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Heating and Cooling Load Differences

mikeferro | Posted in General Questions on

I recently ran a full manual J using Rhvac software on my new house design and am questioning the numbers.

The house will be located in Maine (zone 6A) and face west, which I know isn’t ideal but there are some spectacular mountain views that are worth it. The overall heating load came in at 37,885 btu and cooling load at 81,588 btu.

The cooling load is largely driven by the 840 square foot great room which has a cooling load of 36,685 btu and a heating load of 6,845 btu. The walls will be a R27 urethane SIP and roof a R55 EPS SIP. There will be 240 square feet of westerly facing triple pane windows and 112 square feet of southerly facing triple pane windows both with an SHGC of 0.54.

It looks like the Great Room cooling load is driven by the window area, but I’m questioning if a 36k btu cooling load really makes sense for an 840 square foot area?

I also took a look at an R27 SIP versus an R41 SIP wall and the heating load difference was only 700 btu. Given this, is there any reason to upgrade to the R41 SIP wall?

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  1. Expert Member
    NICK KEENAN | | #1

    When I think of Maine I think of a climate where it doesn't get that hot in summer, and most people don't have AC. It sounds like virtually your entire cooling load is due to insolation, that with a well-insulated house and outdoor temperatures not that hot there isn't really any heat coming through the walls. I have no reason to believe the calculations aren't accurate, but what Manual J doesn't take into account is the possibility of just opening windows. You know your climate, would you be OK with not having cooling and relying on ventilation on the hottest days of summer?

    1. AlexPoi | | #9

      Even in eastern Canada, we need AC in the summer now.

  2. BirchwoodBill | | #2

    Have you plugged the numbers into a spreadsheet and done the calculations to compare the results? I ran into a similar issue with a “free” online manual j tool, which oversized my system. Also adding an automatic window operation system is not that expensive. You can use Home automation to close the windows when the AC turns on.

  3. charlie_sullivan | | #3

    On the R27 vs. R41 SIP, whether that makes sense depends on how much wall area you have. I'm also not sure what your design temperature is but at a 70 F delta-T, the 700 BTU/h difference is corresponds to about 800 sq feet of wall area. That's not a lot. So may be there's an error. But one of the problems with SIPS is that it's expensive to increase the R-value. If you had a double-stud wall, going up from R-27 to R-41 is pretty cheap.

    On your cooling load, some options are:

    1. Don't worry about keeping up with the incoming solar gain with the A/C. Close off that room, open the widows, and if it's too hot for that, go hang out in another part of the house.

    2. Use low solar heat gain windows there.

    3. Electric awnings to shade those windows when needed.

  4. mikeferro | | #4

    Thanks for the feedback on the R27 versus R41 SIP conundrum. The wall area is about 800 square feet because the entire west wall of the space is a curtain wall of windows. So, it sounds like that 700 BTU/h difference is real and useful for comparison.

    I did some aggressive price comparison between an R27 SIP and a higher r-value double stud wall assembly and with the increased cost in building materials and labor lately, the price premium for SIPS is smaller than you'd think. Not to mention, SIPS are an easier enclosure for a timber frame than a double stud wall.

    As far as the cooling load, it sounds like I should try some low SHGC windows to start and re-evaluate the model. Then, perhaps a fully modulating 1-to-1 mini split for the area may be my best option to deal with the varying loads.

    1. charlie_sullivan | | #6

      Am I reading right that your comparison came out that the SIPS is more expensive than the higher r-value double stud wall? If that's the case, I would opt for the double stud wall, but maybe I'm misunderstanding.

      The fact that your wall area is small (mostly windows), it does make sense that the energy difference is small. On the other hand, that should also mean that the cost premium to go up to R-41 is small. If that's true, you might still consider it.

      But that's not the main story here--the main story is the windows. One thing to consider is that that wall of windows will look black from the inside on winter evenings, without some kind of covering. If that covering also provides a way to reduce solar gain in the summer and reduce heat loss in the winter, that could be a win-win-win.

      1. mikeferro | | #12

        Yes, the SIPs are more expensive than a double stud wall. However, I chose to go with them becuase they're easier to install around a timber frame and get the air sealing details right.

  5. Expert Member
    Michael Maines | | #5

    With heating loads that size, considering good envelope R-values, you must have a relatively large house and/or a lot of glazing overall. A SHGC of 0.54 is very high for west-facing glass; I would change that to the lowest SHGC you can get that maintains a decent U-factor and VT value.

    DCContrarian, Maine does not get extremely hot, but we occasionally reach 100°F and have an increasing number of days each year above 90°F. And our relative humidity tends to be high--not DC-high, but higher than most people think. So A/C use gets more common and more important each year. Dealing with sensible loads tends to happen more quickly than dealing with latent loads, so if the energy modeling includes relative humidity set points it might skew the results.

    Edit to add: I just saw your comment #4. An entire wall of west-facing glass in Maine, which has a lot of sunny days, will definitely drive your energy modeling. In case it's not clear, what happens is that the sun heats the house all day, but it's mostly beating down on the roof. At the end of the day, when the sun is low in the sky it blasts any west-facing windows with solar energy. In the depths of winter that can be a good thing, but most of the year it leads to unwanted solar gain.

  6. Expert Member
    Akos | | #7

    Since this is a new house design, it is easy to design in proper shading for the west windows. This will be much cheaper and more importantly WAY more comfortable than getting a hugely oversized AC unit there. With modern windows, once the sun's energy is inside the house you own the heat, interior shades provide limited help.

    Large roof overhangs and louvered sun shades make for a great covered outdoor space. I have something similar on one side of my house and my only regret is not making the overhang two feet longer for better shielding from rain.

    1. Expert Member
      Michael Maines | | #8

      Akos, shading devices above the windows are good for south-facing windows but here in Maine, roughly 45th parallel, the late afternoon sun is low in the sky and hangs there for what seems like forever. I just modeled a house with a 10' deep porch on the west and even that doesn't block sunlight in the shoulder seasons, when overheating is most likely.

      Sliding or rolling exterior shutters are effective, if they fit the design and get used.

      1. Expert Member
        Akos | | #10

        You can't complete eliminate western solar gain but you can certainly reduced with proper shading. Adding on a pergola with properly oriented slats like this can also go a long way:

        The bit of solar gain that remains towards sunset can easily be dealt with by some fast growing trees or an exterior roller shade. Even in terms of energy use, it is a huge difference in having 5 or 6 hours of western sun through the house vs an hour or two.

        It is still a better option than an extra 3 or 4 ton of cooling load.

  7. mikeferro | | #11

    Thank you for all of the great advice and suggestions on how to achieve shading. I ran a .32 SHGC for my western facing windows through the Manual J model and it dropped my cooling load in half! Mondays task will be going back to the window dealer and talking with them about what Loewen or Aluprof curtain wall options they can provide in that SHGC range.

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