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Community and Q&A

Conditioned vs. unconditioned attic quandary

PeteMartin | Posted in Green Building Techniques on

I’ve read hours and hours of material on GBA and other sites about this issue, but am not certain I can apply it all to my proposed design.

I’m designing our retirement home in CZ 4A in mid-northern Arizona. Elevation is 5000 ft, we have 4200 HDD, 1000 CDD, and lots of sunshine. I want to incorporate a lot of concrete for thermal mass, structural integrity, and permanence. Having past experience with hydronic radiant heating, I plan to use an air-to-water heat pump and run tubing through the concrete floor, the lower third of the exterior walls, and in the concrete ceiling. The ceiling will be chilled for summer A/C, but not heated in the winter. The floors will be the opposite situation, heated in winter but not cooled in summer.

In spite of the low number of official cooling degree days, in reality we find we use A/C quite a bit in our local rental home. Therefore I’m particularly sensitive to the need to insulate the attic floor (cooled concrete slab in the summer) really well from the high roofing & ambient temperatures.

I’m analyzing the tradeoffs between placing the insulation directly on the slab (attic floor) vs above the roof sheathing (as a built-up, multi-layer, foam-and-2×4 assembly). The latter definitely seems more expensive and labor-intensive, but I like the idea of keeping the roof’s structure inside the conditioned envelope.

There will be no equipment in the attic, and no ducts up there. The small diameter ducts for DOAS and chilled air for latent heat removal will be run under the ceiling slab, above a suspended ceiling (I like access to pipes, ducts, and wires).

The roof design is all hips, no gable ends, with quite a few valleys. The house is a single story, 2100 SF, with 10-ft ceiling height throughout (no cathedral ceilings).

When I try to work out the details for the conditioned attic option, I’m running into challenges where the main floor conditioned space abuts unconditioned spaces such as exterior porches and the garage. Since the hip roof attic would (simplistically thinking) be contiguous across and above all these areas, I think I would have to build insulated attic walls at the perimeter of the main conditioned space, and deal with the discontinuous roof thickness where the insulation above the sheathing ends.

So that pushes me towards insulating at the attic floor, which seems much simpler and cheaper. But here I become concerned about potential condensation during our humid monsoon season, when outdoor temperatures and humidity both climb for days or weeks at a time, if the insulation above the attic floor is cellulose. So perhaps the cost savings of cellulose versus EPS will be somewhat reduced due to needing to place two to four inches of EPS or polyiso foam directly on the top of the attic floor slab, under the cellulose. And will I be able to adequately encapsulate the 70-degreeF slab in foam to prevent it from coming in contact with warm, moist outside air (which may occasionally have a dew point above 70F)?


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  1. walta100 | | #1

    In my opinion a conditioned attic is a desperate, risky and expensive option that should only be considered if one is stuck because someone stupidly installed ductwork and or equipment in the attic.

    If your house is let’s say 3000 square feet and your concrete ceiling is 9 inches thick your ceiling is a 340,000 pound rock! Do not even think about an earth quake.

    My guess is for half the money you could build a passive house, whose internal temperature would change more slowly than your concrete palace


  2. Expert Member
    Dana Dorsett | | #2

    An EPS monocoque SCIP approach (wire-reinforced shot-crete on both sides of a slab o' EPS) to the construction is probably cheaper than poured reinforced concrete throughout, and would perform well in that climate even at less than the simple code R values, but would be an issue for radiant walls and ceilings.

    If poured concrete is going to be the ceiling it might as well be your roof. You could go with a flat roof w/fully adhered membrane on the slab, with a few inches of roofing EPS under a foot or two of adobe (architecturally appropriate for the region) or south Asian style phuska mud under a brick-tile-paved rooftop patio. The high thermal mass & R of the phuska mud or adobe above the EPS layer combined with the thermal mass of the slab is a benefit year-round. The reinforced concrete roof with phuska mud as the only insulation on top is still pretty standard construction even in some monsoon-prone locations in India:

    Of course that's not exactly your hipped roof look, but a flat roof with parapet is still architecturally appropriate for the region.

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