Upgrading a single/dual-pane window with plexiglass
I was reading about single, double and triple-pane glass, when I found a tip of putting a sheet of plexiglass in front of a single pane window and that way upgrading it to HR value.
I can’t find any numbers regarding plexiglass in front of late 80’s double-pane windows. Can someone give me an estimated efficiency increase like add R-1?
My second question is related to the above.
The total window area is 90% double-pane and 10% single-pane.
I searched for online calculators to figure out how much my average window insulation would increase when only upgrading the single-pane.
Found a few calulators designed to calculate the total wall R of wall + windows.
The results varied greatly. So I looked up the formula behind the calculators and put it in a spreadsheet. A simple weighted average (area and R).
90% of the window is R-2 >>> double-pane.
10% of the window is R-1 >>> single-pane.
My calulation: R-1.8
90% of the window is R-2 >>> double-pane.
10% of the window is R-2 >>> single-pane + plexiglass.
My calulation: R-2.0
90% of the window is R-3 >>> double-pane + plexiglass.
10% of the window is R-2 >>> single-pane + plexiglass.
My calulation: R-2.9
Can someone please check my calculations or even better point me to a reliable online calculator? Likely the calculator should a bit more advanced because obviously the above examples don’t reflect a real situation. My glass isn’t hanging in mid air. It’s in a wooden window frame with some wall around it.
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Replies
Your calculations are right based on your R-value and area inputs.
As far as making a more efficient wall these panels may be helpful. But I would also look closely at air infiltration, the weatherstripping of your windows. Depending on their condition that may be much more helpful. But certainly the panels will help too.
Sorry I don't know of any online calculators that do what you ask. I'm sure others will respond.
You may experience condensation between the panel and the original glass. So if possible attach it in a way that allows for easy removal and cleaning.
If my calculations are correct I use my own spreadsheet. That's more convenient for me.
Condensation is one of my fears. Saving some energy and then having to replace everything because it rots's isn't what I have in mind :-)
I've read there should be small ventilation holes to avoid condensation. But I think that would be very bad for the R-value?
I've used quite a few of the magnetic acrylic panels on both my residence, and commercial building. They work. You want the best air seal possible on the inside to prevent moisture from getting to the glass and frosting/condensing. An air seal issues with the window itself can be hard to stop completely, especially with double hung windows. I'm using these panels with operating (leaky), and non operating (not leaky) windows. In the commercial building, the windows were much newer triple pane casements with very good sealing. The acrylic panels we are using have a magnetic trim that contacts a steel frame on the window interior with about a 2" gap to the glazing. They also drop sound transmission somewhere around 50% (5-7 db). We're in zone 7A, with temps dipping to -35C in winter.
Here are some FLIR images. These are non operating double pane windows. Outside temps were -22 C, wind chill -34 C. Surface temp at the window with the acrylic was 13.4 F/7.5 C warmer. With regard to air leakage, the older double hung sliders on the North side of the house leak enough to bow the acrylic inwards during high wind gusts on very cold days, but the acyrlic panels, along with careful air sealing at the inner frame pretty much stop this infiltration. They have in fact sealed up the house to the point (with other retrofits) that I've have to add an HRV to keep CO2/VOC levels tolerable.
Adding glass, or plexiglass, or even shrink film to an existing window opening is going to improve the performance of that opening by R1. Simple rule-of-thumb is that everytime you add a layer you improve performance by R1.
If the additional layer is reasonably airtight, then you shouldn't have any condensation problems in the airspace created between the added layer and the original window since you are blocking internal moisture out of the space that you created.
Not sure what you mean in your calculations above, but if you want to calculate the improvement when combining materials with different performance values (i.e. if you want to calculate overall wall values when using different window configurations or you want the combined values of several different windows), then you should be using the U value of the different materials for your calculations and not the R value.
U = engineering, R = marketing.
Thanks for the postive info!
Is there a best time of the year to install the panels?
Does installing on a dry hot summer day reduce the chance of condensation during the year because the sealed in air is dryer?
The moisture content of the air is typically lowest on the coldest days of winter.
