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Community and Q&A

Low-Slope Roof: Where to Put Insulation

bradesp | Posted in Energy Efficiency and Durability on

Hi guys,

I have a large connecting hallway in a new addition I’m building that has a vented roof here in Zone 4 (1 hour north of Raleigh).   The small challenge I have is that the micro-attic area over the flat ceiling is so small that I wont have access to this area after we complete construction…   So it got me thinking, is there a better placement of insulation in this scenario.

Here are a few of my questions:

1 – if I move the insulation to the rafters, will the small open space above the sheetrock be “stale” and possibly “smelly” over time since there’s no direct conditioning of this small air space.

2 – Would insulating the flat ceiling (perhaps with dense pack cellulose) give me better “buffer” against the unconditioned air in the micro-attic and avoid any “smells” from migrating to the space below?

3 – Are there practical or performance advantages that either location provides?

4 – Would baffling the rafters and filling the entire space with something like 1/2 foam, or even dense packing the entire space be the more “fail safe” way to treat this space?

Thanks in advance for your suggestions / comments!

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  1. plumb_bob | | #1

    Both types of roof construction require ventilation. If you have a rafter roof, the rafter bays need a ventilation channel between the insulation and roof deck, a 2" vent channel is a good place to start.
    If you have an attic, it also need ventilation. Soffit and ridge vents are typical. If it is properly vented to the outdoors there should be no odours or smells, if you are smelling your attic something has gone wrong.

  2. bradesp | | #2

    Understood, which is why you see soffits and ridge vent indicated on the drawing... In both scenarios the roof is vented... my question has to do with the "stale", slightly unconditioned air sitting above the flat ceiling in the event that I choose to baffle / ventilate a the rafter.

  3. plumb_bob | | #3

    Got it. Spaces should be either conditioned, or unconditioned. Hybrid spaces are prone to problems.
    A traditional cathedral ceiling with insulated rafters and air/VB on the underside is difficult to properly air seal, adding ceiling joists will make the detailing that much harder. I would keep with a full attic, or typical rafter system, and not something in between. If you want a flat ceiling then the fully vented attic is the way to go.

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