What's more important in northern minnesota?  I'm really quite the novice to passive solar fyi.  Anyways, ideally i would get south facing windows with really low u values, and really high shgc....obviously.  Problem is, i'm kind of on a budget, and am not looking to spend a fortune on windows.  So my question is, would it be better in my climate to get a lower u value, say in the high .20's, low .30's....with a shgc in the low to mid .40's?   Or would it be more beneficial in my case to get the south windows with a mid .30's u value, and a shgc in the mid to high .50's?....or since i'm kind of just trying to go half way, is it just *#!@, just don't do it at all if you're doing it like that? haha  Also another question, it's going to be a 2 story house, the thermal mass on the lower level will be the 4" slab....on the upper level though, don't have much.  If i used a dark tile for the flooring upstairs, I'm assuming that probably wouldn't be enough to notice any difference in heat retention?  And too, if any of you know of any window company's that are pretty decent in price for passive solar windows, that would be helpful as well.  I got a rough quote from marvin for windows with a u value of about .3, and shgc of .5.   For 4 to 5, 4' x 4' windows, that would apparently run me about $7-800 apiece.   That's a bit much for me, trying to keep budget for total windows around 2- 2.5 grand.  I think that covers it...but if you have any other advice for a newbie, that would be appreciated as well.

-Thanks

I'd be surprised if Marvin is going to be your price point competitor. First, select windows with a code-compatible U-value. Then, make sure you can get high SHGC glass in them. Those are your southerly windows. All other windows are the same without the high SHGC glass. If you have any window budget left, then you can maximize the window quality in terms of air sealing.

For a passive solar, you MUST model your solar apertures. DO NOT make them any bigger or more numerous than they need to be.

You should have a good enough envelope that temperatures on the second floor should take care of themselves. I wouldn't go for much passive solar on the second floor unless you have a good circulation system.

Like ICF indicated, for the PS fenestration you want the highest SHGC that you can find and the lowest U-factor (highest R-value) that you can afford. You need to properly size the PS fenestration for your climate and building/room heat loss while also accounting for the PS roof overhang design and any local terrain obstacles. Depending on how aggressive you get, you may also need thermal mass to buffer the PS heat gain and need to properly size this too. In summary, PS design involves determining the BTUs for all the possible daily PS/climatic scenarios and designing a building and control methodology that will handle all these scenarios while maintaining your desired constant indoor design temp. We have a free DIY suite of PS design software on our website that you may find useful: 1) Passive Solar Altitude Angle, 2) Passive Solar Roof Overhang Design, 3) Passive Solar Fenestration Exposure, 4) Passive Solar Heat Gain, and 5) Passive Solar Thermal Mass Performance.

Posted By patrick55702 on 06 Apr 2014 10:50 AM

...if any of you know of any window company's that are pretty decent in price for passive solar windows, that would be helpful as well.  I got a rough quote from marvin for windows with a u value of about .3, and shgc of .5.   For 4 to 5, 4' x 4' windows, that would apparently run me about $7-800 apiece.   That's a bit much for me, trying to keep budget for total windows around 2- 2.5 grand...

What type of window frames were you quoted by Marvin?  Wood, fiberglass, or ?  Were these fixed or operable windows?

I paid $482 for a 4'x4'6" fixed window, Marvin Integrity All-Ultrex (fiberglass frame), with Cardinal LoE180 glazing, U-Factor = 0.31, Solar Heat Gain Coefficient = 0.55.

To get better than a WAG the relative passive solar performance of windows with differing SHGC & U-factor specs, download a copy of RESFEN and learn how to use it. (It's not very complicated.)

http://windows.lbl.gov/software/resfen/resfen.html

With your site information you can then plug in the relevant SGHC and U-factors and see how that affects overall energy use to know when the somewhat higher U-factor higher-SGHC window outperforms a lower-U/lower-SGHC window. The optimal window is both local climate and site-factor/orientation sensitive. With some window comparisons it'll be obvious, others not so much.

Unless RESFIN has been significantly improved since the last time we used it, we found it pretty useless for designing a passive solar building. It didn’t properly account for actual roof overhang design or actual local terrain obstacles, and it could only handle discrete North, East, South, and West fenestration orientations as opposed to the actual fenestration orientations. Maybe it has been improved since we last used it. It was certainly easy to use and it generated a nice looking output form...but the RESFIN over-simplified math modelling and assumptions left much to be desired.  In any event, you really need software that can accurately determine the maximum instantaneous PS heat gain and the predicted climatic PS heat gain that occurs during the irradiance period for each day/hour of the year for your actual PS design.

Posted By sailawayrb on 07 Apr 2014 05:54 PM
Unless RESFIN has been significantly improved since the last time we used it, we found it pretty useless for designing a passive solar building.

RESFEN is a disaster when it comes to PS. As you mentioned, the mathematics were bad. Passive House software (PHPP, WUFI) works better but it is not free.

Another glitch with RESFEN is that when one inputs a lower U-Value and all other variables remain the same, the cooling costs INCREASE. How is that possible? All variables are the same; SHGC, AI, and the only change is that the U-Value of the windows goes from 0.28 to 0.13 and RESFEN penalizes you and shows a higher cost to cool!

RESFEN will also penalize you when it comes to running ductless mini-splits as the software is not advanced enough to recognize the heating costs of a gas furnace with duct work vs. an electric ductless mini split.

When modeling homes I was told that RESFEN is antiquated and that to properly do a Passive Home design one has to use a modernized software package.