Need advice on replacing an old leaky roof

One thing I’d add - don’t forget about ventilation. People get so focused on insulation and underlayment that they forget you need air moving through the attic space.

Yeah, ventilation gets overlooked way too often. I’ve seen folks throw money at fancy underlayments and insulation, but if the air can’t move, you’re just trapping problems up there. Honestly, I’m a little skeptical about rigid foam unless you’ve got a real reason - sometimes it just complicates things. Just make sure whatever you do, the system works together. No point in having top-notch materials if they’re fighting each other.

Rigid foam’s actually saved my bacon a couple times, especially in older houses where the rafters are too shallow for enough batt insulation. I get the concern about overcomplicating things, but if you detail it right - tape the seams, keep a good air barrier - it can really cut down on thermal bridging. Had a project last winter where we finally stopped ice dams after adding exterior foam. Ventilation’s key, but sometimes you need that extra layer, you know?

if you detail it right - tape the seams, keep a good air barrier - it can really cut down on thermal bridging.

That’s true, but I’ve seen a few jobs where the foam detailing got rushed and then you’re chasing air leaks for months. Sometimes I lean towards dense-pack cellulose in those tricky rafters - less fussy install, and it still does a decent job with air sealing if you net it right. Just feels a bit more forgiving if you don’t have a crew that’s super meticulous.

Sometimes I lean towards dense-pack cellulose in those tricky rafters - less fussy install, and it still does a decent job with air sealing if you net it right.

I hear you on that. Foam can be great, but only if the crew is really dialed in. I’ve walked into too many projects where someone missed a seam or didn’t tape a joint, and then you’re stuck with callbacks for drafts or even moisture issues. Dense-pack cellulose is definitely more forgiving, especially in older roofs where nothing is straight and you’re dealing with weird framing.

One thing I’d watch out for with cellulose, though, is making sure you’ve got a solid vapor retarder in place - seen a few cases where moisture migration became a problem over time, especially in colder climates. Have you ever tried hybrid assemblies? Sometimes I’ll spec a thin layer of closed-cell foam for the air barrier, then fill the rest with cellulose. It’s a bit more work up front, but seems to balance performance and install risk. Curious if anyone’s had long-term luck with that approach...

Hybrid assemblies are definitely gaining traction, especially in retrofits where you’re dealing with unpredictable framing and existing conditions. I’ve specified the closed-cell foam plus cellulose combo a few times - usually in projects where we’re trying to hit higher R-values without losing too much interior space. It does seem to work well for air sealing, but I’ve noticed the detailing around penetrations and transitions is still critical. One project had issues at skylight curbs because the foam layer was too thin in spots, and that ended up being a weak point for condensation.

Curious how folks are handling ventilation in these assemblies, particularly with older roofs that don’t have much space for vent channels. Are you relying on exterior venting, or going unvented with full air and vapor control? I’ve seen both approaches, but I’m not convinced there’s a one-size-fits-all answer, especially when you throw historic details into the mix...