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I was told that putting grilles between the windows lowers the R value of the window. Is that true?

Debbie in Toronto

The short answer is yes. Sometimes.

Internal grilles are usually made of folded metal tubes. Metals are thermal bridges, as they have high thermal conductivity What that means is they absorb heat and cold, and pass it through to whatever they’re touching on the other side. While internal grilles do not normally touch both the glass panes - so there is no contact from the exterior glass pane to the interior pane - the high thermal conductivity of metal reduces the insulation provided by the argon gas between your panes of glass. Now and then, you can see this reduction in insulation strength by condensation around the grille itself. When we do triple-pane, the overall effect from internal grilles should be reduced, as only one of the glass chambers has them. This means the other argon-filled chamber still insulates your home.

External grilles (aka Simulated Divided Lites) are made of cellular PVC installed outside the glass, and as such should have a smaller effect. The disadvantage, compared to internal grilles, is that they aren't as straightforward to clean. Deciding to go with internal or external grilles will come down to your preference - higher r-value, or lower maintenance.

Tough call.

Thanks for Asking an Expert, Debbie!

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