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Will my crawlspace help temper passive solar gain?

user-930047 | Posted in Energy Efficiency and Durability on

My wife and I are having a 2100 sf house designed based off of rough sketches I put together. We are building in Squamish BC, Zone 4C. I have a relatively simple east/west shape and simple shed roof design. I’m shooting for R30+ wall, R50 roof and am now doing my best to dial my designer back on his suggested window layout. He really likes glass and currently has the overall Window to Floor Ratio (WFR) around 30%.

I am basing my perspective on the house design somewhere between the ‘Pretty Good House’ and ‘near Net Zero’. I will likely have the house modelled shortly to get more firm numbers, but currently I have been using spec sheets from existing Net Zero Homes and other passive solar rules of thumb to guide my revision of the WFRs. I am trying to cut the overall WFR in half.

I should mention that I am more concerned about the negative impacts of too much south facing glass in the shoulder seasons and winter, than I am in achieving a perfect passive solar design. Really, I’m looking to have nice south views, with some sun tempering, without overwhelming the space with glare, overheating, and general comfort issues.

We have to build with a crawlspace to ensure our first floor joists are at a prescribed flood risk mitigation level, so my main question is, do crawlspaces help temper/absorb some of the passive solar gain? I have read somewhere that the 7% South glazing WFR value should be calculated including basement. How does a crawlspace influence the 7% south WFR number? Any other thoughts would be more than welcome!

Regards,
Sean

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Replies

  1. JTyler | | #1

    I can't help with your question, but I am curious where you read that WFR calcs should include basement floor area. Any chance you remember where you came across this info? If this statement is accurate, I need to make some changes to my plans...again.

  2. Expert Member
    MALCOLM TAYLOR | | #2

    Sean,
    I'm afraid this is another post with no useful answer to your question. I just wondered whether your designer has run his glazing ratio through the new energy requirements that came in with recent revisions to the BC building code. They may well determine both glazing ratios and wall insulation levels.
    good luck with your project. Sounds like building in Squamish is really taking off right now.

  3. Dana1 | | #3

    My wall-rat niece in Squamish lives in her 30 square foot SUV at a higher glazing ratio than that, but it doens't have to meet code! :-) (Nice cliffs!)

    A 30% glazing/floor ratio is going to overheat often- not a great idea. Be selective about where and how much window there is. West facing windows can drive your air conditioning loads skyward very fast if you insist on the sweeping "sunset view" look.

  4. user-930047 | | #4

    Jim - that's the problem, I can't find where I read that and hence why I'm looking for some feedback. I'm not going to have any real mass to speak so am looking to learn what effect, if any, the crawlspace would have in tempering passive solar gains. I'm curious so that if I end up in the 8% range I would have some peace of mind. Again - I know the numbers will need to be modelled properly.
    Malcolm - good question. I plan to meet with the designer to shave windows this week. I'll ask him.
    DDorsett - it's a beautiful place to live in a van! My wife and kids might find it a bit cramped. Yes, 30% is too much and I'll be looking to cut that in half. We have a nice covered west facing patio area in mind to mitigate the west sun.

  5. Expert Member
    MALCOLM TAYLOR | | #5

    "We have a nice covered west facing patio area in mind to mitigate the west sun."

    A great, under-used idea.

  6. GBA Editor
    Martin Holladay | | #6

    Sean,
    Q. "Do crawlspaces help temper/absorb some of the passive solar gain?"

    A. No. Using the crawl space to store solar heat would require an active system. I don't recommend that approach.

    Q. "I have read somewhere that the 7% South glazing WFR value should be calculated including basement. How does a crawlspace influence the 7% south WFR number?"

    A. My understanding is different. The basement and crawl space are excluded from these calculations.

    For more information on this topic, see these two articles:

    Cost-Effective Passive Solar Design

    Reassessing Passive Solar Design Principles

  7. user-930047 | | #7

    Thanks Martin. I have read every passive solar article on here several times through. I still can't track down where I read that basements are included, but makes more sense to exclude!

    Is anyone familiar with any rules of thumb for understanding the effects of decreasing SHGC on the 7% south glazing:floor area ratio? (I know, I know - get it modelled already!)

  8. charlie_sullivan | | #8

    A simple SHGC adjsutment would be linear: if you like the 7% rule for a 60% (?) SHGC, then for 30% SHGC, you could say that you are "allowed" 60/30*7% = 14%. But that doesn't really make sense, because the 14% glazing wall with 30% SHGC would lose heat much faster than the 7% glazing with 60% SHGC. So with that high heat loss, you'd be wishing you had high SHGC. TO make it really equivalent, you'd need the U-value to be cut in half as well.

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