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Community and Q&A

Building in heat/cool/humid region

miked56 | Posted in Energy Efficiency and Durability on

We are looking to build a new town home in Alexandria VA. According to the Passive House US site we are in the heat/cool/humid region.
We were looking to make as efficient a house as possible with an eye to price performance (construction cost vs time and energy savings vs investment. House is envisioned to be full basement with 3 stories above, each smaller than the floor below for a total area of 3000 sq/ft. We were thinking an ICF basement with 6″ SIP walls, SIP roof, and panelized interior walls. We’ll be investigating solar panels but won’t be doing extreme items like ground source heat pumps.

Does this seem to make sense for our climate. Thoughts/ideas very welcome and desired.

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  1. Expert Member
    Dana Dorsett | | #1

    SIPs are somewhat easier to air-seal than stick-built, but an expensive path to overall thermal performance. If perfectly designed (no change-orders) and perfectly installed it can save on overall project time, but it's usually a loser on the thermal bang/buck end for higher R assemblies.

    With an ICF foundation under it you can do pretty well with 2x4 or 2x6 framing set with the structural sheathing co-planar with the concrete, and adding insulating sheathing thick enough that the insulating sheathing is co-planar or slightly proud of the exterior foam of the ICF. A typical 2x6 framed structure with blown cellulose or fiberglass comes in at a "whole-wall" R of about R13-14 after factoring in the thermal bridging of the framing and adding in the R of the interior gypsum & structural sheathing. If you add 2.5" of EPS that raises it to about R24, which as good or better than the true performance of a 6" EPS-core SIP. (Don't even consider using polyurethane core SIPs unless the manufacturer will stipulate that the foam is blown only with low global warming potential blowing agents. Almost all closed cell polyurethane is blown with HFC245fa, which has a global warming hit roughly 1000x that of CO2.)

    It's not clear how you would be reducing the size of each successive floor with SIPs cost-effectively either. Even in a framed building it makes the framing more complex, with more thermal bridging and more places for air leaks. A picture or floor plan might make it a bit more easy to advise.

    SIP roofs have to be very well air-sealed at the seams, particularly the ridge, both interior & exterior to avoid moisture accumulation and rot damage from air leakage in winter. A ventilated attic using an energy heel-truss a and R75 cellulose is lower cost higher-R solution unless you just have to have the cathedral ceiling look (in which case you could get there with a scissors truss.)

    Photovoltaic panels will become hands down the cheapest electricity available on a lifecycle basis within 10 years, maybe even within 5 years. In any new construction designing your roof pitches & orientation to take advantage of that is advisable, even if you don't install the PV the same year that you build. Currently the (unsubsidized) cost of installing grid tied PV is between $3-4 in my neighborhood, but it's below $2/watt in Germany. According to NREL analysis your break-even point in northern VA is when it drops below $2.25/watt. If there are subsidies available (including the 30% federal income tax credit that expires at the end of 2016) that can get your out of pocket costs lower than that, it's cheaper than buying electricity from the utility. It's widely anticipated that the $2/watt frontier will be crossed in the US as an average rooftop PV cost before 2020, and it may be well under $1.50/watt.

    Ductless air-source heat pumps (mini-splits) run at comparable efficiency to ground source heat pumps in your climate (at a fraction of the installed cost) and may be an option if your floor plans are reasonably open if you have R25-ish whole-wall values and you have U0.25 or lower windows in any doored-off rooms from the ductless heads. You may need to install some amount of electric resistance heating in doored off spaces with higher heat loads, but that's not to say you have to run those heaters. With a higher-R/lower-U construction the room-to-room temperature differences shrink, making point source heating/cooling options like mini-splits more reasonable.

  2. miked56 | | #2


    Whoa, what a detailed and thought through answer. Thank you very much for your time.
    I'll pass it off to the architect and see how we incorporate you thoughts into the design.

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