I am having Denver Home Depot replace five old double-hung windows next weekend and will watch carefully to see if I can do remaining windows myself.
Home Depot normally removes sash pulleys and squirts insulation through resulting slots to fill gap where sash weights hang. However, I have already removed windows, removed panels from gaps where weights hang, removed weights, and cleaned the entire thing up.
Since the gaps between windows are open, I would like to use batting rather than spray foams and I like the denim batting but am concerned about its resistance to moisture compared to fiberglass.
I suppose I should make sure contractors fully caulk inside of sash weight gaps before inserting denim.
Are there additional steps I could follow to achieve the best possible denim installation and moisture resistance?
Any insulation will work, but expandable foam works best in this application as it provided move R-value per inch plus it has the ability to get into every small area. As you point out, to achieve the same weather tightness, you will need to caulk first, then add your denim fill. Since it's hard to see all the areas surrounding the window, it will be easy to miss some caulked areas. OTOH, the foam insulation will expand to fill everything.
It's better to use the low expansion foam for windows to avoid too much expansion, but hands down the foam spray insulation is a far better product to use in your case.
If I used a spray foam, it would probably be Home Depot's Dow Great Stuff product whose specifications list R3.8/inch, about the same as denim:
If I knew the specific glazing and spacer system of my new windows (sitting in factory), I could find their overall Total Unit R-Value but it does not look like my argon windows will get much over R3 per total window:
On the other hand, if I am computing this correctly, total R-value for the 5.5 inch x 0.872 square foot insulated slots between windows is near R20 (for both denim and Great Stuff polyurethane), so R-value may not be the best criterion for comparison.
Foam 1) forms best seal; 2) is easier to apply; 3) is water resistant; 4) forms hard semi-permanent plastic; 5) is environmentally noxious.
Denim 6) is environmentally friendly; 7) would be easy to repair; 8) maybe poorer at reducing convection; 9) may be affected more by moisture or settling over time.
Tough to decide. I'll keep asking around. Thanks again for your feedback.
You are thinking this out well. The only point I can add is that the denim may settle over time, leaving an insulation gap.
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