Should I seal my yard shed windows with backer rod?

Question

I’m adding trim to three windows in my Tuff Shed (the installer also added a house wrap of that matters).

I went back and forth in adding backer rod in the interior gaps around the windows and decided to add back a rod where I could fit it then completed the trim for the first window, but now I’m having concerns and would rather address things now than down the line.

I filled the top and right side gaps with backer rod about half way to the edge of the gap (there’s still half a gap left), and left the left side and bottom gaps empty since the spaces were too small.

Is the backer rod even necessary since the outside of the window is already sealed? If so do I need to add more layers of backer rod so that it’s flush with the windows, then add the trim?

Answer 1

You asked if the backer rod is necessary. You are working on a shed, so the short answer is "No".

However if the shed is being used as some work area, or a hobby house or such, or if you simply want it to be as air tight as possible, then maybe.

If you are going to be using some type of heating and cooling system, you want it to be efficient. Sealing the gaps between the windows and framing would be desired in that situation.

Backer rod is used to fill large gaps prior to caulking, but also could be used as an insulation from air infiltration as well. Is it needed before you can install trim? Of course no, it is not.

The common and easier solution to fill the gaps is to use expanding foam. That is a code requirement in most areas, when installing windows in a dwelling. However, you have a shed and haven't told us what your final intended use is. If you have insulated the walls and you intend to condition the interior, then you should fill the gaps. If not, it is totally something done by choice.

Answer 2

Never use foam rope for windows.

One of the biggest problems we encountered when we started using spray foam in the '90s was that it was bulging the jambs, causing binding and alignment issues. Foam rope (backer rod) will do the same thing in many cases. It's too stiff for windows unless you're very careful about fitment so that it's barely snug. Since window gaps vary, it's rarely just the right size.

Use low-expansion spray foam instead, which will seal against airflow and help make your window installation more solid without moving it around. It expands slowly and gently to fill the available space without creating pressure.

Note that the stuff sticks to everything, permanently, so take precautions and apply it in small amounts, pausing before adding more.

Answer 3

Adding more layers of backer rod isn't ever the answer. If the backer rod doesn't fill the gap then you either have the wrong size backer rod, or you should be using something else entirely (like spray foam that will fill any size gap).

You'll want to fill the gaps with something. Leaving them open probably isn't a good idea. Even if your windows are sealed on the outside and this is just a shed and not insulated/conditioned like a house, there's another insidious problem you want to protect against: insects. There's nothing worse than opening up a shed for the first time in a month only to find that it's full of wasps. Any gaps you leave will be an opening that insects will eventually find and take advantage of. The fewer of these that you leave, the fewer creepy-crawlies that you'll have to evict.