I'm renovating an old house, and I need to prioritize my work. Let's say we have three heat losses to be compared:

  • an old 2,00 x 1,00 meters single glazed window

  • a recent 2,00 x 1,00 meters window, with 2022 double-glazing standard

  • all little spaces around doors cumulate to the equivalent of a 1 cm² hole between indoor and outdoor.

How can we calculate the heat loss in Watts for these three?

Hypothesis: let's say 20°C indoor, and 0°C outdoor, and a constant very light wind (in Europe) (might be needed for the computation of the air through hole).

  • 1
    The holes will let cold air in, but easy and cheap fix is expanding foam for windows/doors to stop it. Temporary fix for the single is a window insulation kit, some thin clear plastic held on with tape. The double can be left or also have the insulation kit put on.
    – crip659
    Nov 26, 2022 at 15:54
  • 1
    You can go on line and find R-values for the single & double glazed windows. From that, the area of the window, and an assumed inside/outside temperature difference, you can calculate the heat loss in watts. Note that single pane windows have an R-value (US, or Imperial units) of ~1, while a double paned window has an R-value of ~2. So a double paned windows will lets half as much heat escape as a single paned window.
    – SteveSh
    Nov 26, 2022 at 17:52
  • The heat loss through holes or gaps is harder to quantify.
    – SteveSh
    Nov 26, 2022 at 17:54
  • start by measuring temperature near the sources, will give you a initial guidance what to fix first.
    – Traveler
    Nov 26, 2022 at 18:29
  • @crip659 Would you have an image about "expanding foam for windows" (I know similar stuff, but out of curiosity, I'm interested about how it's packaged/sold/marketed in other countries from France). Same question about the "insulation kit", I know adhesive plastic film here, I wonder if it's similar.
    – Basj
    Nov 26, 2022 at 18:55

1 Answer 1


I wouldn't know about the maths, but air leakage and drafts in general are definitely your first priority. Air movement and exchange with the outdoors will be the most uncomfortable and most defeating of your energy efficiency.

A cheap can of Spray Foam or a Tube of Caulk is great only where they can squirt into.

While Weatherstripping around leaky doors and windows is the best solution there.

And, Plastic Sheeting over leaky doors or windows is the worst, as it stops usage of whatever it's applied over and only seals out drafts temporarily. It doesn't last nor stay stuck-up even for a year...usually.

  • 1
    Thanks @Iggy! Being curious, as I'm not a native English speaker, is "Tube of Caulk" the same thing as silicone cartridge? Or is it slightly different?
    – Basj
    Nov 29, 2022 at 7:56
  • 1
    Yes, silicone cartridge or the smaller use toothpaste types that you just squeeze out without any gun or other device.
    – Iggy
    Nov 30, 2022 at 0:36
  • 1
    Ok. Caulk and silicone are the same thing @Iggy?
    – Basj
    Nov 30, 2022 at 8:24
  • 1
    Yep, they're usually just called sealants, but most people don't get that. They can be silicone based or latex based, silicone lasts much longer.
    – Iggy
    Nov 30, 2022 at 17:58
  • 1
    Caulk/caulking is a flexible substance used to fill and smooth over small gaps. Silicone-based caulk is one of the more common types, but there are other formulations which have their own advantages and disadvantages.
    – keshlam
    Apr 28, 2023 at 1:24

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.