I have some rather large aluminium bifold doors in my kitchen and they let alot of heat out.
How do you know? What scientific experiment did you carry out to determine this?
I am tempted to drill holes into the extruded aluminium frames and to spray expanding foam into the holes to give it a greater U-value.
The change will be minor; the frames already trap air in a small chamber and it convects minimally so won't be a reasonable path of heat loss through the frame. The frame itself is a likely sandwich of aluminium, plastic and aluminium, and the plastic stops the inner aluminium being well thermally connected to the outer or the frame being a giant heatsink
If you want to do it so you feel better, consider a two part mix-yourself urethane foam rather than squirty expanding foam.
Expanding foam is urethane two parts already mixed, with a solvent to keep it liquid. When you squirt it out it foams up and the solvent evaporates leaving the foam behind. If the solvent stays too long, eg if you make a wedge of foam too big so the inner solvent can't evaporate, or you squirt it into some place where all the surrounding surfaces are impervious and do not let the solvent evaporate, then the foam structure collapses again. Try it; fill a carrier bag with expanding foam, tie the top and wait X days for it to solidify then cut it in half and look at the quality of the foam
To me it seems a small job
It will until you start, then it'll become a real ballache and you'll get foam everywhere. If your frames are powder coated the foam solvent will eat the powder coat and it'll end up looking gash. You'd be better off removing the door, drilling a hole in the bottom, a hole in the top and using a large syringe to push premixed A/B into one hole and waiting for it to expand and possibly spray out the other hole. Don't drill holes on the face of any part of the frame. Consider too that filling the frame could disrupt its established, designed drain paths and end up with rainwater coming into your house
The metal frame gets incredibly cold and does attract condensation.
Generate less moisture in that room or, as one poster hs already recommended, fit a second insulated layer such as curtains. Condensation won't bother aluminium
I wouldn't do it, but I think before you ruin an expensive set of bifolds you should perform a properly scientific test on a small section of loose frame (ask a local supplier for an offcut), make one wall of a small insulated chamber out of the frame piece and the other walls out of PUR. Fill the frame with water at temperature X (eg 19, representative of the room) and time how long it takes to cool to temperature Y (eg 5, in the fridge, representative of the world). Fill the frame with PUR foam and repeat. See what actual difference it will make and whether it will ever repay the cost of the foam and time
Post it up on YouTube; people love that kinda "I wanna know this but I can't be arsed experimenting it myself" stuff
