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Old 05-24-2011, 09:51 PM   #1
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Sunroom/greenhouse for cold climate


I'm in Alberta. Ag zone 3. 10,000 degree days/year heating season. Winters can get down to -50, but -40 is more common.

I want to make an attached room that is a 3.5 season room.

In Summer, it's a screen room, with enough ventilation that it is barely warmer than outside. (Our summers are cool, 80 is a HOT day.) It's also a place where tomatoes can get ripe. (Our summers aren't hot enough to get ripe tomatoes.)

In Spring it's a green house to start our bedding plants.

In fall it's a place to extend our growing season, get an extra month or so for cool season crops such as kales.

In winter it's empty of plant life. During the sunny part of hte day hot air from the peak is blown into the crawl space to warm the floors of that part of the house. It may be used as a sitting room when the sun is out, but at night, the doors separating it from the house will be closed, and the temp allowed to drop.

The haywire, leaky greenhouse I built with an old shed and a pile of aluminum slider windows on a sunny mid winter day can show a temperature swing of better than 30 C (50 F) degrees a day. Potentially a well built sunroom can be more than this.

Taller greenhouses are easier to keep at a constant temp. It also has a higher temperature difference between bottom and top -- which means potentially more useful heat at the top to channel to the house in winter, or drive air exchange in summer.

But it also means even larger temperature swings for all the window seals.

A taller greenhouse can also intercept more light in winter. Suitable reflective curtains can redirect this toward plants during the bedding plant season. (At noon in midwinter, I have to use my sunvisor when driving south. The sun only gets 12 degrees above the horizon. Noon.

Anyway, to match the lines of the house, I'm making the sunroom internal space 20 feet tall. I have two piles of glass: 75 panes 24 x 22" of 1/8 tempered glass. 52 panes 36 minus a fraction, x 64" of 3/16 tempered glass. All without sashes. The two sizes don't have a reasonable least common multiple, so at this point, I'm looking at using one kind for the walls, and the other for the roof. I may accumulate more glass if I need it.

Glass has a different expansion rate with temp swings than wood. Good window design has bumpers in the channels to allow the window to expand and contract differentially. It also has weep holes for moisture to escape the bottom edge. Which tells me that the gasketing isn't perfect.

1. How do I use this glass properly. I'd like to have double panes -- is it reasonable to make your own sealed units?

2. I'd like a system where if I need to I can replace a pane from the INSIDE. Step ladders & scaffolding are cheaper than picker trucks.

3. Is it reasonable to make my own double pane units?

4. What are the details?
-- How do I get a reasonably airtight/watertight seal?
-- How do I mount glass so that it has room to expand and contract?
-- How do I do the roof so that there are no little edges to catch dirt on the glass?
-- How do I do units that can be swapped for screen units in summer? (I like the idea of making the entire lower level patio doors -- if I can scrounge enough patio doors.

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Last edited by sgbotsford; 05-24-2011 at 10:01 PM. Reason: paragraph breaks
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Old 05-26-2011, 02:53 AM   #2
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Sunroom/greenhouse for cold climate


It all depends on how many photons you get to your particular house. You will need to consult the sun data for your locale (azimuth, angle of the dangle, etc), factor in what type of windows you have (SHGC, R-value, leakage, size compared to floor area), and do a thorough heat loss analysis of your house. This is not simple and not easy. A specialist may have a $2500 meter that can facilitate this, plus his/her "grinding the numbers". I do not think you can do it solo without doing a lot of studying. Good luck.

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Old 05-26-2011, 08:59 AM   #3
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Sunroom/greenhouse for cold climate


I'm confident about the numbers, just from watching the behaviour of the small shed-plus-a-truckload-of-aluminum-slider-inserts greenhouse. Since I'm willing to abandon the solarium for the winter, the only difference numbers will make will be the length of the un-inhabitable season. In addition, the place where the sunroom will attach has a crawl space. In the longer run, moving heat from the peak of the greenhouse into the dirt under the crawl space during the day, and bring it back during the night should give a few more weeks of use as a conservatory at each end of the season.

The reason I've posted on this site is to get ideas on how to detail the roof to minimize air leaks, allow for relative movement between the glass and the support structure, and make maintenance as simple as possible.

Temperatures at the peak of the the greenhouse can be expected to reach 150 F in mid summer on a calm sunny day. In winter they can reach -40. Glass changes dimensions by about 3/32" per foot over this temperature range. So a 5 foot pane could change almost a half inch. (the original figures were 1/32 per foot for a 70 F change -- This will vary with the glass composition. Peak temp is an estimate, based on the degree of temperature stratification in my shed green house when the fan isn't running. Being a firm devotee of Murphy, I'm allowing slop to accumulate in the direction of greatest inconvenience)

Since I've got 2 foot windows too, I may use them for the roof. Easier to handle, and with half the size, I'd end up with half the expansion to deal with, as well has having smaller weights to deal with.

Or maybe I just have to bite the bullet and use twinwall polycarbonate for the roof.
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