I had windows installed recently, and when winter came I realized that they didn't fully seal around windows in hard-to-see places. Blue photo is outdoors and the white/pink photo is indoors. I can see inside the house wall through both gaps and can feel the cold air coming through.
What product do you recommend for finishing this? I was going to use spackle for indoors and silicone caulk for outdoors but I've read conflicting info online and want to check with you.
Firstly, this was an epic fail by your window installer and they should be held accountable if possible. Barring that...
I agree with Ecnerwal, but I'll try to reinforce the notion that caulk is not an air barrier (mostly because you'll never find all the leaks that way). That's not where the problem lies, and that's not where you solve it.
Were I in your socks I'd gently pry the exterior casing off (slice the caulk first), then evaluate. Chances are that low-expansion foam applied around the entire window frame will solve airflow and heat conduction issues. Then, look at flashing for water drainage before reinstalling and caulking in the casing.
It is important NOT to caulk the outside bottom joint. Any water that gets into the window needs to be able to get out.
Spackle is not caulking, and is not a suitable substitute unless you're somehow unable to get caulking but have spackle and have months of winter ahead of you. Inside you typically want some sort of paintable caulking - acrylic or siliconized acrylic, are typical.
Outside, a silicone or polyurethane caulk.
Depending on the type of window (not suitable for old fashioned ones with weights hanging in the walls) you might want to inject some low-expansion foam into the wall cavity before finishing with caulk. Do not use the "regular expansion" (3X) foam for this task, as it is more prone to overexpand and cause the window to bind. You'd cut away any excess and caulk the joint to the wall surface.
At the very least, when you caulk the outside/top of the window, if not using any of the other methods such as remove casing/inject low expansion foam, add flashing, etc, then at least protect from water ingress with more than a bead of caulk: try to use an actual fillet -- a triangle shaped fill of caulk that will direct and shed any water that runs down the wall to the window.
For inside, you can temporarily prevent passage of air, especially if you can't get it stopped with better methods outside, by using anything that stops air passage: rolled up cloth, tape, clay, etc. By which I'm considering temporary measures that make the room livable TODAY! but should be removable without making any mess of the wall or woodwork/window frame.
It would be better to use an integrated approach as outlined above, considering fixes starting around the window within the wall, then both outside and inside as appropriate. But I've seen situations that need a temporary fix NOW, and can be remediated correctly over time...just hope you can get to a 'best' solution.
