After reading good things about them - I decided to try insulation cellular shades with sidetracks for air sealing. They seem to do a good job of insulating the window but the windows that have them get a lot of condensation. It's to the point where I've lost the paint on those windows at the bottom and have had some mold growing on some. Other than wiping them every day in the winter does anyone have suggestions? I may replace the windows someday but probably not now. I have Pella windows.

In cold weather, window insulation belongs on the exterior side.

How would you put these on the outside?

http://www.comfortex.com/cellular/Comfortracks.php

I have had to run a dehumidifier to reduce the condensation on my windows.

Chris

Indeed condensation on the interior side of windows is a problem for insulating shades or curtains. A highly insulating shade will reduce the temperature of the innermost window surface, increasing the chances of condensation.

In my dry climate, I use a humidifier to maintain an indoor humidity of 35% or 40%. However, the Honeywell thermostat that controls both the hot air furnace and the humidifier has an algorithm to reduce the humidification as a function of the outdoor temperature, with a user adjustable amount for the level of humidity reduction. This feature is specifically aimed at reducing window condensation. For example, the humidity set-point is currently 40%, but the outside temperature has dropped to 9 degF, and the indoor humidity is 25%. The steady-state indoor humidity might be even lower at this outdoor temperature for the current setup on the thermostat, but the house has not yet had time to reach equilibrium (the temperature has been dropping quickly).

I have used "Virginia Premium Elastic Sealant Compound" to more completely seal the top hanger of the Comfortex light-blocking, double cell, cellular shades with side seals under the theory of letting the water vapor trapped between the shade and the window condense, but to limit to transport of further water vapor into that space. I think this effort slightly reduced the amount of condensation on the windows, which usually only becomes a problem when the outdoor temperature drops below zero degF. The control of humidity is a more effective approach.

Last night it got down to -5 F (-21 C), and I had about 1" to 6" of frost on the bottom of all the windows this morning. The double-cell cellular shades with side seals had been down all night on all the windows. After raising the shades this morning, it took about 30 minutes to 1 hour for all the frost and water to evaporate. The relative humidity in the house was 20% at the time, the humidity having been reduced from the set-point of 35% by running the HRV, and the thermostat limited the humidifier from running due to the low outdoor temperature and the condensation reduction algorithm.

I am looking into those shades for my windows.

Chris

Would the use of windows that have a lower U-value better handle humidity and keep condensation from forming? Would a triple pane window with a U-value of 0.16 be less likely to form condensation?

Probably not. The glass will still be the coldest surface so thats where the water vapours will condense.
Really the only way to handle the situation is not to close the windows off and let the warm air circulate.
Or as Jon said - insulate the outside the raise the glass temps.

Posted By FBBP on 06 Dec 2013 11:20 AM
Probably not. The glass will still be the coldest surface so thats where the water vapours will condense.

That is clearly an inaccurate statement. Let us accept the fact that colder temperatures on the innermost glass surface will increase the chances of condensation.

Assume an outside temperature of 0 F (-17.8 C) and an indoor temperature of 70 F (21.1 C). The innermost glass surface will be cooler than room temperature, because of the insulation value of the air film on the inside surface. This film has an R-value of 0.68 hr ft^2 degF/Btu according to http://www.coloradoenergy.org/procorner/stuff/r-values.htm. (All R-values will be in this set of units and taken from this same reference.)

So for a single-pane glass window with an overall R-value of 1.10 (U=1/R=0.91), the R-value excluding the inner air film is only 0.42, and the innermost glass surface is 26.7 F (-3.0 C). Add an R-2 shade, and the glass temp drops to 9.5 F (-12.5 C). For an uncoated double-pane window with U=0.49, the glass temp is 46.7 F (8.2 C). With an R-2 shade, the glass temp is 23.6 F (-4.7 C). For a code-min double-pane Low-E window with U=0.35, the glass temp is 53.3 F (11.9 C). Add an R-2 shade, and the glass temp is 31.4 F (-0.3 C). For a high-performance triple-pane Low-E window with U=0.16, the glass temp is 62.4 F (16.9 C). Add an R-2 shade and the glass temp is 47.3 F (8.5 C).

The condensation problem is clearly reduced when using a lower U-value (higher R-value) window, and the reduction in temperature with the shade is less for the better insulating windows. By the same token, the improvement in thermal performance by adding the shade is less significant with the more insulating windows, making the cost of the shades harder to justify.

