The inside: enter image description here

The outside: enter image description here

The installers gave me a window unsealed on the outside but sealed with a rubber seal on inside around the frame (windows are uPVC).

After rain, the damage was done to walls by leaks and the rubber seal inside was blocking the exit for the water so I pulled it out. Unfortunately I'm unable to put it back in place as tight as it was, it's become completely loose after fitting in gap.

The windows are installed on uneven wall opening so there is expected wiggle room.

I prefer to do insulation myself since installers completely disregarded this. Should I try to re-fit the original rubber seal or glue it on, or caulk even though the window wiggles? Ive seen videos where people caulk the rubber seal? Here's some of the damage, bubbled paint:

enter image description here

  • "The installers gave me a window unsealed on the outside" if this is a new installation, call them back (even if its been a few months). Call the company and tell them they have done unsatisfactory work and why.
    – Tyson
    Jan 26, 2018 at 11:35

2 Answers 2


Theres alot to be left desired by this installation. First of all it looks like your in the city and installed into a masonry opening. By the way your window is resting hard on the masonry sill it looks like the installation is what we refer to as brick to brick. Basically your window fits snug around the brick with maybe a quarter inch perimeter gap. Typically when you have this type of installation you would waterproof the back up masonry jambs which is usually brick or a concrete masonry unit with a perm a barrier applied on a primer. S100 sealant would be applied to the masonry sill and a sill pan would go on top of that. The perm a barrier would then drape into the sill with the vertical sections of perm a barrier being terminated with s100 sealant to prevent the perm a barrier from peeling away from the masonry. The idea is the perm a barrier diverts water that gets passed your caulk on the exterior into your sill pan and keep it from getting inside your house. If you cant strap to the masonry then a buck could be built in the pocket and the window could be anchored to the buck providing the buck was properly anchored. Either way a backer rod would be applied to the exterior perimeter about a quarter inch depth and sealant like a sika flex or np1 around the perimeter. Sometimes they leave a couple of 1 inch gaps at the sill to let water caught by the sill pan leak out. On the interior they could insulate around the window with bat insulation or low expansion spray foam although brick to brick openings usually have to much of a gap to spray foam. If I am wrong and this is within frame then just seal the exterior of the new window to the exterior of the old and spray foam in between the two windows on the interior underneath whatever finish trim you will install. But before I would lift a finger I'd call the installer and ask why is my window inadequately anchored and not sealed on the exterior. Sounds like the job is unfinished.

  • I got a cheap window from a cheap company. You get what you pay for, I'll see what they tell me. I'll need to research what you say because I'm not familiar with all of it. The cill outside is messed up, the painters didn't see a need to do anything to it other than paint it white. The window frame that restaurant on wall can't be taken apart to reveal the gaps, it's all one piece hammered into each other or something like that. So I can't spray foam or other.
    – Altoban
    Jan 26, 2018 at 15:36

what's the best way to seal ...

Use a paintbrush to sweep out loose debris in the gap then apply builders mastic externally, use a sealant gun.

Random example:

A Tube Builder's Mastic


  • Overpaintable.
  • Adheres to most common surfaces.
  • Semi drying - remains permanently plastic beneath surface skin.
  • High solids formulation - excellent gap filling properties without shrinkage.
  • Good external weathering properties.


  • Perimeter pointing internally and externally around doors and windows.
  • Adheres to concrete, brick, wood, metals and glass without the use of a primer.

C3 cartridge size. Brown 310ml

(my emphasis)

Also available in white or grey.

Sealant Gun

If you have not done this before, practice on some scrap wood or something similar. It is sticky and can be difficult to clean up if you make a mess. It may help to put masking tape around the area to be sealed. Expect it to take a long time to dry/set in cold weather - keep fingers out.

... and insulate


  • seal the gap externally to prevent drafts.
  • seal the inside in the hope of creating a hermetically sealed air gap?
  • maybe try to fill the void with some closed-cell foam goop? or just
  • not worry too much about insulating it so long as there are no obvious drafts.
  • I was prepared to but realized the window moves when you push against it or pull. And the water damaged the cill and that in turn damaged where the screws are dug into that fix's window onto cill. Either way I'm afraid caulking will be crack when window moves for whatever reason, like strong winds. Also i might want to take it apart to repaint damaged walls.
    – Altoban
    Jan 26, 2018 at 11:05
  • @Altoban, Mastic remains flexible. If your window is indeed moving so much that even mastic cannot seal the gap then your only course of action must be to remove the window and replace it with a better one that is sufficiently rigid when properly fitted. I'd try mastic first. Perhaps inserting wedges would help stiffen things up but if the window is irredeemably hopeless, discard it. Jan 26, 2018 at 11:12
  • Should I use PU foam spray to fill the gap or pre caulk robe? Also should I use mastic inside the room or is it better to keep the rubber seal gasket?
    – Altoban
    Jan 28, 2018 at 8:31
  • Found something local that's "mastic silicone sealant" that okay? The random example provided has it's datasheet state not to use on upvc. "Do not use on Upvc, polycarbonate or acrylic sheet. Use SILICONE 450."
    – Altoban
    Jan 29, 2018 at 16:53

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