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I have a front door with 2 double glazed opaque windows either side which, in summer, let in lots of heat which I want to deflect.

I don’t really want to have that mirror like film on the outside but would like to put something on the windows inside that I could easily put up and remove when the sun was less hot.

What is out there that would do the job?

4 Answers 4

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In your setup, the thermal barrier of significance is the double glazing. Anything on the inside that blocks sunlight will still heat up, and because it's behind the thermal barrier, releases that heat on the inside and continues your problem (this applies to internal blinds, louvres, curtains etc). Your best bet is therefore to apply something externally:

  • horizontal battens (evenly distributed slats that stop the high sun)
  • fixed external sunshading (a single, larger slat)

Anything that blocks the sunlight will heat up, and anything outside of the glass barrier will get cooled down by the outside air.

Failing that, I have used an indoor solution (albeit with some unknowns, see below):

  • cut a 50mm thick extruded polystyrene foam board (XPS, the firm variant; not the panels made of white expanded polystyrene pebbles called "EPS", this cuts very messily) to neatly fit the window frame on the inside (taper it slightly if that's what the window frame requires for a good fit)
  • sprayglue aluminium foil to the side facing the glass (a single coat is sufficient, have the foil handy when you apply the glue)
  • insert into window frame
  • if it doesn't sit in place on its own, apply additional fixation (such as duct tape)

This changes the situation described above, by providing a much better thermal barrier behind the double glazing. The aluminium foil will still reflect a large portion of sunlight away, and it will still heat up significantly, but because the lesser thermal barrier is the glass, most of the trapped heat will leave in that direction.

The unknown here is that I'm concerned the high temperatures reached on the glass side (I reckon at least 60-70C) might at some point result in cracks or other damage. After extensive testing (see below) I have not experienced any negative effects so far, but it's still in the back of my mind when suggesting this.

I've used this setup in my very badly designed, west oriented highrise dorm room to compensate for massive solar intake from a lot of windows in summer for a number of years. It brought temperatures down from 35+ degrees celsius to below 30, so it works quite well.

EDIT: Another solution, based on crip659's answer: take some blackout curtain fabric and cut it to size, then stitch velcro to the edges, and glue velcro backing to the door's window frame. This way there's no air gap that would allow the air on the window size to circulate. It's not even close to as effective as using XPS, but it'll work better than just hanging things inside.

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  • perhaps clarify that heat can be reflected back, by radiation, out through the same glass it entered. You seem to suggest that all heat stays inside, which is not correct. ("because it's behind the thermal barrier, releases that heat on the inside"). See my answer which explains difference between low-e and non-low-e, and radiant vs transference, or check with your building physics colleagues.
    – P2000
    Aug 17, 2022 at 16:27
  • Didn't mean all heat, just that elements inside heat up in the sun and then leave most of that heat inside. It's not a black and white thing... well, I suppose in a different way, it is :)
    – MiG
    Aug 17, 2022 at 16:34
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Blinds, drapes, and/or curtains would do.

Open and close them as needed.

Will reduced the heating quite a bit. Not completely, but the sun will only heat the space between the window and what you use, some heat/warmth will leak out into the room, but guess only about 10 to 20 % compared to full sun.

Many different styles and types to match your home.

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  • Regular blinds, drapes and curtains aren't going to stop 80-90% of heat, it'll more likely be the inverse of that. If you consider the orange lines on this pic to be the curtains heating up, in the case of curtains there'll be something insulating on the left (which stops most of that heat leaving, as opposed to into the room to the right), and openings top and bottom that allow the air to circulate (you're not stapling the curtain onto the door after all). That's a radiator :)
    – MiG
    Aug 16, 2022 at 6:04
  • You have given me another idea though: take some blackout curtain fabric and cut it to size, then stitch velcro to the edges, and glue velcro backing to the door's window frame. This way there's no air gap. It's not even close to as effective as using XPS, but it'll work better than just hanging things inside.
    – MiG
    Aug 16, 2022 at 6:10
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Outdoor vegetation.

Plant one or two deciduous trees in the front yard so that they'll cast a shadow over the front door most of the day. In the summer, when they have leaves, they'll cast their shadow, block the sun, keep the house cooler, help clean the air, give the birds and squirrels some place to live and overall help save the planet.

In the winter, they'll drop their leaves and allow whatever sun there is to shine though the side lights and help warm the house.

Depending on what you're willing to spend, it may take a few years for a young tree to grow enough to provide the shade needed, or you might plant a more mature (and more expensive) tree and start reaping the benefit right now.

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  • It'll be a while before they're of significant enough size to have an effect, but yeah, trees are pretty great for this :)
    – MiG
    Aug 16, 2022 at 19:26
  • I did note that, @MiG. I also noted that one could transplant more mature trees for $$$$$. :)
    – FreeMan
    Aug 17, 2022 at 12:10
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The question specifically asks about inside measures, so let's begin there.

