Insulating Curtains That Cut Heat Losses Through Windows by 50%

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Introduction: Insulating Curtains That Cut Heat Losses Through Windows by 50%

I am a mechanical and environmental engineer, and I help companies pollute less by making a more ...

20 to 50% of all the energy that is used to heat up a typical home during the winter is lost through its windows. This wasted energy increases our heating bills as well as our carbon footprint, neither of which does anyone any good. With this in mind, we designed the Kume curtain: a simple and inexpensive do-it-yourself insulating curtain that can help us save money, keep our homes cozier and be kinder to the environment.

"Kume" in the language of the Mapuche people of Chile means "good." We feel that this name is well-deserved as the performance of these curtains is very "kume" indeed. The thermographic photograph shown above speaks for itself. It was taken early one winter morning and clearly shows that the window fitted with a Kume curtain is much cooler than the one fitted with a conventional decorative curtain - and is actually even cooler than the masonry wall. This can mean only one thing: instead of letting the heat seep out of the house, the Kume curtain keeps the heat in, where you need it.

The benefits offered by Kume curtains are as follows:

  1. They lower heat losses through windows by 50 to 70%, and improve comfort by eliminating cold spots and drafts in rooms. In the first test homes fitted with Kume curtains, heating fuel consumption dropped by 25%.
  2. They are inexpensive. The cost of the materials that are needed to make a Kume curtain typically ranges from US$ 1 to 1.5 per square foot.
  3. They are easy to make. Putting together a Kume curtain does not require great manual skills or much time. For example, assembling a 2x4 ft curtain takes less than two hours. The hardest part of making the curtains is cutting the panels straight and to size. If you use polar fleece for the panels, you don´t even need to hem the fabric, and the whole curtain can be assembled without a single stitch.

Step 1: What Is a Kume Curtain?

It is a roll-up curtain that is composed of four distinct layers.

1) A front panel which acts as the first layer of insulation and seals the perimeter of the window opening when the curtain is closed.

2) A moisture barrier which prevents indoor humidity from reaching the window and condensing on the cold glass and window frame.

3 & 4) Wooden battens which maintain the fabric stretched out and thereby ensure that the curtain fits tightly against both sides of the window opening. The battens also create air pockets which further reduce heat losses hrough the curtain.

5) A back panel which acts as the final layer of insulation and helps seal the perimeter of the window opening when the curtain is closed.

The reasons why a Kume curtain is so effective in reducing heat losses through windows are:

  1. Still air is one of the best insulators found in nature, and the Kume curtain contains a lot of it. First, between the fibers of the thick polar fleece that is used to make the curtain, and second inside the thin spaces that are created etween the front and back panels by the battens.
  2. When closed, the Kume curtain fits tightly against the top, bottom and sides of the window opening. By doing so it traps a layer of insulating air between the glass and the curtain, and prevents the cold air that forms against the lass from seeping into the room.
  3. A Kume curtain basically works just like a good down jacket on a cold winter day. The air that is trapped in the thick layer of down creates an effective insulating layer, and the tight fit of the jacket around your waist, neck and wrists keeps your body heat in, rather than letting it leak out into the cold environment.

Step 2: Where and How Can You Use a Kume Curtain?

1) A Kume curtain can be used on any window whose frame is recessed by at least 1.5" with respect to interior surface of a wall. This is because the curtain works best if it is able to tightly seal the complete perimeter (that is, the top, bottom and sides) of the window opening.

2) A Kume curtain is much less effective when installed on a window that is mounted flush with the interior wall, because the cold air that builds up on the inside surface of the window is able to flow around the sides and bottom of the curtain and seep into the room.

3) When raised, the curtain forms a tight roll whose diameter varies depending on the length of the curtain and the thickness of the material used. For example, a curtain that is 5 ft long and made of two layers of fleece will produce a roll with a diameter of approximately 5". The rolled up curtain does not affect the operation of windows that open outward, sash windows or sliding windows, but it will prevent the operation of windows that open inward. If fitted with Kume curtains, these types of windows will have to remain closed until the curtains are taken down in spring.

Step 3: Where and How Can You Use a Kume Curtain? (continued)

4) Depending on the type of fabric used, Kume curtains can be translucent to opaque. Therefore, they are mainly designed to shield windows at night, when indoor privacy is desirable, outdoor temperatures are lowest, and heat losses are greatest. In certain cases, however, the curtains can also be used during the day: for example, in unoccupied rooms in which daylighting is unnecessary, and in which the windows do not provide any useful solar heat gains.

