How to install radiant barrier on inside window?
SUMMARY: When applying foil-faced rigid insulation to a window from the inside to block solar heat gain:
(1) Which direction should the foil face?
(2) If it faces the window then what’s the maximum acceptable air gap?
(3) Does the air gap have to be ventilated?
DETAILS: I have four plastic windows in a vertical roof/wall which are letting in tons of radiant heat from the sun. Due to the slope of the roof it’s not safe for me to replace them and expensive if I hire the work out. My attempted solution will be to build covers for the hottest days. I taped up foil-faced rigid insulation with the shiny side facing the window, and a 3.5″ gap between the foamboard and the plastic glass, but there was no measurable difference in the ability of the two wall ACs to cool the room. So my questions are:
(1) Should the shiny side face the interior, rather than the exterior? I was trying to bounce the heat back through the window, but I’m sketchy on the physics.
(2) Is a 3.5″ air gap too big? If so, I can install spacers into the window frame, to get my rigid insulation closer to the pane.
(3) Does the air gap need to be ventilated? I taped it up pretty tight. I don’t know whether that affects performance.
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Replies
I would not install a radiant barrier on the inside underneath a window. Once the suns rays have passed through the plastic (I'm guessing this is some kind or greenhouse space?) they are already heating the room and a barrier will just trap that heat against the plastic, possible melting it.
The best solution is to block the suns rays before they hit the plastic on the outside.
It’s a finished attic/living space. Someone unbeknownst to me installed the plastic windows long ago, I would not have done it that way. The outside isn’t comfortable for me to work on because of the slope, and I prefer covers which are removable so there’s light in the milder months. I guess I could ignore radiant heat and focus on conduction with extra air insulation. A cover of 1” of reflective polyiso did practically nothing.
You want the reflective foil facing the outdoors if you are trying to limit solar gain. Any light you can reflect back outside will cut down on indoor heat rise. There will be some heat essentially trapped between the window and the polyiso, but not all of it, and the insulation itself should help to keep the “trapped heat” from getting into the living space.
What you need to avoid is air circulating between the window and the polyiso. Make the polyiso fit tightly, even right against the glass (or plastic :-)) window pane for best results here.
Bill
Thank you very much, but wait, what I've read over and over on GBA and elsewhere (e.g., Dr. Bailes) is that a radiant barrier has to have an air gap in order to work, and that if I press it tightly to a surface then the heat comes in through conduction. Is this case an exception, and if so, then how and why?
Radiant barriers normally do need an air gap to work, but in this case the solar radiation is coming through a transparent window pane, and it can be reflected back out through that same window pane. This would not be the case with something opaque like plywood. Remember that with solar radiation, the visible light itself is part of the energy you’re dealing with, so reflecting it back out with a mirror gets it out of the space.
You can test this theory with a mirror and your hand. If you reflect light through a window onto your hand, you’ll still heat your hand up. You won’t heat your hand up quite as much as you would with your hand in the direct sunlight outdoors though, because the mirror isn’t 100% efficient reflecting light, and the window pane isn’t perfectly transparent, either. The “lost” light heats up the window pane and the mirror, but that’s only a fraction of the total energy that would otherwise have all gone into the interior space. Note that in my example, the light heats up the window pane twice, since it passes through in both directions, but it’s a little weaker the second time because the mirror isn't perfect.
Physics does actually allow for a perfect reflection though, when the angles are just right. This is why glints off of distant objects can be so intense when things are aligned just right to blast you with light.
Bill
Okay, got it, thank you. So you're saying that pressing the barrier up against the glass gives me better reflection of heat at the smaller expense of a little conductive heat?
A tight fit is more for ease of installation than anything else, but it will work as a reflector with or without a gap. You will have a bit more conductive transfer, but I think overall you’ll come out ahead with the tightly fitting piece.
Bill
Michael,
I don't know what you mean by a "plastic window." Is that a vinyl window? A vinyl-framed skylight? A window or skylight with plastic glazing? If there is glazing, is it single-glazed or double-glazed?
If you follow through with your plan, you will increase the temperature of the window frame and the glazing. That may result in warping of the frame or damage to the glazing seals.
By "plastic" I mean that the glazing is either acrylic or polycarbonate, I don't know which. I'm guessing acrylic because it's cheaper. The window frame is a box of cedar 1x4s with the glazing secured to it from the outside, probably with construction adhesive.
If heat is already coming in and I bounce that same heat back out, how am I increasing the temperature? Is it because the heat is flowing both ways?
If you can see the edge of the plastic window pane anywhere, look at the color of the material on the edge. If the color is whitish/clear, it’s acrylic. If it has a noticeable blue tint to it, about like blue jeans, then it’s likely polycarbonate. Note that this works best with a polished edge, if the edge was run through a sander or otherwise made to be “frosty”, both materials tend to look grayish.
Bill
Bill, if it's ease of installation I'm going for, it's easier to have the foamboard insulation cover resting on the 1x4 cedar box that frames the window, rather than trying to cut the sheet(s) to fit inside the box, especially as the box isn't square. That would give me a 3.5" gap from the insulation to the glazing. I'd like to do whichever way reflects out more heat. As I said, when I covered the two window in the main part of the room with rigid foamboard with the shiny side facing the window, there was no difference in how low I could cool the room (79°F). I also have a problem with a poorly insulated attic space between the two windows adding heat to the room, but I expected the cover to do *something*.
I think the insulation would perform best tight against the window pane, but I’m not sure how much different it would be over the frame IF you seal well so that you don’t have air circulating between the window and insulation and out into the room (making the window act like a little furnace).
You may find that the poorly insulated attic space is contributing more heat to the interior room than your realize. I was really surprised in my own home how much heat was coming through a few attic walls, but when I did my insulate and air seal project it made a HUGE difference.
Bill
Thank you, this is helpful.
According to my calculations, which could be wrong, there’s a similar amount of heat coming from the windows and the poorly-insulated attic space. In any event, I plan to address them both, as per my related posts.
I think I mentioned in your other thread that I bought some sun blocking screen that I have been putting on my southish facing skylights for some years. stops the sun before it is inside and lowers glare in the kitchen and home office areas affected.
https://www.homedepot.com/p/Phifer-36-in-x-25-Charcoal-Super-Solar-Screen-Roll-3021116/100552677
Says 90 percent but I doubt that.
It is a trip up the ladder, but maybe worth it?