Insulating *vented* low-slope roof assembly above sheathing
Hello! I have read and re-read numerous articles on this site by Martin Holladay and others to try to get some clarity on my situation, but to be honest I am thoroughly confused. I am a homeowner, not a builder. I wonder if someone might kindly provide some guidance. I have a 1961 house in climate zone 5a (Central PA) with a low-slope (1:12) Duro-Last membrane roof and cathedral ceilings throughout. Here’s the issue: The roof needs to be replaced, and in theory, this presents a good opportunity to add some insulation, and my roofer is offering to add several inches of polyiso insulation above the sheathing. But I want to be careful not to create moisture problems and don’t know what to do.
Here are more details about the structure as it is right now:
1) My home has *vented* soffits, and two Olyvents. The fact that it’s “vented” is what is causing me the most confusion when I read all the articles on this site. Presumably, the current set-up allows for adequate airflow so that the roof sheathing can to dry to the exterior.
2) I can access/inspect the *interior* insulation only via light-fixture openings. There are no other access points. From what I can tell, there is a 3″ layer of fiberglass batt insulation laying against the ceiling. There is no insulation against the sheathing (see pic).
3) There appears to be a very thin (1/2″ thick) layer of foam board between the sheathing and the existing membrane. But other than that, there is no external insulation.
1) Most articles I read seem to suggest that adding 4″ of insulation atop the sheathing will block the existing airflow pathways and introduce moisture problems. Is that correct? Is there any way to avoid this that I can ask my roofer to implement?
2) For example, would using a material that breathes (like mineral wool batts) instead of polyiso above the sheathing provide an adequate route for moisture-laden air to reach the Olyvents and escape?
3) Given that there is already fiberglass insulation above the ceiling with only a couple more inches of free space above that, I don’t believe I can blow-in insulation between the rafters to create the insulation sandwich described on this site. Is that correct? In other words, there needs to be some place for air to flow, correct?
4) Given all this, am I better off leaving well enough alone, and just replacing the roof membrane without adding insulation? i.e., “if it aint broke, don’t fix it.” I hate to waste an opportunity to improve the thermal envelope of my home, but given that the structure seems to be functioning well as it is in terms of no apparent moisture problems, that is what I’m leaning toward doing in the absence of any clarity on the questions above.
Thanks very much for any feedback, clarity, or guidance!!
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