5) They can be used as the primary curtain on a window, or used in conjunction with a decorative curtain, as shown in the pictures above.

Step 4: Standard Components and Materials

For a window opening of width "W" and height "H“, you will need the following components:

1) Front panel

  • Width = W + 0.75", height = H + 0.75"
  • Material = Polar fleece or another thick and flexible fabric that can fit snugly against the sides of the window opening and prevent the passage of air.
  • If the Kume curtain is used as a primary curtain, the front panel can be made with a fabric that best suits the decor of the room (a cotton print for example).
  • You should ideally wash the fabric before making the panels and, as with all drapery, you should consider using flame-retardant fabrics.

2) Moisture barrier

  • Width = W - 0.75", height = H
  • Material = Transparent polyethylene with a thickness of approx. 4 mils (0.004"). You can also use a black polyethylene sheet, but keep in mind that this will make your curtain totally opaque (blackout) even if you use light colored fabric for the front and back panels.

3) Upper batten

  • Section = Approximately 0.5" x 1"
  • Length = W - 0.75"
  • Material = Pine or ideally hardwood.

4) Lower battens

  • Section = Approximately 0.5" x 0.75"
  • Length = W - 0.75"
  • Material = Pine or ideally hardwood.
  • Number = The spacing between the battens should not exceed 20", therefore the number of lower battens needed depends on the height of the curtain.

5) Back panel

  • Width = W + 0.75", height = H + 0.75"
  • Material = Ideally polar fleece or another thick and flexible fabric.

6) Fasteners (not shown)

  • For the back panel we recommend using staples, while for the front panel, which is visible from the room, we recommend using thumb tacks or upholstery tacks. If you are concerned with having these fasteners rust and stain the fabric, you should use stainless steel staples and rust resistant tacks.

Notes:

A) The front and back panels are a bit bigger than the window opening. This is because these pannels need to completely plug the window opening when the curtain is closed.

B) The choice of materials was mainly guided by our desire to make these curtains very affordable and simple to make. This is why we chose polar fleece for the curtain panels (fleece is inexpensive, insulating, widely available, and requires no stiching to be transformed into curtains), and polyethylene film for the vapor barrier. However, many other types of materials can be used to assemble a Kume curtain. For example, the curtain shown in the last step of these instructables was made using a back panel made of dark blue fleece, a vapor barrier made of transparent polyethylene, and a front panel made of dark blue fleece on which we sewed a flowered cotton print. As you can imagine, this curtain was totally opaque and worked great as a blackout curtain in addition to keeping out the cold.
If you would like to use other types of materials, or even a different structure for the curtain (for example, using a quilted or interlining fabric to fill the gap between the front and back panels), please keep in mind the following points:

  • The fabric chosen for the front and back panels should ideally be moderately thick, tightly woven to restrict the passage of air, and flexible so that it can accommodate itself tightly against the sides and bottom of the window opening.
  • If you decide to use a very thick material for the front or back pannels or for the moisture barrier, please consider what this will do to the size of the roll that you will end up with when you raise the curtain.
  • The curtain should have a moisture barrier placed behind the front panel. Without a vapor barrier you will get a lot of condensation against the cold glass, with a good vapor barrier you will get none (or almost none).
  • The moisture barrier should be made of a thin, flexible and non-breathable material or fabric. It can be of any color, but if it is opaque it will transform your curtain in a blackout curtain even if you use light colored fabrics for the front and back panels.
  • The curtain should fit in the window opening and seal the full perimeter of this opening to prevent the cold air that forms against the glass from seeping into the room.

Step 5: In This Example We'll Assemble a Curtain for a Window Opening That Is 24" Wide and 48" Tall

The materials needed for this curtain are:

  • A 24.75" x 48.75" front panel (in this example it is made of thick white cotton canvas)
  • A 23.25" x 48" sheet of transparent polyethylene with a thickness of 4 mils (0.004")
  • An upper batten made of pine with a cross section of 0.5" x 1" cm and a length of 23.25"
  • Three lower battens made of pine with a cross section of 0.5" x 0.75" and a length of 23.25"
  • A 24.75" x 48.75" back panel (in this example it is made of yellow polar fleece)
  • Thumb tacks to fasten the front panel and plastic sheet to the battens. We use thumb tacks for aesthetic reasons since the front panel is visible from the room.
  • A stapler to fasten the back panel to the battens

Step 6: Trace the Position of the Battens on the Plastic Sheet.

