What constitutes a "heavy curtain" for energy efficiency?

June 11, 2020 4:56 PM

Everywhere on the internet is happy to tell me that I need "heavy curtains and pelmets" to properly insulate my windows, but does not say what that means. Is a single layer curtain made from a fabric with a blockout backing "heavy"? A two layer curtain with front fabric and a separate blockout lining? Or a full-on thermal insulated curtain? What's the minimum I need to largely eliminate heat loss and gain through my windows, and what are the pros and cons of different options?

For context, I'm replacing all the window coverings in my currently rather chilly 1980s brick veneer house. Looking to make the house both warmer in winter and cooler in summer. I'm in cold/temperate Australia (regular light/moderate frosts in winter, -4˚C, hot summers to 40˚C) and have single pane aluminium framed windows (I'm also going to install some of that shrink film secondary glazing).
Beyond thermal insulation, I want curtains that are fairly long-lasting and washable, and not super expensive (I can sew them myself if need be). So:
- What is a sufficient curtain thickness for thermal insulation in a cold temperate climate, and what materials will make for the most durable curtains?
Bonus questions:
- How tall a pelmet is required to prevent airflow from behind the curtain? I understand that they are essential but I also think they are super ugly, so I'd like to minimise the visual impact with smaller pelmets if possible. I'm planning to make wood pelmets and paint them the same colour as the walls.
- Are modern s-fold/wave curtains (with a pelmet) as insulating as older style pencil pleat curtains? I would have thought that perhaps the air leakage from around the waves at the top might be higher.
Final note: I know that honeycomb blinds are a more energy-efficient option (not to mention more straightforward), but they are pretty expensive here and nearly all my windows have existing sheer roller blinds, or are sliding doors, so I think curtains will be a better option.

posted by neatsocks to Home & Garden (9 answers total)

Chilly Australian weather board here,

We've had some success with the spotlight thermal backed curtains. Definitely cut the heat out in summer more than other types of curtains. We have sheers and left up the old blinds too. (Also better for light blocking) caution there are curtains advertised as "thermal" that aren't that great at blocking heat/light.

Seem to keep the warm in too, but that's more about closing up gaps rather than fabric type. Making the rails go beyond the window if possible and fabric to the floor.

We haven't gone with pelmets yet, instead we're investing in in-wall insulation.
posted by freethefeet at 5:46 PM on June 11, 2020

What's the minimum I need to largely eliminate heat loss?

Brick and vacuum and something reflective.

I don't really understand your question. To prevent heat loss you need thermal insulation. The amount of thermal insulation curtains provide is directly proportional to their thermal insulation properties. Thickness and weight are correlated but just buy curtains that insulate well.
posted by turkeyphant at 5:56 PM on June 11, 2020

Call or drop into a Spotlight store and one of the sales staff will talk you through all the different options and give you some costings so you can shop around.
posted by Thella at 6:07 PM on June 11, 2020

The more curtain you have (thicker, longer) the more insulation you'll get. The main thing is to make sure the curtain fully covers the window with no gaps. You'll want it to hit the ground, and to be wide enough that it comes around the edges to the wall on each side. You can attach a hook or (better) something like velcro to the wall and curtain to make it seal against the wall if you like.

Multiple thinner curtains layered on top of each other are better than one thicker curtain, because you'll trap air in between the layers, which is an insulator.

We lived in Canberra and now in Sydney and have always had the Spotlight blackout curtains, but in Canberra we also had a second thinner curtain underneath.

Alternatively (or as well) you can have honeycomb blinds under the curtain, or apply a film to your window to imitate double-glazing. (Bubble wrap works on a budget, if you don't mind losing the view).
posted by lollusc at 6:16 PM on June 11, 2020

IKEA also sometimes has decent blackout curtains, if I recall, and I think we had those in one house and they were good.
posted by lollusc at 6:18 PM on June 11, 2020

Throwing some thought at the problem will outperform just throwing money at it. Two distinct problems, keeping heat in, and keeping heat out. Keep heat in, insulation - walls, DG, curtains, etc. Keep heat out, stop the sun shining in/on. That means eaves, reflective blinds, or awnings. It also means you need to deal with external walls soaking up sunlight and transmitting it into the house (shading, insulation). If the energy from the sun is not being reflected, it is being absorbed and heating that - this should be your basic line of attack for summer.

