Are code-breaking BPI builders causing trouble for unvented roofs?

Andrew,
Here in the SW where most houses have “flat” roofs, the correct way is to install open cell under the decking AND rigid foam over the decking. If you are going to install closed cell under the roof sheathing, you better pray that your roof never leaks; you will have a very hard time finding where the leak is from bellow and I would not chance it with an old roof and probably poor water management detailing.
Another option you may have at this point is to install blown-in insulation, cellulose or fiberglass, but you need to install roof venting; this application is the most common here in the SW.

Martin, you will not pass inspection if you don't have roof venting. It can be on the roof or it can be in the parapets, but either way, it has to be there.

Thanks for the comments on vented/unvented. Additional thoughts on the unvented DPC solution in climate zone 4? Given the need to take down the ceiling for a proper air-impermeable solution (e.g., CC foam), a DPC solution will be around 4x cheaper. Is it just risky, or is it gambling?

I think you are under a mis-impression. Philly rowhouse roofs slope down from front to rear and it is only in the rear portion that the cavity is too small for crawling around and using barriers for air sealing. That rear section is dense-packed, but it is open to the front section.

There isn't much concern about condensation because the vast majority of these houses are quite air leaky and have very low indoor RH during the winter. The low RH and fairly mild winter conditions mean that you don't have to worry about interior moisture migration into the roof cavity. If you moved this same house to Montreal and cut the air leakage rate in half, then you should worry.

The bigger concerns with dense-packing the rear portion are things like roof leaks (which cause damage no matter) and knob and tube wiring (for which the circuits can be tested pre and post retrofit).

The weatherization programs in Philly have been using this approach for many years with a good track record. I wrote a guidebook on this approach back in 1993.

The cost for a foam-based approach is prohibitive- at least for weatherization-type programs that need to show a reasonable payback for the program measures.

Michael,

Thanks for the local perspective! I have a peculiar circumstance in that the ceiling is high in the front and low in the back, so despite the slant there are no points with a large cavity. There are a couple of holes in the ceiling that I can peek into to confirm this.

I'm doing this work together with substantial air sealing in the rest of the house, so air leakage should be cut more than in half.

Since I'm not in climate zone 2 or 3, code writers deemed winter conditions to be cold enough here to require air-impermeable insulation. Are you disagreeing with code in saying that climate zone 4 can get away with less air impermeability and still avoid condensation problems even in a optimally air-sealed house?

Thanks!
-A

Andrew-

You have an unusual rowhouse -- at least from my experiences having lived and worked in Philly rowhouses for 15 years. It would be challenging to dense pack such a large cavity and I would be concerned about the structural integrity of the ceiling.

But in terms of moisture build up and code, I think that code, by necessity, is based on a variety of assumptions and CYA requirements, If I were you I would look at your specific house. It isn't the climate zone as much as the combination of how cold the roof deck will get vs. how moist the air will be in your home. If you seal your home very well and don't provide much ventilation and end up with high RH in the winter, then I'd be a little more concerned. Of course, you could just install an exhaust only ventilation system and you'd pretty much remove the potential for moisture migration into the roof.

In addition, those old roofs are really built -- thick rough-sawn board sheathing with several layers of built-up roofing can add up to fair amount of R value above the inside surface of the deck. There's also a tremendous capacity for seasonal moisture storage in that deck -- it's much more forgiving than plywood. In my research, I tried to create moisture problems in Philly rowhouse roof decks and we couldn't get the moisture content much past 9%.(although the homes didn't have very high RH).

I'm still not sure what you should do with your 18 inch roof cavity -- dense pack doesn't seem viable to me.

Michael-
That's good reasoning, my house definitely has an old thick roof deck, plus maybe some extra roof decks on top of that.

What is your concern about DPC? I have all new wiring, so knob and tube isn't a problem. And I imagine roof leaks would be a problem no matter what. Is the concern that 18" is too much or too little space?

My concern is that 18 inches is too much cavity to fill with dense pack properly and that the resulting resulting weight may also be an issue for your ceiling.

Michael:

I like your Philly flat-roof cellulose approach, and it looks like you have good data to back it up. So I'll suspend my concern about the approach of local green builders.

As far as my house goes, I've had some contractors take a look and they haven't raised any concerns about using dense-packed cellulose. Still, I guess I'll just go with a spray foam solution, it sounds like there's a lot of different kinds of risks with the cellulose.

-Andrew

Andrew- Much credit to you for actually looking up the code! Let me first say i am a spray foam insulation business owner. You are very correct in your finding that DPC is not an air barrier so it is not allowed by code in your non-vented roofline situation.
There is a long history of failure due to DPC in these and similar situations. As a matter of fact i heard about these failures at a NESEA seminar in Boston a couple months ago given by one of the contributors right here in this thread! Even after the failures being noted and breezing over the code like it didn't exist the message was use cellulose but be careful. I could not believe my ears to be honest.
The code is written that was for a reason and you are the intelligent one for heeding the warning and avoiding an impending failure! Congrats and good luck.

I would ask what shape your roofing is in. If you will need a roof in the next 10 years[highly probably with BUR or torch down] you can do it in EPDM and insulate from the top as much as you like. Iso board is much cheaper than spray foam at around 50 cents a board foot. 4 inches 2 bucks a square foot

You will still need to do some air sealing no doubt.

Stuart-

You obviously misheard what was said in that NESEA session -- the message was definitely not "use cellulose but be careful". The message was that densepacking compact flat roof cavities in cold climates can cause a lot of trouble. The failures that Terry and I both talked about occurred in specific circumstances -- particularly in very cold climates and very wet sites. In addition, a distinction was made between fully dense-packing the entire assembly vs dense packing just a section. If you don't have a cold climate or a wet site, then there is a very long track record of successfully employing dense pack. Many thousands of roofs have been treated that way.in Philadelphia over the past 20 years and I have never heard of an instance of moisture migration from the interior creating any roofing problems. .

It's usually a good idea to try to follow code -- but due to a variety of factors (e.g., inertia, lack of data, desire to maintain simplicity), codes often end up with requirements that are unnecessary or are only really needed under specific sets of circumstances. Codes shouldn't be considered holy scriptures but guidelines that need to be continually challenged and re-assessed based on sound research and building science.

Michael,

What does Very Cold mean to you as relates to this recommendation?

Jesse-

Very cold means Minneapolis or Montreal -- where problems have been found in about 10% of the dense-packed roofs. Philadelphia is not very cold. Boston ...I'm not so sure.. a tight house or a wet site, I'd worry. When in doubt, don't take a risk. The Philadelphia approach is based on lots of research in that very housing stock and climate.

Michael,

Thanks again for your sage words. Any thoughts on alternatives I could look at aside from removing the ceiling and spray-foaming? I definitely don't want to take out the roof right now.

-Andrew