R-value of insulated shades
Hi Guys,
Thought you all might be able to add some insight. We needed shades for our recently built Net Zero home, and we ended up buying double honeycomb style.
They are marketed as “insulated” shades, and they are motorized, so we won’t forget to put them down during the day in the summer, or during the night in the winter. The claim is that they can provide an R-Value of nearly 3, which seems crazy high to me.
Source:
https://cellularwindowshades.com/content/33-rvalue-and-insulating-cellular-window-treatments
Is this accurate? Can a window shade nearly double the R-Value of my window? This is a large window… nearly 9′ * 4.5′, so there’s a lot of glazing involved. These shades will cover all the windows on the main floor. Will this be enough to lower my winter heat pump electricity usage?
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Replies
Believe it or not, the R-value is reasonable.
Comfortex did some of the early research on thermal performance of cellular shades, with CARB (as indicated on the website of the url you cite), and I am pretty sure that Andy Shapiro developed a method for assessing R-value of Comfortex shades similar to what CARB did.
And the IR images in the material on the website? Those were taken at my house in early work I did (maybe 2010?) with researchers from Lawrence Berkeley National Labs.
To get up in the R-3 - R-4 range, you definitely need airtight cellular fabric and sidetracks (in the research we did, we used sidetracks on blackout shade fabric).
There used to be a website http://www.efficientwindowcoverings.org but that all got absorbed and recently disappeared with the Attachments Energy Rating Council (AERC). On that website, is a link to the new AERCalc software that LBNL developed to assess the energy performance of window coverings, including cell shades. I have not used the software but I bet its use would support, approximately, the thermal performance asserted by Comfortex.
Peter
If they are not vapor proof, you will probably get condensation on the glass.
Since windows are so poorly insulated compared to other parts of the building thermal envelope, you should notice a decrease in heating requirements during the winter if you use tight fitting window coverings that increase the window's R-value/U-factor significantly. You could estimate that difference by calculating the approximate area of the covered windows, the two different (approximate) R-values, and using a conductive heat loss formula
BTUs = SQFT X (temp-indoor minus temp-outdoor) X hours divided by R-value
You might want to try to seal the top rail to the window frame to reduce air leakage there. Maybe add something below the bottom rail if you notice air movement between the bottom and the window frame.
In my case, there was a very noticeable difference when you raise and lower the shade during very cold weather. Sometimes ice formed on the bottom inside of the large window glass when using R-7 pop-in insulating shutters, which were not airtight at the corners (due to water vapor condensing then freezing on the R-2 glass).
Also exercise caution if there's significant solar gain with the shades closed, heat buildup could degrade window glass seals over time.
There are these articles.
http://www.cchrc.org/sites/default/files/docs/JGB_windows_article.pdf
http://www.cchrc.org/sites/default/files/docs/window_insulation_martin.pdf
http://www.cchrc.org/sites/default/files/docs/window_insulation_final.pdf
And this product.
https://www.windowquilt.com/
Side tracks certainly seem to help.
Tom is right: for both cell shades and window quilts, they can be "airtight" but also vapor permeable and particularly overnight, you can get condensation.
From my own experience with the cellshades: the blackout shades are very low vapor permeability and so when "sealed" with sidetracks, condensation is minimal. With other types of cell shades and window quilts, I am afraid that "morning mopping" is part of the routine, but frankly, it's manageable...
Peter
I can see why you would be skeptical, with silly statements like "Energy is always trying to expand. In the winter, the warm air inside your home naturally moves toward and out your windows, so that it can expand into cold, open air." and "A typical window has an R-value ranging from 0.9 to 3.0. To put this in perspective, the insulation that is used to build a home today has an R-value in the low 40s" appearing on the page about cellular shade R-Values.
Looks like the window quilt option is no longer available
Anyone know of any alternative?
Also mounting a motorized cellular shade between a plexiglass interior storm deals with both moisture & air infiltration (making side tracks no longer necessary)?