It seems like there are a lot of somewhat differing opinions about the best way to insulate and/or vent my attic spaces. I have a 1910 2.5 story house with three separate attic spaces behind knee walls. There is already blown cellulose insulation in the atticc floors and old roll (not pink) insulation behind the knee walls. There is nothing on the underside of the roof however, so the temperatures in there have been well over 130 degrees many days this summer. The problem is that our new HVAC will not cool the third floor to <~90, and I think that it is because of the heat flux between the knee walls.
Some more info:
- AC is zoned, and even if only the 3rd floor is cooling, there is still no improvement
- All hip rooves – no gables
- HVAC uses attics for (insulated) duct runs feeding the 2nd and 3rd floor
- You can see daylight from inside the attics through 1/2″ cracks in the eaves. This was the only “vent” in them until I cut several 6 x 12 openings into the eaves last weekend (with the intent of installing an exhaust fan in the eave). I decided that was too dangerous – fire hazard – and stopped after the first attic space.
- The roof ridge running the length of the house (above the 3rd floor ceiling) if vented.
I am considering insulating the underside of the roof and/or installing 3 roof-mounted attic fans. Can someone please offer some advice?
Thanks, Lew
Replies
You have the floor of the 3rd floor space insulated. That puts your third floor space outside the heated envelope of the house...
Don't know what good the kneewall insulation is doing you right now. With no insulation above the room you may as well not bother trying to heat or cool the space.
If you want useable space you need to insulate the roof. There are a number of different options which everyone will argue about. As it happens I just tore out the plaster and laths in my "3rd" floor space in preparation to inslating the place this fall. My house was also built circa 1910. Previous owners had already insulated the floor area with cellulose.
You could vent from soffit to peak of your cathedral ceiling and put vent holders in to make sure that you had air flow. Then put batts or whatever and then vapour barrier etc... Since you didn't put any profile information in I have no idea what kind of climate you are in. Everything I have to say applies mainly to a heating climate.
Another option you have that is generally acceptable is to spray polyurethane directly on the roof deck from peak to the bottom of the roof deck and if I were doing it I would also spray the "floor" of the soffit area right back to the framing for the kneewall and seal up the edge of the floor from that soffit area. This brings the whole kneewall area into the building envelope. Note, that you can often get inspectors to go for this but it is not a sure thing.
I'm doing mine with cellulose and building down the ceiling 9" to allow the extra room for the large amount of insulation I want (R60, owen sound, Canada, 7500 heating Fahrenheit degree days).
In any case the main thing you need to do is pick a spot for your building envelope and make sure that you have a complete air seal between the outside and the inside. Then insulate. That also means you don't have useable space on the outside and you won't have trouble heating the inside.
If you air seal and put good insulation up then you won't have problems with too much heat. The insulation will stop it from coming inside. The air sealing and humidity control during the winter should be your main defences against condensation on the bottom of your roof deck, not the venting.
I guess that my info got blown away with the recent web upgrade. I'm in Louisville, KY where it's hot & humid in the summer & semi-bitter cold in the winter.
I don't think that the attic floor insulation goes much past where the attic space ends tna dht elivable space begins (see drawing.)
I'll probably go ahead and hang the insulation on the rafters and be done with it. If I can only get a few years of life from an attic fan, it's not worth it. Hard to believe that they can't make something a bit tougher...
From your sketch, it would seem like your number one heat source is the heated roof deck. The roof deck past the knee walls just radiates heat into the 'fractured' attics--but where it's over the 3rd floor, it's heating the insulation in the cathedral ceiling. That radiation goes through the insulation eventually. It's also heating the rafters directly, which is likely heating the ceiling wall borad directly, too.
You have a complicated insulation solution. Foaming the roof deck might help, by helping to control how hot that overall roof mass gets.
Hmm, any way you could separate the cathedral ceiling from the roof deck? By spanning separate ceiling joists under/between the existing rafters, perhaps? I'm thinking (writing) out loud here, trying to decouple your ceiling from that hot roof deck. May not be very feasible.
Given the complexity ofyour insualtion needs, and the price of re-engineering parts of your house, what you may actually need is a separate, micro-size a/c unit for just the thrid floor. Sounds wacky, don't it? But say you can find a system that installs for $12-1300--that's a lot cheaper than new roof decks, more insulation, kluddged together attic fans nad the like.
Is it an ideal answer? No, just a possible one. Why did that occur to me? Because there's about zero chance I'm thinking that this room is ever cold in the winter. Ok, half the year done, all you need is summer cooling . . . bing! But, that's my sort of twisted thought process, too--others differ.Occupational hazard of my occupation not being around (sorry Bubba)
Yeah - I puzzled about this for a long time myself. Before we had HVAC installed, we cooled (very effectively) the 1st & 2nd floors with two window units - 1 and 1.5 tons respectively. The temps did start to creep up occasionally until I opened the windows on the third floor to let a breeze come through. This maintained the 3rd floor temp at the outside temp instead of that plus 10 or 20 degrees. After we did that there was almost no heat gain to the house.
Sooo, when the HVAC contractor suggested 6+ tons to do the whole house I said no way. Now our 4-ton system cools the 1st two floors reasonably well, with the third floor thermostat turned off. Unfortunately, we've had nearly 50 days with 90+ temps outside, which is unusual for KY. I do not want to put in a separate system for the third floor given that we just spent $14K putting this one in (including ducts & auto dampers).
Also, with the foil/foam system, I'm wondering if my roof life will be sacrificed. Seems like the roofer told us that we needed to vent the roof (they added some, but only to the portion above the 3rd floor ceiling) to improve the life of the shingles.
