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Is there a cumulative R-value effect in ceiling + roof insulation?

Stephen Edge | Posted in General Questions on

If I have an R-30, cold roofed, thermally isolated, conditioned attic (400 sq ft, rarely used living space for guests) and R-30 ceiling insulation over the main 1,000 sq ft living space, does that equate to R-60 for the main living space under the ceiling?

Both levels will have independent Monitor heating units. Both levels will have adequate mechanical ventilation and makeup air inlets. In zone 6. I’m aware that this is not up to code but I’m curious. What are the pros and cons of this set up?

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Replies

  1. Lucas Durand - 7A | | #1

    How tightly air-sealed are the two spaces from each other?

  2. Stephen Edge | | #2

    They will be well sealed from each other. Should also note that it has high heel, energy trusses.

  3. Eric Novotny | | #3

    A picture would be helpful but my understanding is that insulation values are only cumulative when they occupy the same plane.

  4. Cramer Silkworth | | #4

    Stephen,

    If I understand the setup correctly, you've got insulation between two conditioned living spaces. And if you're going to keep both at the same temperature, then the insulation between the two spaces is basically irrelevant - there's no heat flow across the ceiling if there's no temperature difference. However, if you intend to heat the attic space only occasionally, then the "effective" r-value of the ceiling + roof, as far as the lower living space is concerned, will be somewhere between the ceiling's r-value and the ceiling plus the roof r-value - depending on how air tight everything is. The tighter it is, the closer to the combined value.

  5. Stephen Edge | | #5

    Thanks Cramer. That makes sense and it's interesting.. The code doesn't account for this real world scenario, but it affords for more head room in the attic, and may be cheaper to accomplish. I still need to run the numbers.

  6. User avatar
    James Morgan | | #6

    Cramer's got it exactly right. However don't expect a code inspector to accept your assurance on the way you plan to use the space, if that's a consideration.

  7. John Klingel | | #7

    With a gap the size of an attic between the two insulation layers, would the total R = the sum of the others (basically) be an over-estimate? I imagine the convective loops in there will be relatively detrimental. Any thoughts on that?

  8. TJ Elder | | #8

    I think the heat flow here depends on delta-T, the temperature differential across the insulation in question. If the temperature in the attic (when unused) is halfway between the indoor and outdoor temp, then the delta-T across the first R-30 boundary (attic floor) is half what it would be from indoor to outdoor directly. That could mean the heat flow into the attic is similar to a single R-60 boundary with the full delta-T (indoor to outdoor). But if more heat than that leaks into the attic (so the temperature is closer to the downstairs thermostat setting) then the rate of heat flow will be greater.

  9. Stephen Edge | | #9

    I think I'll be able to do a good job of air sealing the ceiling (attic floor). There will be no can lights and it will have 10 inches of cellulose. The tricky part will be the straight stairs that lead to a seperate attic entrance which has a hotel like connector door to the main floor. Three doors in one set of stairs..

  10. Aj Builder, Upstate NY Zone 6a | | #10

    Cramer has it. You benefit when the upstairs is not used and not heated and well closed off from below.

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