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We recently installed balcony skylights in our western-facing, semi-finished attic "bonus room". Part of the ceiling was cut away and LVL beams were instaled at the hips and joining rafters to create a 12' vaulted ceiling. Despite blackout shades in the windows and a mini-split, it was unsurprisingly 95° in the summer prior to any insulation.

The area where the ceiling is still intact will be walled off and an inline vent fan with an intake will be installed at the peak to draw the ambient hot air to the outside (the red circle in the photo). Baffles were installed between the rafters. There is spaced sheathing that allows air flow laterally across the walls/roof. Styrofoam sheet insulation is going in the rafter bays.

The plan is to put interior intake vents in the bottom corners to draw in cool air from the room, through the baffles, and exhaust it into the room at the top. The expectation is that the nearby exhaust vent will draw the hot air outside. We'd plug the vents in the winter.

Is this reasonable? We're concerned that venting at the peak back into the room will just fill it with super-heated air, leaving it hotter than if wed just tried to button up all the walls and prevent any air flow. The fan is thermostat triggered and is reasoably powerful but didn't do much during construction when there were no walls or insulation.

The walls will be constructed with the same tongue and groove visible in the photo so some gaps between the roof and walls will be inevitable. We are also planning a thermal curtain for the skylights in the hot summer months (Portland, Oregon).

To vent or not to vent? Or something else entirely? Thanks for any guidance.

cross section

enter image description here

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  • Are you grabbing conditioned air and throwing it outside? It seems like you would eliminate this problem as you add insulation.
    – KMJ
    Oct 24, 2022 at 19:35
  • The conditioned air would draw in at te bottom of the wall,, mix with the heated air in the baffles, and pass back into the room at the peak. There would be a vent to the outside near the vent also. So yeah. I suppose you could say that. The walls are only 4" thick so they're warm to the touch even with insulation though.
    – Laramie
    Oct 24, 2022 at 19:49
  • I cannot see what you have so I will take a SWAG and say this would be the time to add a ridge vent and vents on the soffit. That air would flow between the roof sheeting and your wooden ceiling. Be sure your styrofoam is approved for the application in your area.
    – Gil
    Oct 24, 2022 at 19:54
  • I'm losing track of what you are describing here. It sounds like you're pulling air from inside the envelope up through the roof assembly and then out, and you're considering venting it back in to the structure. Both of those are bad ideas. I suggest you read inspectapedia.com/ventilation/Roof_Ventilation_Specs.php and also provide a cross-section of what you are proposing, so we can all be on the same page here.
    – KMJ
    Oct 24, 2022 at 20:03
  • I should note that the house is 1906. There are no soffits or any feasible way to add vents to the outside below the eaves. The roofers are also long gone so adding any new exhaust vents is also prohibitive. I added a cross section to try and clarify. It's understood that drawing air from inside the envelope and venting back into the room is not great, but it seemed preferable to not venting at all, especially given there is an exhaust fan in the wall at the peak.
    – Laramie
    Oct 24, 2022 at 21:40

2 Answers 2

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Thanks for adding the diagram.

To me, this looks like a plan to use expensive, conditioned air to ventilate your roof assembly. That seems like a really poor idea. The air is coming from inside the envelope to between the insulation and roof. I would do anything practical to change this so that you're bringing that air in from the outside instead, then vent it out at the top either with a ridge vent or a normal roof vent. The current approach is going to result in a really inefficient building. You're effectively bypassing your insulation if you vent back in to the house, and you're effectively dumping out your conditioned air if you vent outside of the house. There's really no winning here with the current setup as you describe it.

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  • "There's really no winning here..." sounds spot on. Appreciated. I've added a drop ceiling to contain the wall exhaust for exterior venting, but there is still no feasible outside intake. The best I could come up with was a vent in the floor behind the pony wall with a channel between the floor joists that butts into an unused service chimney shaft to the basement. I opened a small vent in the joist there. The key question is whether baffling is worse than just sealing the walls if the air is sourced from inside.
    – Laramie
    Oct 30, 2022 at 19:55
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In summer, the roof which is exposed to the sun will be much hotter than outside air. It then heats the outside of your insulation and this heat eventually finds its way inside the room.

The usual solution is to create airflow just below the roof (black arrows), so it can be cooled from both sides. That keeps the air on the outside of the insulation to a lower temperature.

enter image description here

If the holes are large enough, natural convection will provide good airflow without needing extra fans. Grilles are absolutely required to prevent wildlife from getting in (wasps, bees, rodents, etc).

Since the bottom of the roof will also radiate heat on the insulation, I'd add an infrared reflector on top of the insulation (below the air flow) to mitigate this.

This has no consequence in winter: since the roof tiles aren't insulating, the air below them will be at the same temperature as the outside air anyway, so the airflow won't waste heat. However it will keep the bottom side of the roof dry in case there is condensation, which prevents your wood from rotting.

Now you say: "There are no soffits or any feasible way to add vents to the outside below the eaves."

If the roof is built with ceramic tiles, then the space between the tiles will let lots of air through and serve as ventilation. You could add more details to the question. In any case, it would be preferable to use outside air for this ventilation.

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