To put that in perspective, I live near Canberra, and have literally no blinds or curtains. I have wall insulation, tiled floors, and north facing floor to ceiling windows double glazed. I DO have a summer heat problem, the ground outside my northfacing windows heats up, and radiates heat into the house. I am dealing with that by changing the ground to absorb less heat - lawn, ground cover foliage etc.

I don't know what window film you are referencing, be careful that a summer benefit is not a winter problem, or vice versa eg if it reflects the summer sun, you lose the solar gain in winter.

Finally, on blockout curtains, do you want to live in a dark house in summer? I wouldn't, and it should not be necessary. It may be necessary in winter, especially after the sun has set, but only if you haven't found a smarter way of keeping the heat in.
posted by GeeEmm at 7:54 PM on June 11, 2020

Best answer: If you've already got sheer blinds, one plastic backed layer of fabric will probably be enough. I'd go pencil pleat/pinch pleat because I've found that the extra gathered fabric makes a difference, and get/make them long enough to almost touch the floor. For the doors, you could add another layer, but I think having a separate lining is more about being able to choose from a wider range of fabrics, or bulk up thin curtains. You can add linings pretty easily to curtains that have curtain tape, if you decide you need to later.

Full blockout are more insulating in my experience (my mum called them "triple pass", but I'm not sure how wide-spread that terminology is), but for a bedroom I prefer something that lets a bit of light in - easier to wake up in the morning.

If you hate pelmets, I'd install the curtains, and then do a smoke test to see how much air movement you have. I don't think they have to be very high to be effective, just overlap with the curtain at the top.

For summer heat, outside shading is more effective than indoor, if you can manage it.
posted by kjs4 at 12:14 AM on June 12, 2020

Best answer: I am in the UK so similar winter temperatures but not summer. When I think of a heavy curtain, I think of fabric that is closer to sofa upholstery fabric and probably lined. Not necessarily blackout. If you have the pelmet fairly snug fitting to the pole and curtain set up then that will probably help with the gap more than the height would. Agree that you want to make sure the length and width of the curtains is generous. It's also helpful to think about the time of day you open and close the curtains in winter, as it makes a difference if it's already dark before you close them.
posted by plonkee at 12:52 PM on June 12, 2020

Response by poster: Thanks for all the advice. I'll definitely look at sealing gaps around the curtains - will definitely try out a smoke test. I'm leaning in the direction of DIY curtains lined with a triple-weave backing (many people have suggested "thermal curtains", e.g. from Spotlight, but those are not washable).
For future people confused about curtains, these are the other things I have learnt:
- 1-pass,2-pass, and 3-pass all describe various thicknesses of plastic backing on curtains. Foam backings are also now available for better thermal properties. Triple-weave fabrics or linings are a washable alternative that is mostly blockout.
- well-fitted heavy curtains and pelmets increase the R-value of a window by up to 220% (unfortunately the source of this number is an Australian industry magazine that I can't access), which is comparable to honeycomb blinds.
- Actual insulated curtains/window quilts can increase the R-value to >4, apparently (so probably only useful in a) a very cold climate and b) a house where your walls are better than R=4, i.e. which you'd struggle to find in Australia)
- "heavy curtains" really means "heavy lined curtains" in the energy efficiency tests.
- the performance of plastic-backed "thermal curtains" is slightly lower than heavy lined: study here. Thermal curtains are more specifically designed to keeping summer heat out.
posted by neatsocks at 9:38 PM on June 12, 2020 [1 favorite]