Make sure that:

  • The top edge of the upper batten is aligned with the top edge of the plastic sheet;
  • The bottom edge of the lower batten is aligned with the bottom edge of the plastic sheet; and
  • The remaining battens are evenly distributed over the height of the plastic sheet.

Step 7: Slip the Battens Under the Plastic Sheet and Use the Marks Traced on the Plastic Sheet to Align Them Properly.

Since the length of the battens is equal to the width of the plastic sheet, the ends of the battens should also align with the sides of the plastic sheet.

Step 8: Lay the Front Panel Over the Plastic Sheet.

The front panel is slightly longer and wider than the plastic sheet, so when in place:

A) The top of the front panel should be aligned with the top batten;

B) The bottom of the front panel should extend by ~0.75" below the lower batten and base of the plastic sheet; and

C) Each side of the front panel should extend by ~0.75" past the ends of the battens and sides of the plastic sheet.

Step 9: Use Tacks to Fasten the Front Panel and the Plastic Sheet to the Battens.

  • Place a tack within 0.5" of the ends of each batten.
    Use enough thumb tacks between both ends of the battens to properly fix the fabric and plastic to the battens.
  • On the upper batten, the spacing between the thumb tacks should be between 4" to 8".
  • On the lower battens, the spacing between the thumb tacks can be increased to double the spacing used on the upper batten.

Step 10: Flip the Curtain Over.

Step 11: Place the Back Panel Over the Partially Assembled Curtain So That It Rests Directly on the Battens.

The back panel has the same dimension as the front panel, so when in place:

  • A) The top of the back panel should be aligned with the top batten;
  • B) The bottom of the back panel should extend by ~0.75" below the last batten and base of the plastic sheet;
  • C) Each side of the back panel should extend by ~0.75" beyond the ends of the battens and sides of the plastic sheet.

Step 12: Use Staples to Fasten the Back Panel to the Battens.

Place a staple within 3/8" of the ends of each batten. This will ensure that the material maintains its full width and touches the sides of the window opening.

Use enough staples between both ends of the battens to properly fix the fabric to the battens.

You are done!

Step 13: For Additional Information ...

Refer to the web page www.kumeproject.com for instructions on how to mount the curtain and create the tie to hold the curtain in the rolled position. Other options for assembling the curtain are also described on this page.

This web page provides the curtain assembly instructions in French, Spanish and Italian, as well as in metric (mm) and US customary (inches) units.

Step 14:

We hope you like them.

The Kume team

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117 Discussions

The mean free path of an air molecule at STP is around 2.5 cm.

Gaps smaller than that permit air molecules to jump through the gap and transfer energy from one side to the other. The smaller the gap the greater the heat exchange.

Dead air spaces should be designed to be 2.5 cm or larger. Larger gaps of around 7 cm or larger will permit convection in the gap which will also reduce the insulating factor.

Covering a window with any kind of plastic - paper - tinfoil - whatever that creates a dead air space of 2.5 cm to 6 or 7 cm will produce a dramatic insulating effect and typically reduce heat loss through the window by 40 percent or so.

Translucent or transparent material should be sued so that sunlight can enter during the daylight hours.

Plastic drop cloth works well and is typically translucent. A $9.00 roll will do an entire house for a couple of years if you throw it out each spring.

But why do that?

These curtains are not a bad idea at all. They will do the job, but they are not required for window insulation.

2 replies

VendicarD - Here are my comments to your post.

1) The mean free path of air molecules at STP is approximately 0.00001 cm.

2) The optimal thickness of an air gap for insulation is approximately 12 mm (about 0.5 inch). This is the reason why this is the distance between glass panes that is normally used in the millions of double-pane windows that are manufactured each year around the world.

3) You are perfectly right when you say that covering a window with a plastic film greatly reduces heat losses, however the logic behind using insulating curtains is as follows: Since most windows are covered with curtains, why not use these elements to further reduce heat losses through the windows? Regardless the kind of windows you have (single-pane, single-pane with plastic film, double-pane, double-pane with plastic film, etc.), covering them with effective insulating curtains will keep your house warmer and your heating costs down.

