What is new exterior insulation detail if I keep old windows?
I was planning on ordering ThermalBuck today for my 1979 Pella casement windows and realized two things. Thermal Bucks are designed for Rough Opening (RO) dimensions. They assume you are replacing windows. When I did searches here and elsewhere for exterior insulation “old window” detail all I found was there is no ROI$ in replacing windows that are tight and double pane. There was much about adding more glazing layers- I want to insulate the perimeter of these R holes in my walls. Perimeter losses are large in wall penetrations.
Surely there is a drawing or Common Off The Shelf (COTS) kit to acomodate two (2) inches of exterior insulation around almost flush windows. I have caulked and insulated to where there is only 1/4 inch reveal top and sides. I’ve seen old windows recessed to decrease wind shear of dead air space.
Is there a Common Off The Shelf fix?
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Replies
Charles,
I don't know of any "common off-the-shelf" solution.
The trickiest part of your plan will be to make sure that the water that dribbles off your window sill is conveyed to the exterior. You need sill extensions. This detail needs to be thought out carefully, because a lot of older windows have water-entry problems at the two lower corners of the windows.
The solution you choose will depend on the exposure of these windows to wind-driven rain (obviously, homes near the ocean are the worst-case scenario) and the width of your roof overhangs (the wider the overhang, and the closer the overhang to the window head, the better). If you can slip some sloped metal flashing under the existing window -- between the window and the rough sill -- that would be a good start.
As Steve noted, you will also need exterior jamb extensions and head extensions, which can be fabricated from metal flashing or a durable material like cellular PVC trim. Whether you end up with a watertight installation or not depends on the quality of your details and your skill.
The safest approach is always to remove the existing windows and flash the rough openings properly, but that's expensive.
I needed to create a clean edge at the window openings for my installation (cement siding, strapping, 1 inch of foam, etc.). Matt Risinger had a detail on his blog that I ended up copying. It involved using aluminum to form a border around the opening and then butting the other materials against it. (Here is the link: https://www.youtube.com/watch?v=l9O9bdOVSH8.)
It made for a clean installation. Would this approach work for you?
Steve, thank you for pointing me to Matt Risinger; his blogs are insightful and professional. Martin, yes thank you, I need sill extensions. This up grade was sparked by what you have observed, "because a lot of older windows have water-entry problems at the two lower corners of the windows." The previous wood replacements have been "cellular pvc trim" from HDepot and Azek boards. Then I was only concerned about stopping the rot. Now on this North wall DOE Zone 3 moist, I want to do repair and what y'all are advising; however, my cellular pvc trim is honkin massive! Patrick Horton- my associate and contractor knows how to use vibratory cutter to remove vertical and horizontal trim on these windows and replace wood with PVC.
But, to get the insulation=R where I want it, must I build up each piece from solid cellular pvc and then mill out the core to be replaced with poly-foam? The pvc will protect the insulation from the elements. Hope my pictures clarify:
Charles,
Your photos don't clarify the issue for me.
PVC trim is just trim. It's not flashing. You can't depend on PVC trim to keep water out of your wall.
To keep water out of your wall, you have to start by flashing the window rough opening. If you can't do that in a retrofit situation, you are taking a risk.
On a web forum like this, I can't determine whether or not you have the needed flashing skills to tackle this project. But you need to have a fundamental understanding of flashing methods, and ways to direct water away from the rough opening and to the exterior side of the WRB or siding, to succeed in this endeavor.