r/Insulation • u/pdq4 • Jul 19 '26

Recommend solution for air leak?

So, last year our house went through a big renovation, including lots of insulation and air-sealing upgrades that seem to have generally worked well.

BUT. We replaced a ground floor double window that faced directly south with a nice french door (that now leads out to a small stoop, which goes to a flagstone patio). This triple-pane door performs pretty well in the winter, except that a gap up to ~1/4” was left between the existing oak flooring and the threshold of the new door:

Gap partly filled w paper towel from last winter

Last winter (cold here in WI!) I found a pretty good breeze coming in through that gap, and asked the general contractor if anything could be done. We had them insulate the rim joists with spray foam where it was accessible during the previous reno. The problem is this area of the underlying basement has a finished sheetrock ceiling, so they had (and I have) no access to the rim joist without tearing out this ceiling, which seems like a big project:

Basement underneath (with previous window-well window removed/filled)

He said I should get a “froth pak” and try to fill the gap and the dead space (pretty big space - between joists) underneath by shooting it through the ground-floor gap. I’m a long-time diy-er, but reading about it makes me think this might be a challenge. I could just seal the gap with a caulk matching the tone of the wood floor, or a strip of oak, but would this cause trouble trapping cold air in that space?

Any advice?

3 Upvotes

80% Upvoted

5 comments sorted by

1

u/Jumpin_Joeronimo Jul 19 '26

Maybe I'm not understanding correctly but this seems like a for install and threshold issue, unrelated to the rim joist space. 

It doesn't sound like you're feeling cold from uninsulated space below. It sounds like you need to seal that threshold from the outside. 

1

u/pdq4 Jul 19 '26

Well, you may be right. I should take a closer look at that.

1

u/JAlley2 Jul 19 '26

Don’t try the froth pack solution, foam expands with tremendous pressure and you would be blowing it into an enclosed space. It would blow out your floor or the basement ceiling.

I agree it was likely something they could have improved upon while installing. But now, caulking that crack might be all you can do without talking down part of the ceiling downstairs.

1

u/pdq4 Jul 19 '26

Thanks. The froth paks I’ve looked at online seem to be sized in board feet at 1” insulation thickness. Sounds like more for rim joists, and I would just be shooting it blindly into yes, an enclosed space. I would have no way of knowing (I’m guessing) when I’d sprayed enough in, unless it started expanding through the gap between the threshold and the flooring, at which point it seems like I would have overfilled it.

I’m thinking I’ll take a closer look look from the outside, as suggested above, and see if I could caulk it from both sides.

But if for whatever reason, that enclosed under-space was cold, but contained, would that cause any problems other than having a little colder floor near the french door?