Earthships

Very much this.

 

SHGC is percentage of possible solar heat transmitted.


Clear glass has SHGC of about 0.85
US made double pane lo-e windows have a SHGC of about 0.3
Some passivhaus rated triple pane lo-e windows have a SHGC of about 0.5
Polyolefin plastic film has a SHGC of about 0.9 (which is what I use in interior storm windows). It does more therefore than just eliminate (actually reduce) drafts. It is adding another layer of still air (interior storms add TWO layers of still air).

This is why you and I want the triple panes, and Sagebrush wants the doubles (if we are basing in SHGC alone).

 

A window with a U value of 0.3 (R = 3.3) will lose 0.3 * 6890 Heating Degree Days (HDD) = 2067 BTUs / ft^2 / year, of heat loss.

HDD is for Kellogg airport.

 

Sure. Things get tricky when you start adding coatings, etc. For example the difference between soft-coat lo-e and hard-coat lo-e, is very important (particularly for SHGC, as opposed to visible transmission).

 

Oh, sh1t. I completely missed this. How dumb of me.


Thanks for pointing it out.

 

Blue, I'm sorry but I have introduced an error in your calculation (but congrats on getting the hang of it)

I did not realize that the U values I was quoting were for American units. It might be easier to first figure in btu, then convert to Wh per time_period. E.g,
1000 HDD in your coldest month, 0.5 U value double pane clear glass window:
1000*0.5 = 500 btu per SF per month
= 500*10.76 = 5380 btu per square_meter per month
= 5380/30 = 180 btu per square_meter per day

Convert btu to Wh:
180/3.14 = 57 Wh (per square_meter per day)

 

OK, now that we have heat loss (pending Corwyn' s blessing of the arithmetic), let's look at heat gain for a double pane:
The tables are from PVWATTS. I set

• 'DC rating' to 0.8 to mean an 800 watt irradiation
• Derate to 0.7 to mean SHGC of 0.7
• South facing
• Vertical orientation

Reading from the AC column, in January you can expect 59 kWh of solar heat gain that month, or about 2000 Wh a day from each square_meter of window. This also suggests to me that triple pane are not worth the money, even in your locale. However, Corwyn may yet find another error ...

Addendum: PVWATTS was set to a derate of 0.77 by accident -- I meant 0.7. So heat gain will be about 10% less -- 1800 Wh a day in January from each square meter of window.

 

Definitely end up in watts, since PVwatts uses those units.
It's just that the heat loss spec for American bought windows are in btu/hour*SF*F

 

I am hijacking PVwatts for our use. It was labeled to let people estimate AC electricity production from PV panels, but the physics are analogous. The reason to use it is two-fold:

The trigonometry of the moving sun is accounted for; and
local cloud cover is estimated from historical data.

If you want to play with other scenarios, just change the SHGC in the Derate field, and orientation if that changes. Voila!

Energy is energy

 

I think code is only meant in this case to size a furnace. Part of the idea behind PassivHaus (PH) is to do without (or with less) conventional AC, and plow the savings into insulation and HRV (or ERV.)

So sure, you can use 75F as your steady-state interior temperature to model heat loss from your windows; but without a furnace how will you get there ? That is why Corwyn (and PH, for that matter) use software to model heat gain, heat loss, and thermal mass.

 

Btw Blue, you might also want to consider the issue of weight with your windows. Figure 3-pane will weigh some 50% more than 2-pane. My 6 x 4 feet fixed windows were pretty damn heavy, even for two people to install.

 

On the subject of orientation ...

I didn't have a choice, my house is built and the best wall for sun gain faces SE. That has annoyed me for a while, but it has actually turned out to be (or will be) a very good thing (TM).

This picture is from inside the house*, and hopefully you can see the slope up to my neighbor's home. It is about 30 ft from my windows to the wall in the background.



For next winter I am going to 'plant' what I call a 'solar garden.' I envision a forest of small poles with reflective material attached. This way my windows will have heat gain from direct sunlight in the AM, reflected sunlight in the afternoon, and a combination of both in the early afternoon hours from morning to dusk. I can turn my home into a solar oven, if I wanted to


In warmer seasons I will rotate the poles to 'off.'
Since I can rotate as many or few poles desired, I have easy control of heat gain year round.

*The eagle eyed will notice that the window trim is not finished. My wife has decided that saws do not belong in the house, so completion is waiting for warmer weather.

 

Blue has remarked a couple of times that fenestration is not an important part of PH. I think that is in general not true, but it does depend on climate. I know that if I lived in a really cold and cloudy climate and was building from scratch, I would seriously consider a ground heat pump.

 

I apologize for not shutting up ... this topic is dear to my heart.

There is a fundamental difference between heating up a room to a comfortable temperature using solar gain, and using sunlight for comfort. It can be experienced by placing a thermometer outside in the shade and then in the sunlight.

In my sunroom it is chilly in the early hours of the day but extremely comfortable because we sit in direct sunlight until we get too hot or the room heats up -- whichever comes first
The room starts out the day at 54-58 F in the shade, but about 80F opposite the window. By the time the direct sunlight is gone (to be prolonged however, as I mentioned), the room temp is about 65F and it stays comfortable (for us) another hour or three.

Cloudy days of course require a different solution.

From my perspective, the problem with many an opinion on the internet about windows is that people assume that the source energy is internal to the building (e.g., a furnace) so they emphasize the U value over the SHGC and ignore the benefit of direct sunlight.

 

Exactly.

We will yet turn you into a modeler