Glazing performance for St. Paul retrofit

[DMcEvers]

I have a project in St. Paul, MN where I will be adding a considerable amount of south facing glass to a residence. I will also be adding several small windows to the east and west facades for ventilation and daylighting. I want to minimize heat loss through the windows but maximize solar gain, what glazing options should I consider? As I understand it, a lower U value window is often used on the west, north and east elevations and the south facing windows are optimized for solar gain/net energy gain. This will the start of a phased project to make this 1940's vintage bungalow more comfortable and energy efficient, what are my window options? How many panes, location of coatings, type of coatings, gas fill, window type, etc. I will also be doing a major upgrade to the wall and ceiling insulation levels along with very comprehensive air sealing for the new work.

Thanks,
Doug

[jesper]

Doug,
I'm in no way an expert, but I believe you have it right.
I'm working on a project for a future Passive House in Maine and I just played around with the number on the PHPP, here's what I found:

I'm using triple pane Thermotech windows with Low e coating on surface #3 and #5 (exterior is #1), 90% argon, 10% air filled.

On my south facing wall I'm using glazing with solar heat gain coefficient (shgc) 0.61 and a u-value of 0.9
For a total area of 12.7m2 (around 110sq.ft.) my total heat loses are 2005 kWh pr year, gains are 3880 kWh pr year. ( a net gain of 1775 kWh pr. year)

Now if I change my southern windows to the windows that I intend to use on all other wall (shgc .37, u-value .71)
I get the following result:

My loses are reduced to 1743kWh pr. year, but my gains just went down to 2354 kWh pr. year.

I have one west facing window with shgc .37 /u-value .71. (glazing area is about 5.5 sq.ft.)
Loses are 100kWh, gains 58kWh.

If I change my western window to shgc .61/u-value0.9

I can increase the overall gains... Loses 114kWh, gains 95kWh, but...

I believe this will lead to problems with overheating in the summer when the lower sun (harder to shade) comes pouring through the windows in the late afternoon. The same I suppose is true for the eastern side. And on the northern side you obviously just want the windows with the lowest u-value.


Jesper

[DMcEvers]

Thanks Jesper,

You have confirmed what I thought to be true for cold climates, the lowest possible U-value for east, west and north elevations and optimum net solar gain for south facing windows. The more I research windows it seems the more confused I become, a number of window manufacturers don't give a low-e type choice for optimal south facing windows. Any insight on low-e Jesper?, hard coat vs soft coat and located on which surface for south windows? What should I expect for a SHGC for a dual pane low-e window to maximize solar gain for the south elevation? Or is tri-pane clear glass a possibility for south windows?

Thanks again,
Doug

[Stephen T]

Jesper,

What happens if you put the high solar gain low-e on the north?

The reason i ask, is that we've done some HOT2000 runs (Canadian software). We get a lower heating energy cost with a high solar gain low-e on the north rather than the intuitive choice; the better insulating, lower SHGC/G low-e. Interestingly to us, the answer is the same when we use London, England weather data instead of Ottawa, Canada weather data. (Admittedly, the margin in London is much much smaller than the margin in Ottawa.)

To be clear, in all cases the north facing windows are net energy losers; just less so with the poorer insulating higher SHGC/G low-e.

Would be interested in knowing what result you get w/ the PH software......

[Mark Siddall]

Stephen,
As I think you know this is the net-solar gain or energy balance of the window that is playing a part. This needs to be determined for the window as a whole not just the glass. Under pinning the PassivHaus window is the net-solar balance and in this context a useful design parameter is the S-factor:

(typ. solar radiation at surface during winter)*0.75
S = ----------------------------------------------------
Heating degree hours

U - S * g

Whilst the climate based S-factor is reasonably fixed within a given region/climate for the purposes of design assessment for glazing the lower the S-factor the better the energy balance. (Unfortunately the S-factor relates to the Uw not the Ug; so it makes it hard to compare the real benefits of improving the centre pane U-value at the expense of the g-value.) That said it is possible to develop a notional understanding that allows one to compare the relative merits:

Min S-factor for a PassivHaus window (Central Euopean climate) = 1.6 W/m2K

PassivHaus:
Uw = <0.8
Min Ug = 0.7
g-value = >0.5
notional centre pane S-factor = 1.4 W/(m2K)

Option 1
Ug = 0.51 W/m2K
g-value = 0.6
notional centre pane S-factor = 0.85 W/(m2K)

Option 2
Ug = 0.44 W/m2K
g-value = 0.38
notional centre pane S-factor = 1.16 W/(m2K)

The option with the best energy balance is therefore Option 1 as it achieves the lowest S-factor. At a regional level the S-fcator is a useful concept but on a site by site basis issues such as over shading and the like have an impact upon the final energy balance of a window.

Mark

[Stephen T]

Mark,

The situation i described in my previous post was based on whole window characteristics.

If i may attempt to paraphrase your post, window energy efficiency must consider not only heat loss, but also heat gain. More specifically lowest U doesn't mean lowest heating bill. (end of paraphrase)

This 'falls out of' your simplified approach as well as detailed analysis like that which is part of the PHPP.

If i can, i'll plug the Canadian ER approach which, while no substitute for detailed analysis, does predict the relative ranking of window energy efficiency choices. Like the whole house runs we did using HOT2000, the ER approach predicts that a high solar gain low-e on the north, saves more energy than the better insulating low solar gain low-e.
My question was, does the PHPP produce a similar or a contrary result ?????

Hope that helps