Remediating blocked soffit-to-ridge venting around skylights in a cathedral attic
nick_vk
| Posted in Energy Efficiency and Durability on
I’m seeking feedback on a venting solution for a new construction project in cimate zone 4B.
The Setup:
- Roofing: Metal roof over plywood deck.
- Assembly: Hybrid conditioned attic. R-49 fiberglass batts will be wire-hung from the 2×4 top truss chords, capped on the interior with taped Tyvek.
- Venting: Continuous heavy-duty rigid baffles to maintain a 2-inch air gap from the soffit to the ridge.
- The Obstruction: There are four skylights already installed (two on each side of the ridge) that completely block the linear bottom-to-top airflow path within those specific truss bays.
The Proposal:
For what it’s worth, I asked an LLM (Gemini) to suggest a solution to propose to the architect and this is is what it came up with:
For what it’s worth, I asked an LLM (Gemini) to suggest a solution to propose to the architect and this is is what it came up with:
- Drill a cluster of three 2-inch holes horizontally through the 2×4 top truss chords immediately below each skylight header, and another cluster of three 2-inch holes immediately above each skylight header.
- Holes will be vertically centered in the 2×4 members (leaving ~3/4″ of wood on the top/bottom edges) and spaced 4 to 6 inches apart on center to protect structural integrity.
- Terminate the rigid ventilation baffles 6 inches before these hole clusters so air can detour horizontally into the adjacent, unblocked truss bays.
My question: Is this a sound approach for handling the blocked bays, or are there better ways to handle skylight obstructions in a hybrid attic like this? I can’t get rid fo the skylights at this point – the ideal is to get the builder to do something helpful before the insulation is installed. Thanks in advance for any advice.
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Replies
Nick,
You can't drill any holes in truss chords without the suppliers approval. The chances of them doing that with 2" holes is remote at best - and I doubt whether smaller ones will be adequate.
Hopefully someone will chime in with a remedial solution. Unfortunately I can't think of one, except to detail those truss bays as unvented, and use spray foam.
Thanks Malcolm. If I get pushback on spray foam, could the same thing be accomplished with rigid foam if the edges were well sealed? Does the foam need to fill the full depth of the truss bay?
Nick,
Yes you can do what they all cut & cobble with foam board, or use spray foam. Neither has to fill the truss bays, just provide enough r-value.
https://www.greenbuildingadvisor.com/article/cut-and-cobble-insulation
I'm in the north edge of zone 5 in the land of 2.5 story houses. There are a lot of skylights and most are not properly vented. They seem to hold up fine, so I wouldn't worry about perfect venting, but aim for good enough.
The means each bay that dead ends at the skylight should be vented either at the soffit or the ridge. I would also do an extra careful detail of the warm side air barrier around those to minimize air leakage.
These rafters also have some permeability to you will also get some additional drying through the rafter bays that are fully vented.
If still worried, add a box vent over the middle rafter to the skylight. This way a single vent can cover both bays.
Spray foam will also work but I would not do cut n cobble as the foam layer is not adhered to the roof deck and any moisture that makes it there will now be trapped.