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Exterior deck over a conditioned basement

Fiddlehead | Posted in Energy Efficiency and Durability on

Seeking thoughts / suggestions for an exterior deck above a conditioned basement (1 story house over CMU foundation, with a little corner deck punch  created under the roof trusses). Zone 6 – NW Lower Michigan, snowy and wet. 

I’m the arch on the project, proposing a double stud 12″ thick wall system on Main level, insulated with Timber Fill. Need to insulate the deck floor (basement ceiling) – looking to do so without dropping the ceiling height downstairs too much. Don’t want to use spray foam. Cant make floors thicker – this is an addition to an existing structure, where we want to keep the floor heights the same. Also looking to keep the deck door 3-4″ above the floor…
Wanting to insulate as much as possible and prevent moisture intrusion through deck door.

Anyone have any tips?

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Replies

  1. Expert Member
    DCcontrarian | | #1

    This kind of construction is not uncommon here in DC, everything is built to the maximum zoning will allow and underground doesn't count toward setbacks or floor area ratio. The basement needs a roof, then support for a deck, and then the decking.

    There's no magic. You need R60, with spray foam you'd need 10" for it. The roof has to have some pitch, the support for the deck needs some space. That's going to take what it's going to take. If that doesn't leave you with enough headroom, can you lower the floor in the basement?

    this is an addition to an existing structure, where we want to keep the floor heights the same. Also looking to keep the deck door 3-4″ above the floor…

    In snow country I would think you want to step down 7" or so from the interior to the deck. Unless you have like a 4' overhang.

  2. Malcolm_Taylor | | #2

    Fiddlehead,

    Decks with living spaces below are conceptually just flat roofs you can walk on. Flat or very low slope roofs can in some circumstances be vented and use permeable insulation, but those require very deep air spaces above - which you don't have room for. So being limited to impermeable insulation your choices really are spray-foam, or foam board above the sheathing.

    You may find these links useful:
    https://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs
    https://www.greenbuildingadvisor.com/article/low-slope-roof-assembly-failures

  3. walta100 | | #3

    It does sound like you are in a tuff spot.

    The ceiling like any other ceiling need to meet your local code R38-60.

    It does not sound like you have much room to fit in the required R value.

    Is much as I hate foam insulation the one thing it does better than anything else is provide the most R value per inch.

    Something is going to have to give if you are going to be code compliant.

    Walta

  4. Expert Member
    DCcontrarian | | #4

    I've seen it done where there's a step up from the inside floor to an outside deck. It's a little goofy but shouldn't be any different from stepping down the same height. A lot of houses in the city have flat roofs with a deck on the second or third floor, which is where you tend to see this. I suspect it's popular because if you frame the walls the same height and make part of the floor a deck over roof, that section is always going to be higher than the interior space resting on the same walls.

    It does mean that the door sill is at the level of the deck, I'd only do that with a big overhang.

  5. gusfhb | | #5

    Well, while I respect that people don't want to use foam, a sealed deck over conditioned space is a tricky thing to get right. It is done wrong a lot. A leak at some point is almost inevitable. A roof people don't tend to walk on every day and do stupid things.
    A leak with foam is a nuisance. A leak with fluffy stuff is a disaster
    My 50 year old house with 2 foot overhangs had rot at the doorframes at the [unsealed] deck because in the 70s people stopped believing in metal flashing.
    This all leads me to think , keep the deck low, the door high, flash it very very carefully.

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