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Average heating load

forcedexposure | Posted in General Questions on

Hi there,

What is the average heating load per square foot of living space for new housing construction in New England? This would be average new construction that is built to code and without additional effort put into the building envelope beyond what is required. 

TIA, Kris from Arlington, MA

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    Heat load isn't a function of floor area- it's about exterior surface area and the type of area (window area is a lot lossier than wall area, wall area lossier than roof area, etc.) and the indoor to outdoor temperature difference.

    As lousy as BTU/hr per square foot rules of thumb are, a typical reasonably-tight 2500' house built to IRC 2018 code levels without excessive amounts of window will usually come in around 30,000 - 35,000 BTU/hr (12- 14 BTU/hr per square foot) @ 0F outdoors, 70F indoors, with many exceptions on both sides of that range. In Arlington MA the 99% outside design temp is something like +8F, so you'd be looking at something like 10-12 BTU/hr per square foot.

    It doesn't take a rocket scientist or building scientist to come in lower than that on a new design, with some attention paid to the shape of the house, and window size & type, even at code-min U-factors. A more complex shape has more exterior surface per square foot of interior space, and more thermally bridging framing elements. Keeping the footprint to no more than 6 corners helps, as well as keeping the window/floor area ratio <15%. A lot of new housing construction in my area has lots of bump-outs, dormers, & corners, as well as a lot of glass crippling the potential energy efficiency & comfort.

    1. Deleted | | #2

      Deleted

  2. walta100 | | #3

    Sounds like your house would be well above the average, I do not see any point of comparing your house with colonial structures and everything in between.

    If you want to correctly size a HVAC system I say pay for an aggressive manual J calculation from in someone not bidding to install the equipment.

    You may find this article interesting.
    https://www.energyvanguard.com/blog/air-conditioner-sizing-rules-of-thumb-must-die

    Walta

    1. forcedexposure | | #4

      Good morning Walta,

      Thanks for the response! I do understand the value of a Manual. However, i am not asking about this for my home or any particular structure.

      Rather, I'm trying to determine how much energy on average might be required, annually, for single family new construction that is built to Stretch code... using 100% electric heat.

      This will not be used for a particular building project, but rather for the formation of new guidelines that seek to limit the carbon emissions of new buildings and gut rehabs.

      Is there any kind of average "rule of thumb" sf annual heating load estimate for new construction?

      Sincerely, Kris

      1. Expert Member
        NICK KEENAN | | #12

        There really aren't rules of thumb. And to the extent there is a model code currently, or things like LEED certification, they revolve around process rather than outcome. So the model code dictates how much insulation you have in your walls and how good your windows have to be, but it doesn't care how many square feet of walls you have or how many windows. The code has specifications for skylights, which is ridiculous, skylights are such an energy hole they shouldn't be allowed at all. Specifically, where I am the roof has to be insulated to R30 but the skylight has to be only R2 (U=.5). And I can have as many as I want. It's easy to think of scenarios where a house that doesn't meet code uses less energy than one that does. For example, a house with an R25 roof with no skylights vs one with R30 roof and several skylights.

        What you're talking about is actually an outcomes based measure of efficiency, BTU/SF. It would allow the house designer to get there any way they want, whether by an efficient design with modest insulation or an inefficient design with extreme insulation. People in the business won't like it because it's not the way they've been trained to think.

        I'm more supportive of the idea of mandating efficiency. There is a structural problem that often the people making decisions about efficiency -- the builders -- are not the ones who have to live with the consequences of their decisions. Mandatory codes are one way of addressing that problem. I feel that the model energy codes that have been added in the last few updates to the IRC are a tremendous positive step.

        For another approach, see the thread I just started, "Principal, interest, taxes and insurance -- but not utilities?"

  3. krom | | #5

    Sounds like you shouldn't be involved with the "formation of new guidelines" That should be left to those who actually know what they are doing

    1. forcedexposure | | #8

      I've learned a lot from this site and really appreciate all articles and the comments from the broader community. It's an absolutely incredible resource.

  4. user-723121 | | #6

    What are the specifications of the Stretch Code? Air tightness is very important for energy efficiency. With to code insulation, Btu's per square foot per heating degree day in my experience correlates to ACH50. A home with 5 ACH50 will use roughly 5 Btu's/sf/hdd.

    1. forcedexposure | | #7

      Hi Doug,

      Thanks for the response.

      What i'm seeing pertains to HERS ratings, not specifically air tightness:
      "maximum HERS index allowed will be 55"

      best wishes, Kris

      1. user-723121 | | #9

        To understand building performance you need to know the nuts and bolts of heat loss, thermally and from infiltration. ASHRAE describes this in detail over the years in their Handbook of Fundamentals. This will explain all with regard to calculating R-Value for given materials, foundation heat loss and heat loss related to infiltration along with solar tables for the US.

        Knowing ahead of time the energy usage for a proposed building is quite easily estimated, putting it all together in the construction takes time and effort.

  5. walta100 | | #10

    Given that your interest, I suggest you model a few homes using a computer program called BEopt.

    https://beopt.nrel.gov/home

    When I modeled my house, given my energy costs today, my guess at inflation and borrowing costs, it looked to me like the most recent model codes were very close to the break even cost point over the next 20 years. Your numbers are going to be very different than mine.

    Is it really green to force someone to spend their money on insulation, windows or equipment that is unlikely to recover its costs before the building likely to demolished or rehabbed?

    Walta

    1. forcedexposure | | #11

      Hi Walta,

      Thanks for the link to BEopt. I'll investigate.

      What we have in mind applies to new construction and gut rehabs.

      In my neighborhood, we are seeing $500k single story 50s and 60s ranch homes razed by developers who leave the foundation in place and build up, increasing the living area by 2.5 times. They add dormers, breaking up the roof line and reducing the likelihood of future PV panels. The developers sell these homes for upwards of $1.2 million.

      There are a myriad of questions here. Does new construction like this use the same amount of energy as the original, leakier and less insulated home? How many years will these new homes last, before, as you say, they are demolished or rehabbed? Would their carbon footprint and energy bills be reduced for the future residents if they were built 100% electric? Does anyone build with electric baseboard resistance heat? Is it less expensive to build a home with ASHPs than it is to build with a gas heating system?

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