Large uninsulated glass windows, Jalpan Mexico © Daniel FriedmanWindow Insulation Improvements
How to insulate windows from the interior

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How to insulate windows from the inside:

this article describes options for adding insulation on the indoor side of windows in an older home. Insulating panels, solar shades, or interior storms can significantly reduce heat loss through window openings.

But attention to air leaks, and where foam panels are used, attention to fire safety are important as well.

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Options for insulating windows from the inside

Antique window that woudl benefit from an interior storm (C) Daniel Friedman

Reader question: what's the best way to add insulating foam panels at the inside of windows in an older home?

First, thanks for your terrific website, I refer to it all the time. I own a 180 yr. old PA. stone farmhouse with 18 in. deep windows wells and poorly installed replacement windows.

Because of the cost for new or authentic windows, I was thinking about sealing them off the window wells with foam panels that would be covered with curtain material or something similar as to be more aesthetically pleasing.

Photo at left, an antique two over two window shown from inside. This photo is from one of our inspections and is not that of the reader who writes below. - Ed.

[Click to enlarge any image]

My idea would be to remove the 2in r10 panels by day and reinstall each night. (This sounds like a pain already.)

They would be positioned in the window well in line with the interior wall to have say 16 inches of air space between it and the window. Is the biggest concern condensation on the inside of the wood windows and are there any other concerns that I should be aware of?

Would the glass shatter due to the cold? Other than the daily removal and replacement of the panels, does this sound like a good idea to you? - J.S. Morristown PA 1/24/2014

Reply: great question, here are some options and some warnings about adding window insulation indoors

Foil faced foam board insulation from Dow, Tuff-R (C) Daniel Friedman

My friend Stu Tucker did something like this for his A-Frame home that had a lot of glass. He cut foil-faced 2-inch solid foam panels that would press-fit into the window frames at night but could be easily removed for daytime natural lighting (and for the view).

Our photo (left) illustrates a typical foil faced foam insulating board, this product is Dow's Tuff-R® insulation.

Fire hazards from exposed foam insulation

Watch out: if you leave foam insulation exposed indoors in a living or occupied space, you are violating building code and creating a fire and smoke hazard if you leave unprotected foam panels exposed in an occupied building space.

Details are at FOAM BOARD FIRE COVERING where we explain the fire and dense smoke issue.

So the biggest concern with exposed foam insulation is safety, not condensation on the windows.

Effects of insulation on window condensation

Uninsulated windows at Frank Lloyd Wright's Talesin West, Arizona (C) Daniel FriedmanWhat about the effects on window condensation (on the window glass interior side) when you install an insulating panel?

Actually you are stopping the movement of moist interior air against the cold glass and thus will be reducing, not increasing, the condensation rate; the insulation itself also reduces the condensation rate as well - we've pushed the dew point outwards.

Our photo illustrates interesting un-insulated windows at Frank Lloyd Wright's Taliesin West in Arizona.

Make some prototype insluating panels

I suggest making a couple of window insulating panels as a prototype. Use foil faced insulation because of its additional reflective value (and it looks better from the outside too) - and trying them out.

If you find you like the idea you could spray-glue-laminate a layer of drywall on the interior or room side of the panels to meet modern fire codes and to reduce fire risk.

That way if you or someone else should leave the panels in place for a time you are not creating the fire and smoke hazard I cite above.

Alternative methods for improving window insulating properties without using foam panels: interior storms or solar shades

An alternative to press-fit foam insulation in windows are more permanently installed solar shades that can also improve the window's R-value. We discuss these at SOLAR SHADES, LOW-E EFFECTIVENESS.

At STORM WINDOW PLASTIC CHOICES we describe a different approach - adding interior storm windows - for those openings whose energy efficiency you'd like to improve but where you also want to have a view or admit light.

Finally, keep in mind that insulating the window opening space between jambs, while an improvement, will not address air leaks through the surrounding window frame nor will it address the absence of insulation in older homes whose windows operated by ropes and sash weights.

Importance of sealing air leaks when insulating windows

Sketch showing window sash weight & rope installation details (C) Daniel Friedman Window exterior rot where no storm was installed (C) Daniel Friedman

Watch out: while an interior storm window may fit very tightly in the space between the window jambs, head and sill - leaking less air than some exterior storm window designs, keep in mind these cautions:

  • Exterior storm windows provide additional storm protection for the window itself. On some older buildings we find more severe leaks and trim or sill rot on older wood windows that have no storm window protection. Keeping such windows caulked and painted can reduce this risk.

    My photo (above right) illustrates this problem. A metal window installed in a wood-framed opening used insulated glass.

    The modern and nearly indestructible metal window lulled the property owner into thinking this was a maintenance-free building. It was not. Heat losses, condensation, and the absence of an exterior storm conspired with absence of inspection, caulking, painting and maintenance to lead to rot and leaks into the wall cavity below.
  • Air leaks around the window frame, especially on older sash-weight and rope type windows (photo at above left), may remain a significant source of heat loss.

In my opinion if the window openings are leaky, that heat loss overwhelms the radiant heat loss through the glass openings themselves. At AIR LEAK SEALING PROCEDURE we discuss the importance of sealing air-leaks around windows and doors.

Tools for looking at air leaks more thoroughly are discussed beginning

If you discover that indeed the windows are leaky in your home - referring now to the window sash and surrounding frame, you'll want to review this companion

Where to Buy Window Insulating Products & Interior Storm Windows

Interior Storm Window Sources

  • Allied Window, 11111 Canal Road, Cincinnati, OH 45241, Tel: (800) 445-5411, Email:, Website: , provides interior storm windows,
  • ClimateSeal™ storm window products, Climate Seal™, Division of Acoustical Surfaces, Inc, 123 Columbia Court North, Suite 201, Chaska, MN 55318 Tel: 877.773.7379, Email, Website:
  • Glass interior storm windows, Innerglass Window Systems, LLC., Innerglass Window Systems, LLC, 15 Herman Drive, Simsbury CT 06070 Tel: 800-743-6207, Local 860-651-3951, Website: . The company provides compression fit interior storm windows\
  • Indow Windows, 2267 N. Interstate Avenue, Portland, OR 97227, Email: , Tel: 503-284-2260, Website: provides interior storm windows
  • Larson Doors, providers Insider windows. Larson Manufacturing Company Brookings, SD, USA, Tel: 800.352.3360, Website:, Quoting the company's website:

    Insider™ windows install on the inside of existing windows. They cut energy loss by up to 50%, reduce drafts by up to 48% and cut outside noise. They’re perfect for older and historic homes because they don’t change the look of your home. Since they’re attached from the inside, they will not weather, and can be easily removed for the summer months or for cleaning.
  • Window Saver Company, 103 Noland Court, PO Box 2650 Lyons, CO 80540 and also at 177 East Riding Drive Carlisle, MA 01741 , plexiglass interior storm window kits, Website: , Tel: 800-321-9276, Email: , - quoting: Most likely you will save about 30% on your heating and air conditioning bills.

Solar Shades Designs, Information, & Product Sources

Storm Window Articles

Continue reading at WINDOW / DOOR AIR LEAK SEALING HOW TO or select a topic from the More Reading links or topic ARTICLE INDEX shown below.


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WINDOW INSULATION OPTIONS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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