I note that one question still remains. Do the Kuma curtains perform much better than plastic film?


If still available to you, the Flir (or other Thermal Camera) can clearly demonstrate the value of the insulation. The plastic sheeting is much cheaper than the insulation. If there not a huge difference in R-value between plastic sheeting alone and the Kuma curtains, I think many people would prefer a very low-cost solution if it was almost as good.

Would really love to see the thermal imaging for comparison!

Great invention! It cut down my energy costs by a lot!!

Awesome write up . Time to add a new project to the list :)

I had been thinking of something similar to this design for rooms/doorways that are seldom used
in our house. Planned layers are: window -> 2" gap -> mylar with silver facing window -> double-layered corrugate cardboard -> double-layered corrugate cardboard -> mylar with silver facing room. The idea is I would keep this up year round. In summer, the mylar would reflect sunlight back out of the house.

Some questions:

1) I looked at the R value of cardboard, and it seems higher than polar fleece, so hopefully this will be effective. Your thoughts? (I have done a few international moves so just happen to have double layered cardboard out the wazoo)

2) Learned from your Instructable that I would need fabric around the edges to create a tight seal. Anything other words of guidance/caution?

3) Brandon mentioned a cracked pane if not taken down during the day in winter, so I guess there goes my idea of keeping it up all the time, at least during the cold.

4) Does the plastic sheet HAVE to go behind the first layer of polar fleece? Or would it be just as effective as long as I keep a gap between the mylar and the window? (My sills are 5" deep. I was thinking of affixing this ~2" away from the window from the top of the sill)

3) Also, is there any additional benefit in putting another mylar layer on the back, silver part facing the room, to reflect the heat of the room back and conserve even more energy? Or should I not bother?

Thanks for the fantastic instructable, I am so excited to start on my project!!!!

2 replies

Hello GEGJ1

1) I am sure that a thick piece of corrugated cardboard
should be a better thermal insulator than a layer of polar fleece. However, I
would be a bit concerned about humidity, especially if you are planning to
leave this panel up for long periods of time. Unless your assembly is extremely
tight and prevents the passage of even a minimum amount of humidity, you will
end up with some condensation on the glass. In the case of the curtain, this is
not much of a problem because the condensation either evaporates or is wiped
off when the curtain is rolled up in the morning. However, in your case, it
will probably accumulate over time and soak into your cardboard.

I may be wrong, so please give it a try.

If you do end up with a problem with condensation, I would
try a water resistant insulation material such as an extruded polystyrene
board. They are not very expensive, have a very high R-value, and are not
affected by humidity.

2) In my case, I use the extra width of the curtain fabric to
press against the sides of the window opening and prevent the passage of air.
This is a good solution for a curtain that needs to be rolled up every morning
and rolled down every night, but if you have a more permanent installation you
could use other materials to seal the perimeter of your insulating board (e.g.,
rubber foam strip).

3) Please see my answer to Brandon’s post.

Also, if aesthetics are not a problem (and they should not
really be if you were planning to put a shiny mylar facing the window), you
could consider spray painting the glass white. The paint has to be on the
outside pane of your window and block nearly all of the solar radiation. If
this is not an option and you are very worried about cracking your windows
(which I assume are double pane), you should try to find another option to
shade your windows/doors from the outside.

4) The function of the plastic in my curtains is to act as a
vapor barrier. A vapor barrier should be placed closer to the “warm” side of
the insulated assembly (i.e., closer to the side which faces your indoor
space). Since you are planning to have a layer of mylar facing indoor, this
would act as the vapor barrier on the warm side of the assembly (i.e., you don’t
need to add an additional plastic sheet).

Please note that for insulation purpose, it is generally
recommended to have an air gap that is between 0.5 to slightly less than 1 inch
(your 2” gaps are too big).

5) Sure, a radiant barrier facing the inside of the room
would be useful from a thermal standpoint. On top of that, it will act as a vapor
barrier and prevent moisture from penetrating into your cardboard and
condensing once it reaches the cold side of your assembly.

Good luck!

Oh my gosh, Padigm, *thank you* for your thoughtful, in-depth replies to all my questions. Helps me immensely. I will report back with my findings once I am done (may take me a few weeks, I have a 3 year old and not as much free time as I would like...)

