What's the real R value of a good window?
Last Post 02 Jul 2012 10:05 AM by Lee Dodge. 8 Replies.
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strategeryUser is Offline
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29 Jun 2012 03:55 AM
Or does it even really matter?

Suppose I'm looking to upgrade my north-facing windows from a double pane to a triple with low-e coating. My double pane window is a bit too cold and it bothers me because it's in my bedroom. I have my house well air sealed, so as far as I can tell the only way to improve the comfort of having to be next to that window on a cold winter night is to replace it. What factors should I be considering?

Re-siding the house is down the road a ways. I might like to replace my two north-facing windows first.
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30 Jun 2012 01:28 AM
Posted By strategery on 29 Jun 2012 03:55 AM
Or does it even really matter?

Suppose I'm looking to upgrade my north-facing windows from a double pane to a triple with low-e coating. My double pane window is a bit too cold and it bothers me because it's in my bedroom. I have my house well air sealed, so as far as I can tell the only way to improve the comfort of having to be next to that window on a cold winter night is to replace it. What factors should I be considering?

Re-siding the house is down the road a ways. I might like to replace my two north-facing windows first.

On cold days/nights, poorly insulated windows basically suck the warm air out of the room and create that cold spot near the window.

Most residential windows in the USA are probably around an R-1. Good windows would have a R-5 or higher. > R-5 should be what to look for.

Also make sure the window is air tight as not to create cold drafts. Avoid sliders or single or double hungs. Casements are better in sealing air.


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30 Jun 2012 03:12 AM
According to DOE:

"Common ENERGY STAR windows only have an R-value of 3. Increasing the R-value from 3 to 5 reduces average heat loss through the windows by 40% and saves consumer money on energy bills.”


Lee DodgeUser is Offline
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30 Jun 2012 12:30 PM
strategery-

If there is room inside the windows, you might consider adding double-cell cellular shades with side seals to your exiisting windows as a less expensive fix for the bedroom chill compared to replacing the windows with triple-pane windows.  If your windows are not low-e, then the R-value for double panes is about R-2, and the double-cell, light-blocking, cellular shades with side seals have an R-value of around 2.0 or greater (http://www.residentialenergylaborat...hades.html), so the combined R-value would be about 4 (or U=0.25).  These shades for a 40"x60" window cost about $130 if you install them yourself, which should be much cheaper than a good-quality, triple-pane window.

These shades also dramatically reduce the convective air flow that is downward across the windows, and can be improved further by using something like Permagum to seal any openings around the enda of the side seals.
 
I have these shades throughout the house, and I can sleep comfortably next to the windows with it at -15 F (-26 C) outside, although my windows are triple-pane and low-e.
Lee Dodge, Residential Energy Laboratory, in a net-zero source energy modified production house
strategeryUser is Offline
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02 Jul 2012 03:15 AM
Some more questions-

So a triple pane window argon filled is *probably* and R-5?

Given that the wall around that window is about R-12 - R-15, that glass is ALWAYS going to be a weak spot where the conductive heat loss occurs, correct?

I know in passive solar designs, they try to limit windows on the north side for 2 reasons: 1) glass on the north side receives little to no direct sunlight and therefore no beneficial solar gain during the winter 2) the glass is a weak spot of the wall and is susceptible to greater conductive energy loss

I really only will have one opportunity to get these two windows right when making this replacement. Should I go for the very best window I can afford for these two north-facers? or is it not that big of a deal as long as it's a double pane low-e with good air-sealing?
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02 Jul 2012 05:58 AM
Posted By strategery on 02 Jul 2012 03:15 AM
Some more questions-

So a triple pane window argon filled is *probably* and R-5?

Given that the wall around that window is about R-12 - R-15, that glass is ALWAYS going to be a weak spot where the conductive heat loss occurs, correct?

I know in passive solar designs, they try to limit windows on the north side for 2 reasons: 1) glass on the north side receives little to no direct sunlight and therefore no beneficial solar gain during the winter 2) the glass is a weak spot of the wall and is susceptible to greater conductive energy loss

I really only will have one opportunity to get these two windows right when making this replacement. Should I go for the very best window I can afford for these two north-facers? or is it not that big of a deal as long as it's a double pane low-e with good air-sealing?

Most good triple pane windows will be in the R-5 through R-7 range, although Serious Windows makes a dual pane window with R-5 through R-9 (very expensive). They way they get these values with dual pane is by utilizing heat mirror technology. Some people question the longevity of this technology but from what I read it is reliable. Although European manufacturers find this technology flawed and don't use it, they stick with triple pane.

R-Value is only ONE part of the equation. Windows must also be AIR TIGHT. A leaky window in winter will let in the cold and let out the warm interior air. The reverse is true in summer. The other part of the equation is SOLAR GAIN. If you are in a heating dominated climate, having a lot of south facing windows that have a high SHGC will help heat your home during winter. Even though at night the windows have a negative impact on the home, the overall SHG makes up for it as it cuts your heating costs during the day.

Windows also serve the function of letting in natural daylight and venting the home (if operable). North windows SHOULD be minimized but not eliminated as they provide indirect light during the daytime. Nobody wants to live in a cave, let me clarify that, most people don't want to live in dark caves.

Your question has a lot of factors. How long do you plan on living in the home? I would get pricing for each design (dual and triple pane), check the air sealing ratings, and then make a choice based on your budget and goals. Either way, you are looking at a long term ROI.

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02 Jul 2012 06:34 AM
This is a bit off the wall but a friend of mine, 20+ years ago, made his south windows about 3" apart and used a Beam vac to fill them up with styrofoam beads at night or when it got too hot in the house. His heating bills were quite low, cooling was almost non existent and the system could be automatic or manual plus the beads could be replaced if they yellowed too much.

The interesting part of it was that the light was very nice in the day with the beads in the windows.  I don't know what the R value would be as he had single pane glass. Double would be quite something.
www.BossSolar.com
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02 Jul 2012 08:10 AM
Mike; Insulated glass is used for bothe heating and cooling conditions, once you get past 1" the space invites solar gain and may be good for heating, but bad for cooling
Chris Kavala
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Lee DodgeUser is Offline
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02 Jul 2012 10:05 AM
Posted By strategery on 02 Jul 2012 03:15 AM
Some more questions-

...snip...

I really only will have one opportunity to get these two windows right when making this replacement. Should I go for the very best window I can afford for these two north-facers? or is it not that big of a deal as long as it's a double pane low-e with good air-sealing?
If you really want "to get these two windows right," download a free copy of RESFEN at http://windows.lbl.gov/software/resfen/resfen.html and do your homework by computing heat losses with your old windows and different proposed new windows.  You could also use BEopt available for free at http://beopt.nrel.gov/ which will also do the optimization for you, but I would judge it a little more difficult to use.  I would expect that this analysis will tell you that, as you said, it is not that big of a deal as long as it's a double pane low-e with good air-sealing, but why not get the real answer by doing the calculations?  I would guess that the break-point might be $20 to $30 per square foot of window, or for sleeping comfort, you could get there much cheaper with window coverings.  

You might also check out the calculations included in the earlier discussion in this same section titled "Going from a U .20 to .24 Worth the extra cost."  peteinny was asking a question very similar to your question. 

Lee Dodge, Residential Energy Laboratory, in a net-zero source energy modified production house
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