other ideas to create a vented over-roof for our hip roof.
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Also consider something like "Cobra® Hip Vent". Perhaps as a supplement to stack vents.
I'm a little nervous about using that style of vent...Not sure how well they work when covered with 2 feet of snow. Here in Ottawa, you almost never see ridge vents...the common wisdom is that they don't work with the heavy snow loads. That might just be a folk tale but one can imagine how they wouldn't work - especially if the attic is really well insulated so the exhaust air isn't warm enough to melt snow.
Hi Patrick,
I realize the hip style of your roof complicates matters but I think you will find this reader’s plan for venting an over-roof—and the related comments—valuable: Does this plan for a vented over-roof make sense?.
Thanks for the pointer Kiley. Lots of good information there.
I also found this article super helpful:
https://www.greenbuildingadvisor.com/article/how-to-install-rigid-foam-on-top-of-roof-sheathing
So far no concrete guidance on how to deal with the hips though.