Posted By FBBP on 06 Dec 2013 11:20 AM

Or as Jon said - insulate the outside the raise the glass temps.

That begs the question. Do jonr or FBBP use external shades on their own houses. If not, why not?

Lee- Bear asked "and keep condensation from forming?" That is what I was answering to.
We were at -31ºC this morning and my triple panes had condensate and even a little ice so I would suggest that it is an accurate statement. In the realm of relativity, I would agree with what you have written. The better the window the less condensate is an issue. But it remains an issue.

I would not use external shades or shutters as the can be problematic in extreme cold and wind. Most are ugly to my eyes.

A while back there was some studies that indicated that unless the seal of the inside shade was 100%, the heat loss actually increased do to convection scrub of the glass.
But is you could get 100% seal you wold move the condensate point to the shade. It would now be the coldest point but with a high dew point.

Lbear stated: "Would the use of windows that have a lower U-value better handle humidity and keep condensation from forming? Would a triple pane window with a U-value of 0.16 be less likely to form condensation?" You answered "probably not."

I presented data that clearly show that a triple-pane window with a U-value of 0.16 IS less likely to form condensation than a window with a higher conductivity. "Less likely" does not mean never. It means over the range of outdoor temperatures, will condensation form on windows less often with the lower U-values. The answer to this question is "yes." Further, adding insulating shades will increase the number of conditions where condensation will form on the windows, but a lower U-value window will reduce the number of conditions resulting in condensation compared to a higher U-value window.

FBBP also states, "A while back there was some studies that indicated that unless the seal of the inside shade was 100%, the heat loss actually increased do to convection scrub of the glass." Please provide the references for these studies. I took measurements for double-cell cellular shades, both light-blocking and light-filtering shades, both with and without side seals, and under all conditions the shades increase the overall insulation value of the window plus shades compared to window alone. In none of these cases was the seal 100%, although the light-blocking shades with side seals do an excellent job. These measurements and analysis are provided at http://www.residentialenergylaboratory.com/r_value_cellular_shades.html.

Posted By FBBP on 06 Dec 2013 02:55 PM
...snip...
I would not use external shades or shutters as the can be problematic in extreme cold and wind. Most are ugly to my eyes.
...snip...

Exactly my point! So why would you recommend them to the original poster?

Posted By Lee Dodge on 06 Dec 2013 02:02 PM

So for a single-pane glass window with an overall R-value of 1.10 (U=1/R=0.91), the R-value excluding the inner air film is only 0.42, and the innermost glass surface is 26.7 F (-3.0 C). Add an R-2 shade, and the glass temp drops to 9.5 F (-12.5 C). For an uncoated double-pane window with U=0.49, the glass temp is 46.7 F (8.2 C). With an R-2 shade, the glass temp is 23.6 F (-4.7 C). For a code-min double-pane Low-E window with U=0.35, the glass temp is 53.3 F (11.9 C). Add an R-2 shade, and the glass temp is 31.4 F (-0.3 C). For a high-performance triple-pane Low-E window with U=0.16, the glass temp is 62.4 F (16.9 C). Add an R-2 shade and the glass temp is 47.3 F (8.5 C).

The condensation problem is clearly reduced when using a lower U-value (higher R-value) window, and the reduction in temperature with the shade is less for the better insulating windows. By the same token, the improvement in thermal performance by adding the shade is less significant with the more insulating windows, making the cost of the shades harder to justify.

Would putting the triple pane window more towards the inside of the window buck area help with U-Values and condensation?

Building Science Corporation released a study that hinted that "innie" windows are less likely to condensate than "outie" or flush mounted windows . Building Science PDF

This from GBA: (emphasis mine)

Thorsten Chlupp, a builder in Fairbanks, Alaska, agrees with Hebert that innie windows are best. In an article published in the Journal of Light Construction, Chlupp explained his preference for innies: “Windows can be installed either at the face of the sheathing — in a recess — or out at the face of the wall. From a performance standpoint, a recess is better, because the window is somewhat protected from wind-washing and the interior glass is more easily warmed by the heat in the room. By contrast, windows installed at the face of the wall are in an interior recess, separating them from the warm air inside (especially if a curtain is drawn) and exposing the outer layer of glass to cold wind. I’ve observed that in extremely cold weather — when it’s 25°F below zero, for example — frost tends to form inside windows installed at the face of the wall, whereas frost rarely occurs on inset windows. I’ve installed windows both ways, but because of the frost problem I now do only recessed installations.

