Should I use PIR or polyiso boards for my garage conversion?

I'd go with polyiso for a garage conversion, honestly. PIR's fine, but polyiso gives you better thermal performance per inch of thickness - we're talking R-6.5 to R-6.8 versus R-5.8 to R-6.2 for PIR. That matters when you're converting a garage since you're usually working with limited space and want solid insulation. Polyiso also handles temperature swings better, which is important in a garage where you might have cars coming in and out creating draft and temperature changes.

The main thing to watch is that polyiso needs a vapor barrier on the warm side in most climates, and you definitely can't leave it exposed - it needs drywall or other protection from UV and physical damage. PIR's a bit more forgiving if you mess up the install, but honestly if you're doing it right either way, you won't notice the difference in ease. Make sure whatever you pick is rated for your climate zone.

One thing I always think about is long-term durability. Both hold up fine in a garage setting, but get your local building codes checked first - some areas have specific requirements for garage insulation, especially if you're converting it for living space. That'll probably dictate your minimum R-value anyway, which might push you toward polyiso just to get better efficiency without going thicker.

Polyiso's solid if you're actually using that garage regularly - which is the real question to ask yourself first. Like, if you're converting it into a gym or office space where you're spending time, better insulation matters. That said, the R-value difference people mention is real but honestly not *that* dramatic in practical terms. You're looking at maybe half an inch of material difference to hit the same R-value, which isn't nothing but it's not a dealbreaker either.

Where I'd lean polyiso is if you're dealing with a tight space and want to maximize interior dimensions without going thicker on the walls. I've got a home gym setup and every inch counts when you're trying to fit equipment. Also, polyiso tends to be easier to work with IMO - it cuts cleaner and fits snugger between studs. The downside is it's more moisture-sensitive than PIR, so if your garage has any humidity issues or isn't super well-sealed, you might have problems long-term. PIR handles moisture better, which honestly matters more than people think in a garage environment.

My take: if you're building this right with a vapor barrier and proper ventilation, go polyiso and save yourself some cash compared to premium PIR products. If you're in a really damp climate or kinda skeptical about your garage's ability to stay dry, PIR's the safer play. Either way, seal your seams properly - that matters way more than which board you pick.

I'd push back a little on that thermal performance gap - honestly, in real-world conditions on a garage conversion, the difference between polyiso and PIR isn't going to make or break your project. Both are solid choices, and the R-value difference is pretty marginal when you factor in installation, air sealing, and everything else you're doing.

Here's what actually matters more for a garage: moisture. If you're in a humid climate or your garage sees temperature swings, PIR handles moisture exposure better than polyiso. Polyiso can absorb water and lose performance if it gets wet, which isn't ideal in a conversion space where you might have condensation issues. PIR's more forgiving that way. Also consider your budget - PIR usually costs less, and for a garage, that savings might be better spent on proper ventilation or a decent vapor barrier setup. The R-value difference between them is basically splitting hairs when you're insulating a garage anyway.

My honest take: measure twice, think about your local climate, and pick whichever one fits your vapor strategy and budget. Either one will work fine if you install it right and manage moisture. I've seen plenty of garage conversions do great with both materials.