Solar screens vs. foam board for those brutal west-facing windows?

I did the same thing with some R-3 Thermax boards on my south-facing bedroom windows in Phoenix. The temperture drop was immediate, but I started seeing some condensation and acutally noticed the glass was scorching to the touch in the late afternoon. I'm using dual-pane low-E glass, and my big worry is that the foil is reflecting heat right back into the air pocket between the pane and the foam, wich can definately blow out the seals or cause a stress crack. To mitigate that, I backed off the tight fit and left a one-inch air gap at the top and bottom to allow for some convection. It keeps the glass much cooler than when it's sealed tight against the frame. You might also want to check if your polyiso is touching the glass directly. Even a small spacer can help. It's a balance between absolute R-value and not having to call a glass repair company for a $400 replacement.

You’re definately playing a riskier game than you realize with that polyiso board. By trapping the heat inside the frame, you’re essentially creating a convection oven between the glass and the foam. I tried a similar setup with some 1-inch R-Matte board on my west-facing slider in Houston, and while it kept the room cool, I ended up with a blown seal and a foggy window within 14 months. The thermal expansion on the spacer material just can't handle those localized spikes. A much safer approach is to focus entirely on exterior mitigation or breathable layers. Instead of the foam, I upgraded my setup to custom-fit exterior solar shades and a high-heat rejection ceramic film like Gila Heat Shield on the glass itself. The film handles the UV and IR without reflecting it back into a stagnant air pocket like foil does. If you need more than the Sun Guard 90 screens, look into the 95% density fabric or even manual roll-down shades for those peak afternoon hours. It keeps the glass much closer to the ambient outdoor temprature and protects your window seals from failing prematurely.

You're defintely risking a blowout with that setup. If you want to block the heat without the thermal stress of foil-backed boards, you should switch to Indow Window inserts. I used their Sleep Panel grade on my west-facing office windows last summer and the difference was night and day. They press into the frame with a compression seal, creating a much safer dead air space than DIY foam. It keeps the heat out but doesn't bake the glass like polyiso does. Plus, they look way better than raw insulation board when guests come over.

You're way braver than me. Honestly, I'd be more worried about the fire code than the glass. Most polyiso isn't supposed to be left exposed inside 'cause it's pretty nasty if it ever catches. If you're dead set on blocking that heat, why not just move the insulation to the outside? I've seen folks build "shutters" out of that same foam board and pop 'em over the exterior frame with some heavy-duty Velcro. Blocks the sun before it even touches the pane, so you aren't turning your window into a furnace. Plus, your living room won't look like a total science project.