Lee - how does your test allow for the convectional scrub?
It would be interesting to see the values at the top and bottom behind the shades. Even the top and middle un-averaged temps might begin to tell the tale.

If air can get in at the top and out at the bottom you will still see a warmer air temp then if no shades where present, but with continual air wash from the adjacent room, even with warmer temps at the sensors, there can still be an overall heatloss to the building. It might well be the air movement is being taken as increase r values for the shade.

The reports I am try to re-find showed that this was the case. When the shielding material had very low r value, it still appeared to increase the performance or R value of the shield but it was because the warm air was drawn in at the top and as it cooled, it kept pumping the room air through the window space at a much greater rate then if there was no shield. This air cooling was a net heat loss to the building that did not occur when there was no shield. And of course it dramatically increases condensation.

Lbear asks: "Would putting the triple pane window more towards the inside of the window buck area help with U-Values and condensation?"

I have not made measurements comparing these options, so I can only speculate on the possible results. The air film on the outside of the window is given an R-value of 0.17, while the R-value on the inside surface of the window is estimated at 0.68 by http://www.coloradoenergy.org/procorner/stuff/r-values.htm. The R-value on the outside of he window is less than on the inside due to the forced convective heat transfer from winds, as opposed to the weaker natural convection on the inside surface. Moving the window inward would tend to increase the R-value of the air film on the outside surface, but it would never reach the value of 0.68 on the inside surface. And the value on the inside surface might be reduced as the window is exposed to stronger natural convection as it approaches being flush with the inside walls. So we might guess that the R-value on the outside might increase from 0.17 to 0.35 when it is well recessed. For a total R-value of 6.25 for your triple-pane windows, corresponding to a U-value of 0.16, changing the outside film coefficient by 0.18, or 3%, would not be significant.

Another possible effect would be that moving the window inward would allow the heat transfer through the wall to short-circuit reaching the outer part of the wall, and instead lose heat into the air in the recessed window area. This would tend to slightly heat the outer part of the window, and could be a larger effect than the change in the air film coefficient, but is hard to estimate. Of course, the overall heat transfer through the walls would increase due to this short-circuiting.

There are some other disadvantages to making a window flush with the inner walls. In that case, shades could no longer be used inside the inner window spacing, and you could not use side seals on shades, so reducing the convective heat transfer off the inner window surface would be more difficult. Some people don't like the looks of innie windows, complaining that they look like windows in a mobile home with thin walls. Indeed I had friends who paid a carpenter to remount their windows to convert them from innie to outie windows for that reason. In the case of very thick walls, perhaps one could slightly recess the windows and still have some recess on the inside also.

Bear - mounting the windows out of the wind stream should reduce the scrub effect on the glass, unless this causes additional turbulence.
The interior window being more warmed by the inside air would prevent some condensate/icing but would increase the actual heatloss.

When windows are mounted to the outside in an ICF wall and then covered with shade/drapes, there is a large cavity produced. This allows for not only the convection as discuss above, but also convectional currents inside this cavity. That is, there will be a cool downdraft along the glass and a warm updraft along the shade. This will produce even greater heatloss. As you move the window closer to the shade (innies) there is no longer as much room for these two separate currents to pass each other and therefore the air movement should slow down. This is the same reasoning as why putting a larger air gap in sealed units does not necessarily increase U values. Overall, I'm not sure there would be a large difference in the actual performance of innies or outies but innies are much harder to flash properly.

If you are using foam bucks (and I know you will ;-)) there may be a slight advantage in having the window frame more protected by the wall.

FBBP-

The heat transfer from a warm room to a cool window is due to natural convection (assuming the HVAC system is not blowing directly across the windows). The heat transfer rate is reduced when the air velocity is reduced. Shades will tend to reduce the air velocity across the window, especially if they are well sealed with side seals. The shades add restriction to the free flow of air. That is what the theory says, and that is what my measurements have shown. If you have measurements that contradict this, please share them. Shades and drapes also reduce radiative heat losses from the room to the window.

Edit: FBBP: This air cooling was a net heat loss to the building that did not occur when there was no shield. And of course it dramatically increases condensation.

Condensation releases heat (8K btu per gallon) - so you don't want it occurring on a window.

Jon - of course you don't want condensation (unless its a boiler) but that is what happens when there is a draw of air past the window.