Window Insulation Help

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Window Insulation Help

#1 Post by Chittt66 »

Hi all, new to the forums, thank you in advance!

I’m going to be replacing my first floor windows. To give you an idea of the job: I have exterior trim around the current windows I’ll be removing, removing the old nail fin vinyl windows, and installing new nail fin windows and trim. I have the procedure worked out, but I have 1 problem I can’t get the right solution for.

On the inside of my windows, there is no trim around the window; it buts up right along side of the interior drywall and only a bottom wood sill for decoration. After I install the windows, I won’t be able to use spray foam insulation from the inside to seal for thermal/wind. What are my options?

What I’ve thought of so far is using a sill seal foam and wrap the window before install. Hopefully finding one with adhesive on one half and about 2″ wide. Someone else suggested drilling holes in the nail fin from the outside and spraying foam in.

Any advice would be great.
Thank you!

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Re: Window Insulation Help

#2 Post by HomeSealed »

Have you verified that the depth of the old window is the same as the new? Its common for the older new construction vinyls to have a more narrow frame (depth) which will create an issue for you. In that case you'd have to cut back drywall which is messy and often leads to more patching etc. Most pros will downsize a fin-less replacement to fit inside the drywall return which really fixes all of the potential issues.

If you are certain that(depth) is not an issue and your only concern is how to insulate, then the two choices are:
1) Cut out the drywall return and corner bead anyway to allow for proper insulation and then add casing and jamb extensions.
2) Try your best with boogers and bubble gum. You can get it mostly sealed by back caulking the drywall and nail fin, using a foam wrap, foam through holes, etc, but its almost inevitable that there will be a corner that leaks air or some other cold spot. Maybe some of the other guys have had better luck with that, they've always haunted me personally.

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Re: Window Insulation Help

#3 Post by TheWindowNerd »

You can remove the old, dry fit the new, have someone inside push the unit out 1/2", spray foam around the unit, push the window back to install depth, fasten with nail fin, tape fin to house, trim cured excess interior foam and caulk interior. Not that hard with a progun for the foam.


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