Placing interior blinds or paneling may have a positive effect and help reduce the heat.

Use blinds with a light colour, or heat reflecting blinds which have a metallic colour.

Hanging blinds is far easier than planting trees or installing shutters. And inside measures are not exposed to the elements like wind storms and rain.

Why could interior measures indeed work? Read on...

Radiant Heat

The reason why it's hot is -as you probably know- the infrared light from sunlight, transferring through the glass of the side lights.

You can stop this heat energy in radiant form by blocking it on the outside or sometimes even on the inside. Whether the inside block works depends on the infrared transference of the glass of the side windows.

enter image description here

Image: https://glassed.vitroglazings.com/topics/how-low-e-glass-works

Low-E Glass

Low-e glass (Glass with low emissivity, "low e" or low thermal emissivity) operates as an infrared barrier. It the infrared component of sun light entering in the summer, and in the winter it reduces the loss of heat from the house.

Low-e reflects radiant heat by reflecting the infrared band of light, and it operates both ways: from the inside and from the outside.

One type of coating (low-e coatings) reduces the emission of radiant infrared energy, thus tending to keep the heat on the side of the glass where it originated while letting visible light pass. This results in glazing with better control of energy - heat originating from indoors in winter remains inside (the warm side), while heat during summer does not emit from the exterior, keeping it cooler inside.

Ref: https://en.wikipedia.org/wiki/Low_emissivity

Low-e is not U-factor

Low-e is not to be confused with U-factor. The U-factor indicates the heat transfer from one side of the window to the other side, based on contact with air on both sides. The "e" factor indicates the amount of radiant heat (e.g. sun light) transferred through the glass.

Low-e is attained by an infrared reflective coating which retains inside heat by reflecting infrared light. This is useful in the winter on dark cold days, as well as in the summer on hot sunny days. Regrettably, low-e also reduces solar gain in the winter.

All glass has an "e" property.

Window Film

Window film, as you suggest, operates the same way, by reflecting infrared light. Film can be applied to the outside as well as the inside of a window.

Adding additional reflection on the outside will work to reflect infrared light from the sun before it reaches the glass and the house interior.

Additional reflection on the inside will reflect infrared light from the outside, through the window, and back to the the outside. In this process, some of the infrared is absorbed as "heat" by the glass, causing the glass and film itself to heat, and this heat radiates back inwards to warm the room: the window and film are hot to the touch.

Blinds and Low-e

Taking this a step further, if the glass in the side window is low-e, placing additional reflective material on the inside may have limited effect: the infrared light is reflected many times between glass and the blinds or paneling, while some of it is absorbed as heat by the glass and some of it by the blinds or paneling. Only a small portion is eventually reflected back outside.

To our detriment, the low-e is doing its job, and it is not letting the heat on the inside radiate out and escape.

How effective the interior reflective material will depend on the circumstances, among which the "e" property of the existing window itself.

Try it

You can experiment easily by meausring the temperature in the room on a sunny day with and without the temporary placement of interior material like sheets of white paper.

Most likely it will have a positive effect, and if so, you could just apply interior blinds or removable panels or even possibly some curtains.

Curtains

Curtains and blinds will have a different effect, depending on the "e" property of the glass: blinds tend to reflect more, curtains tend to better insulate the air pocket between curtain and window. Curtains insulate better and thus have a higher R-factor (lower U factor) than blinds.

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  • There is less than the full blast of solar radiation that you will get outside, but don't forget that the amount of solar radiation in summer is immense. Keeping the sun outside (trees, external sunshading, tropical roofs etc etc etc) still is by far your best option. Anything interior should be considered last measures, for example because the building owner prohibits modification of the shell. Think of it this way: whatever you can prevent from coming in, you do not have to spend effort getting back out again.
    – MiG
    Aug 16, 2022 at 19:34
  • @MiG yes, but the OP specifically asked about inside: "would like to put something on the windows inside", so let's begin there. Hanging blinds is far easier than planting trees or installing shutters. Inside measures are not exposed to the elements.
    – P2000
    Aug 16, 2022 at 22:39
  • I think the framing challenge is very important for this question though. And I would really not mention curtains and interior blinds as having a significant thermal effect, see my comment on crip659's answer :)
    – MiG
    Aug 17, 2022 at 16:00
  • @MiG we have blinds and near-white curtains for our non-low-e windows and they are all very effective keeping out thermal heat from sunlight on the south, east and west side.
    – P2000
    Aug 17, 2022 at 16:04
  • Probably is highly dependent on the local climate, structure of the building and your personal tolerance, but I suspect other measures (especially external ones) might give your own home a far better heat avoiding performance than these though :)
    – MiG
    Aug 17, 2022 at 16:10

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