A wall assembly that is at R-50 and when 15% window glazing is installed using double pane Energy Star rated windows at R-3 will experience a 70% reduction in R-Value and equal a R-15 whole wall average. Going to triple pane R-8 glazing will increase the whole wall to R28 average. A significant difference.

R-50 Wall with double pane R-3 windows (15% glazing) = R-15 whole wall average
R-50 Wall with triple pane R-8 windows (15% glazing) = R-28 whole wall average

(corrected #s)

That is not correct.

Assuming R50 is the whole-wall value, all thermal bridging, it's U-factor is about 1/R50=0.02. The U-factor of the "R3" window is U 0.33. The average U-factor of the whole section of wall is:

(0.85 x 0.02) + (0.33 x 0.15)= 0.0655

Which makes the average R-value (1/0.0655)= R15

With the R8 window the U-factor of the window is (1/R8=) U0.125

...making the average wall U-factor

(0.85 x 0.02) + (0.125 x 0.15)= 0.03575

...for an average R of (1/ 0.03575=) R28

It doesn't quite cut the heat loss of that window/wall assembly in half, but it almost does.

Of course this performance is only going to reflect the heat loss. During daylight hours there will also be radiant gains, depending on the radiant spectrum incident on the window and the selective reflectivity & absorption of the window coatings & glass.

Of course this performance is only going to reflect the heat loss. During daylight hours there will also be radiant gains, depending on the radiant spectrum incident on the window and the selective reflectivity & absorption of the window coatings & glass.

Yes, those are the same R-values that I got, R-15 and R-28.

And as Dana1 says, it can be a shortsighted view that windows can only be a heat loss. My windows are a net heat gain, not loss, in the winter due to windows with a high SHGC and a sunny climate, and I don't need A/C in the summer due to overhangs, shades, and cool nights. In fact, I added an extra window on the south side to increase passive solar heating.

All low-E windows with a U-factor of about U-0.35 or less are a net heat gain during the winter months, independent of the number & type of low-E coatings.

But with insufficient thermal mass to manage it, over-glazed houses still use a lot of heating energy, overheating during sunny days, and needing heat input to maintain temp at night. If it were EASY or CHEAP to build passive solar houses using a lot of U-0.35 glass home heating energy use would have long since dropped to inconsequential levels. The PassiveHouse approach tries to balance it all out, with large but fairly low-U windows on the south side, and only enough glass for daylighting elsewhere (or at least that's how all of the cost-optimized PassiveHouses end up.)

DOE2/BeOpt models the gain/loss aspects reasonably well.

My apologizes on the incorrect R-value for the R8 window wall. Journal of Light Construction & Hammer and Hand made the blunder and I did not check the #'s before posting it: JLC 2011 PDF

So here are the corrected stats:

R-50 Wall with double pane R-3 windows (15% glazing) = R-15 whole wall average
R-50 Wall with triple pane R-8 windows (15% glazing) = R-28 whole wall average

Still it is a significant amount from a double pane vs triple pane window. I also did not address the passive solar heating aspect which others touched upon. From a whole wall R-Value point of view, the triple pane windows make a difference, almost doubling the whole wall R-Value.

Utilizing passive solar and triple pane windows is where the green energy builds are currently heading. Double pane R3 windows on the north, east and west sides are energy penalties for the most part. Installing windows on the south side only will make a house very unappealing and dark. I found it very interesting to see how much a double pane window can reduce the whole wall R-Value of a R-50 wall down to a R-15 wall.

In a passive solar, you can put windows on the North so as to avoid that dark problem, but take care to control their size. Even smaller windows will let light in without giving up too much heat. When designing, the trick is to put as much of your non-window containing rooms like closets up against the North side.

Even the best windows significantly reduce performance unless you can successfully use them for passive solar gain. A good floor plan that puts some of the kitchen, bedroom, and bathroom walls not requiring windows on the north, west, and east exterior walls can mitigate the unappealing concern. You can use solar tubes (although we like to minimize holes in roofs) and lighting to further enhance the appearance without throwing heat out the windows.

On the southern facing rooms having an exposed concrete slab works better for passive solar than a carpet covered floor, correct? Can one have too much exposed interior concrete?

What is the increase of heat rejection caused by the third pane? Each pane rejects as certain amount of light/heat. Highest solar gain would come from a single pane (as would heat loss.)

Posted By FBBP on 01 Nov 2013 10:26 PM
What is the increase of heat rejection caused by the third pane? Each pane rejects as certain amount of light/heat. Highest solar gain would come from a single pane (as would heat loss.)

A lot depends on the manufacturer, glazing spacing, and of course the SHGC. Guardian and Cardinal glass are your main two window glazing manufacturers.

European windows like Intus use wider glazing spacing (helps R-Value & STC ratings) and one can get SHGC of 0.61 in triple pane. Triple pane windows can have STC ratings of around STC 47.

Posted By ICFHybrid on 01 Nov 2013 07:02 PM
In a passive solar, you can put windows on the North so as to avoid that dark problem, but take care to control their size. Even smaller windows will let light in without giving up too much heat. When designing, the trick is to put as much of your non-window containing rooms like closets up against the North side.

Even on the north side there's a small but real net gain above the nightly losses with any window with a U-factor south of 0.35, but the higher peak heating load of the north facing rooms along with the extremely tepid gains leads to more comfort issues if you overglaze the north side. There's no point to putting more north glazing than is necessary for daylighing (or meeting code-egress issues for bedrooms), and the passive solar benefit is much greater on the south side (no rocket science here.)

Posted By Dana1 on 01 Nov 2013 02:42 PM
Assuming R50 is the whole-wall value, all thermal bridging, it's U-factor is about 1/R50=0.02. The U-factor of the "R3" window is U 0.33. The average U-factor of the whole section of wall is:

(0.85 x 0.02) + (0.33 x 0.15)= 0.0655

Which makes the average R-value (1/0.0655)= R15

With the R8 window the U-factor of the window is (1/R8=) U0.125

...making the average wall U-factor

(0.85 x 0.02) + (0.125 x 0.15)= 0.03575

...for an average R of (1/ 0.03575=) R28

It doesn't quite cut the heat loss of that window/wall assembly in half, but it almost does.

Of course this performance is only going to reflect the heat loss. During daylight hours there will also be radiant gains, depending on the radiant spectrum incident on the window and the selective reflectivity & absorption of the window coatings & glass.

Is R3 an average double pane window performance?
Is R8 a very good triple pane window performance?

How does Ug translate to R* or to U-factor?  I see values of 1.5 and down to 0.5 for passive windows.  So, these numbers are not a direct 1:1 correspondence

u value is the inverse of the R value so an R-8 window = 1/8= U-.125 which is an excellent window. R-3= 1/3= U-.33; typical double glazed. "typical triple glazed windows are U-.20 = 1/.20= R5.