Thanks again for this great Instructable, you've made a whole lot of people's lives better!

I am wondering if, in fact, something like this could be used to reduce the heat coming into the house from sunny exposures, as someone commented below. I think you would have to use different materials, something reflective. Would you have any ideas? I love what you have done here and would love to tweak it a bit for our hot California summers.

3 replies

I have two southern exposure windows in two bedrooms. I bought some 70% shade screen on the internet, 3/4 PVC and some pvc clips that'll hold the shade cloth to the pvc pipes. Hung them from the rafters about a foot from the windows. No more hot rooms in the summer. Did the same thing over the driveway, cool cars instead of sun blasting them all day, making them hot as well as uv damage to paint.

You are spot on. The best way to reduce the solar heat gain through a window is by shading it from the outside.

During the summer, these curtains can definitely help reduce the heat load coming from windows that are exposed to direct sunlight. Having a reflective panel (e.g., aluminized fabric) facing the window would be best, but this might create some aesthetic problems since you would be able to see the reflective material from the room when the curtain is rolled up.

An intermediate solution would be to make the exterior panel from a light colored fabric (e.g., white or off-white). As a matter of fact, if you used light colored fabrics for both panels, you would end up with a translucent curtains that would allow light into the room when closed. In this way, you would be able to keep much of the heat from direct sunlight out of the room while still allowing the room to be illuminated with natural sunlight.

The first curtains I installed in my house had a dark blue panel facing the window, but even so they were pretty good at blocking the heat from direct sunlight when closed (I live in Chile where summers are just like in California). However, this dark blue backing also blocked off all of the light and basically had the same effect as a blackout curtain -- the room was pitch dark.

Dang it! Had to make one and goof it up to find this. There is a discrepancy between your listed msrmnts. and your assembly instructions--- In step #4, you state this:

1) Front panel (and/or back panel)

  • Width = W + 0.75", height = H + 0.75"

3) Upper batten Length = W - 0.75"

4) Lower battens Length = W - 0.75

Then, in step #8, comes the problem----

C) Each side of the front panel should extend by ~3/8" past the ends of the battens and sides of the plastic sheet.

That should read "should extend by 3/4" past the ends of the battens", should it not? The panels are 3/4" wider than the actual window opening, for an overlap/tuck of 3/8" on either side, and the battens are 3/4" less than the window opening, for an allowance of 3/8" on either side. In other words, the width of the panels is 1-1/2" wider than the length of the battens, so that, when the battens/plastic are centered on the panels, there should be a 3/4'' overhang on either side, as opposed to 3/8''.

Clear as mud? The devil in the details and all......

Fortunately, the "goofed" panel is going in a corner window where only one side of it will be visible. Shouldn't affect it's operation. Onward with the other 5!

Great shades, looking forward to the finished project!

1 reply

You are right! Thank you for pointing out that mistake in the assembly instructions. We had checked and rechecked them, but never saw that mistake. We have corrected it in the instructable, so hopefully those coming after you will make perfect curtains.

Thank you once again, and please post a picture of the curtains you made.

Cheers!

Using Mylar film (space blanket) for the moisture barrier should reflect more heat back into the room.

3 replies

You are perfectly right, using a space blanket as vapor barrier should improve the design by reducing heat losses by radiation. The thickness of the blanket or vapor barrier doesn't matter, as long as the material is tough enough to resist handling (daily rolling and unrolling) without breaking.

By the way, the instructions for these curtains are available in metric and US measurements in English, Spanish, Italian and French at the following website

www.kumeproject.com

Cheers!

I was thinking the same thing, but are the space blankets thick enough? The instructable calls for 4mils thick plastic. The space blankets that I have are very thin ... well like a Mylar balloon.

It is a moisture barrier, as long as it stops moisture a slight dıfference in thickness should be unimportant.

I had to look up the meanıng of mils as I was not familiar with the term. It seems to be an Americanism for a thousandth of an inch. what the rest of the world would call a thou. I would rather sick to metric measurements to avoid confusion.

If you live in a 9m square room with just a window and exit, insulating the window and the door, will you die from room oxigen deprivation?

1 reply

CO2 toxicity would be your first problem, not oxygen. Does your sealed room contain any calcium oxide (quicklime)?