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Passive Solar: is a tweak to increase orientation towards the south worth it?

Randy Bunney | Posted in General Questions on

Friends,

Bald eagles and osprey are busy snatching fish from the lake. Migrating pelicans and loons are stopping by to feed. And we our readying for our May 15 passive solar cabin groundbreaking at 46 degrees N in Minnesota.

We are razing an old 500 sq ft cabin to make way for our 1,100 sq ft cabin that folks on this forum vetted previously regarding heat pump sizing. https://www.greenbuildingadvisor.com/blogs/dept/qa-spotlight/designing-hvac-system-cold-climate

The current lakeside cabin that family members have been using since post WW II is oriented at 210 degrees (30 degrees west of due south).

Q. Would shifting the new cabin 2 to 4 degrees produce any appreciable solar gain? I can understand that I am asking a qualitative question when referencing “appreciable.”

I don’t have the means of quantifying the solar gain, albeit I know there would be some, while also slightly decreasing unwanted summer heat gain from the cabin’s two small western-facing windows. My thinking is that every little bit helps in the systems game of cumulative gain in building performance.

I hired a passive solar consultant whose findings indicate that we have the correct ratio of southern glazing to thermal mass, the correct roof overhangs to protect from unwanted summer solar gain, and adequate insulation on all 6 sides of the structure.

My hesitancy in shifting orientation is that I don’t want the shift to be jarring or even noticeable to my wife who grew up in the old cabin.

The design load modeled early in the design phase had us around 25,000 to 27,000 BTU, as I recall. We have made so many changes, including to the wall assembly and increasing insulation under the slab on grade (from 2 inches to 4), and improved windows; that the previous modeled heat load is now a guestimate.

Sun Chart
I have attached a sun chart from an earlier study that I did. The chart projects summer and winter solstice sun penetration into the cabin at 30 degrees west of due south (the cabin’s current position) and 20 degrees. The chart demonstrates how the sun enters the cabin later in the morning at 30 degrees but still has excellent sun penetration. The chart input anticipated floor to ceiling 9 ft walls. However, our final design has 8 ft walls, so I assume sun penetration depth will be reduced.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Randy,
    My own advice: orient the house for the site (the views of the lake), and make sure that your wife is happy. Those factors are more important than the very small amount of energy use associated with the 30 degree rotation.

  2. Randy Bunney | | #2

    Well said, Martin. Thanks.

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