Window Frame Insulation

by Josh on December 21, 2009 · 3 comments

in Bedroom,Projects,Windows

In my long-running bedroom project, the next piece that I am tackling is the restoration of the two double-hung windows.  Recently I thought of a great way to insulate the weight pockets next to the window frame. And then I thought of a different idea that would actually work.

But before I get to that, let’s talk about the weight pockets themselves. Old double-hung windows use a system of ropes, pulleys and weights to balance the sashes and hold them open.  The weights move up and down in empty spaces alongside the frame. These empty spaces are commonly a source of drafts if they aren’t properly sealed to prevent outside air from moving inside. Replacement window companies make a lot of money promising to eliminate drafts like these, but better sealing and insulating of the weight pockets can make old windows much more comfortable to live with.

Which brings me to my first idea: put each sash weight inside a length of pipe in the pocket space and then seal the pipes in place with spray foam. I really thought this was great because it would absolutely air seal the weight pockets and stop drafts.  Unfortunately, I found that the diameter of PVC pipes necessary to contain the weights would be too large to fit inside the weight pocket. Drat!

My second approach was more coventional: just seal the corners of the weight pockets with spray foam and fill the top and bottom window frame spaces with foam.

For this project, I used Great Stuff expanding foam “window and door” formulation because it sets up soft and won’t deform with window frame. This foam was easy to pick up at my local True Value Hardware, where I found other brands and formulations to choose from, too.

Foam insulation sash weight pocket

As the picture above shows, the weight pocket won’t be insulated per se, but the foam at the corners should seal the joints between the boards to keep air from moving through the space.

When I sprayed the gaps on the top and bottom of the windows, I was able to completely fill the spaces with foam.  Once the foam had set up, I needed to trim the overfill at the edges. I tried a few different tools to shave this overfill, but the thing that really worked great was a hacksaw blade.

Trimming foam insulation

The blade was flexible to bend into odd places and the fine teeth cut the foam cleanly without shredding or compressing. The result was a smooth foam edge that should provide some great protection from drafts around the window frame when the window restoration work is finished.

Foam insulation trimmed

Disclosure: I was one of five bloggers selected by True Value to work on the DIY Squad. I have been compensated for my time commitment to the program and my DIY project as well as my posts about my experience. I have also been compensated for the materials needed for my DIY project. However, my opinions are entirely my own and I have not been paid to publish positive comments.

{ 3 comments… read them below or add one }

Joe December 22, 2009 at 3:43 pm

Wow! I had the EXACT SAME IDEA of using the pipes, but I too realized that the pockets aren’t much bigger than the weights themselves. I also thought of carving some styrofoam blocks to put in around the weights, but I haven’t followed through on that idea yet. I think I’ll try the Great Stuff on my next window project. Or maybe my current one if I ever get around to finishing it. 😉


Josh December 24, 2009 at 2:16 am

Great minds, eh Joe? If you come up with a better solution for those weight pockets be sure to share it!


John Leeke January 1, 2010 at 3:11 pm

Back in the 1980s and 1990s I discovered that a lot of the windows we sealed up with foam in the 1970s energy crunch had rotted out after 10 or 20 years. The cause of the rot was definitely the foam, which trapped moisture in the wood.

We though we were so smart, but Mother Nature and the Test of Time proved us wrong.

It turns out that an important part of the traditional window design is all these air spaces, which help keep the wood dry and decay free. A little dry air from the outdoors trickles in through the joints and carries away the moisture. This is even described in some of the old building trades manuals.

In the early 1990s we stopped sealing and foaming around the windows, and now, twenty years later, we find that those windows are doing just fine–no decay.

We now do a little sealing around the interior side of the window casings, where the edge of the casing meets the plaster, install weatherstripping around the sashes, etc. And we do seal at the exterior window casings, to keep liquid water out, but to let a little air in.



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