@tt, the air in the window cavity is not sealed…so it does not matter really. You may want to do some work though to caulk interior trim etc to minimize air leaks.
Chaulk is an option too. I don't know the name of it, but there's soft 'thick' tape that seal the panel when it's screwed down.
You can install the panels anytime, whatever residual moisture that you might trap in the airspace will equalize on it's own.
Also, although it may not feel like it, cold damp winter air generally contains much less moisture than hot dry summer air. 80% Rh at 40°F has about the same moisture level as 15% Rh at 90°F, or 8% at 110°F.
I should have remembered that from science class. Cold air can't absorb as much moisture as warm air.
Not looking so nice, but would putting a small pack of that moisture absorbing stuff between the windows a good idea?
@t_t, Dessicant is not necessary. I have used these panels now for over 10 years, over about 45 windows of various qualities from double hung operating "wind blows through them" to new triple pane casements. The only time I've had any frost show up on the interior glass is if a panel has come away from the frame introducing a large air leak. The installer caulked corners of the magnetic trim (the trim that fits on the acrylic) and even when that corner caulk is removed, the windows are fine down to -35 C. As you might expect, starting with a triple pane casement does not require as much attention to a good air seal as a leaky double hung.
I will add that the installer I used for the residence did a very good job of installing the interior steel L trim, making sure the corners lined up and caulking them to the frame. He also added a bit of caulk at the 45" corners on the magnetic trim. The trim that is fixed to the acrylic has held up extermely well. The trim they used is similar to this product: https://www.first4magnets.com/other-c89/magnetic-trim-secondary-glazing-attachment-kit-2-metres-p22231#ps_0_23562
Trim like the product above just slides onto the the acrylic. The product I linked above includes a self adhesive steel strip so you could build inner frames of wood and affix the steel strip to that. My guess though is that it is not white, so would not look as nice as steel, painted L trim. It looks like a lot of the online "kits" now just sell adhesive magnetic tape that you would adhere directly to acrylic.
With regard to plexiglass, I believe acrylic is used to prevent yellowing, and I can confirm that the panels remain optically clear for a long time. If you're looking for kits, I'd take a good look at this company as their seal incorporates a bellows to manage panel bowing at cold temps. This is an issue I've seen at very cold temps (below -25 C) with ridgid trim...a panel will occasionally pop a corner. This company's seals would manage this:
https://www.climateseal.com/products/thermal-series/
If you have an assembly of mixed materials, where each material has R-values of R1, R2, R3, etc., and percent areas of A1, A2, A3, the R-value of the assembly is given by:
R=1/( A1/R1+A2/R2+A3/R3+ etc)
To simplify the math, U-factors are commonly used, where U=1/R. That gives:
U= A1U1+A2U2+A3U3+ etc.
Once the U-factor of the entire assembly has been calculated you invert it to get the R-value.
If you look at windows they are commonly rated in U-factor.
@tt, I plugged my temp difference values (from the FLIR images I posted) into this method : https://woodgears.ca/physics/r-values.html
Tweaking on the outside temps a bit (it was -22 C, but wind chill values were -34 on that day) if I use -30 C as an outside temp, then I get R 2.6 for the double pane window with no acrylic panel. This is in line with a double pane sealed unit with zero air leaks (not operating). With the acrylic panel in place, the calcs indicated R 7. This is consistent with data I've seen from measurements taken on new triple glaze (non operating, full lite, door glazing panel) in the same home. In other words, performance adding the magnetic inner "storm" acrylic panel to the double pane window is quite similar to a sealed triple glaze unit. The difference has to much higher with an operating double hung window where air leakage is very high, particular with any kind of differential pressure. The interior panel substantially reduces air leakage if installed properly.
I have a large glass brick "window" which was rather cold inside. I put a frame into the outer rough opening and inserted two double pane plexiglass panels into that frame (one in front of the other). I picked plexiglass and not Lexan because it is more vapor open to avoid condensation issues - also this one here is quite transparent (90%). The attached picture was from the first stage with only one panel.
No problem so far and a warm(er) inside surface.