To further complicate things, there was an article in the paper from the Indiana natural gas authority announcing that gas prices may be 3x last years rates. So NOW I'm wondering if I should even install the foil shield until Spring (it will be nice to have that radiant gain over the winter...). Wonder if I should just wrap my house in aluminum foil come April?
I'm wondering if my roof life will be sacrificed. Seems like the roofer told us that we needed to vent the roof
That's one of those "facts" about roofing that confuses me. My roof and roof deck--it's a reasonably homogenous mass will run up into the 135º-145º range in the summer, swinging down to only the 80's at night. That's with the bottom of the deck & the rafters all exposed. So, adding insulation, entraining air that helps resist heating, that's supposed to make the roof deck and/or shingles "too hot" to warrantee?
Oh, and Mongo's code stipulation, I'm thinking, is that some exposed foam has to be covered in GWB in some jurisdictions. But that's supposition on my part.
Occupational hazard of my occupation not being around (sorry Bubba)
Actually, foam should be covered in all applications/all jurisdictions, whether code or not. And I have to believe it's code just about everywhere by now. The foam is just far too flammable to leave exposed. (Remember that NJ nightclub fire several years back where about 200 folks died? Foam was the primary problem there.)
Foam was the primary problem there
I remember that too, but, IIRC, it was the upholstery foam that was the culprit. I know the UK fire types are working for (or have achieved) a ban on the standard foam used in furniture for being nearly an accelerant and also giving of toxic fumes under combustion.
The two-part insulation foam, especially the closed-cell, I want to remember as having a relatively high flame tolerance. I can't grab a reference this very second (of course)--just a rememory right now.Occupational hazard of my occupation not being around (sorry Bubba)
My impression was that it was "styrofoam", installed as a backdrop on the stage.
it was "styrofoam", installed as a backdrop on the stage
Could be that, too--there's a lot of expanded foam used in stage work.
It's been a while since I saw the fire fighting technology "lookee here!" stuff, too. Just too much stuff to remember some days. <sigh>
Edit to add, the Iso board may also be what I'm remembering as being more resistant to open flame (or perhaps it's direct heat).
Occupational hazard of my occupation not being around (sorry Bubba)
Edited 9/15/2005 9:25 am ET by CapnMac
"In June 2000, club owners Jeffrey and Michael Derderian bought $575 worth of packing foam -- the kind typically used to pack gifts and line musical instrument cases, said Aram DerManouelian, president of American Foam Corp.
A similar amount of flame-resistant foam would have cost about $1,200, he said.
Flame-resistant foam would not have prevented the fire, but it might have slowed its spread, he said. The fire ran through the wood-framed former Italian restaurant within minutes after on-stage pyrotechnics ignited the sound-proofing foam. "
From the videos I watched, this foam was a foam rubber "egg crate" style, like you might put on top of a hard mattress. Totally different creature from the insulation foams.Rebuilding my home in Cypress, CA
Also a CRX fanatic!
this foam was a foam rubber
Yeah, that's the stuff, upholstry foam--it's now the bane of fire safety types as it is both very common and very flammable.
Furniture biz has been quietly changing over to different chemistry foam (that's more expensive and less comfy)--but there's a lot of the stuff out there.Occupational hazard of my occupation not being around (sorry Bubba)
Re roofing lifetime, I suspect that the main problems are not related to overall roof temperature, but rather temperature differentials. It's especially bad for it to be a lot warmer in the attic than on the surface of the roof, as this can cause the rafters to expand (lengthen) without corresponding expansion of the sheathing and roofing materials. The result is cracking along the sheathing joints.This is actually more apt to occur in the winter than in the summer.
Different foams have different smoke and fire spread ratings.There is at least one brand of polyiso foam with the foil facing that is approved for exposed use in unocuplied spaces; ie unfinished attics and crawl spaces.
Keep in mind that, in general, there's no one "right" way to do this (though certainly plenty of clearly "wrong" ways). Lots of tradeoffs.
One tradeoff with regard to the roof-mounted fans is that they have a limited lifetime of maybe 3-5 years. At that point the bearings go out and the motors must be replaced. For this reason, passive venting is often better.
Even with fans you need adequate air intake along the eaves, ideally spaced evenly vents so that the entire attic receives a regular supply of fresh air.
With the temp swings up in the attic, it's apparent that the radiant gain due to solar hitting the roof and then beign radiated into your attic space is overwhenling your HVAC system.
You need to minimize the radiant gain in the roof plane.
You need a barrier to stop air infiltration. Stopping air movement will increase the effective R-value of what insulation you use, as well as stop moisture vapor from going from your living space and into the roof framing.
The downside? Right now moisture vapor is most likely being carried up into the attic, then exiting out through the leaky structure., Were you to seal the lid without taking into account the moisture generated within the house, then you could end up with moisture problems within your sealed attic space. Not good.
Consider moisture sources withing the house and make sure they are accounted for as best you can at the source. Basement, baths and kitchen. Use canned foam or fire caulk as required to seal holes between floor platforms...holes for wiring, plumbing, etc. You want to minimize and discourage air movement throughout the house. Your HVAC system should be the one doing that.
Trying to seal a roof (floor.knee/roof) as shown in your drawing is notoriously difficult. The transitions from floor to kneewall to rafters are often ineffective in new construction, nevermind trying to do it in a remodel.
Easier, more effective, and kinder to your HVAC ducting and equipment would be to bring the entire attic space, including behind the kneewalls, into the house's thermal envelope.
Foam, whether it be foam sprayed into the rafter bays, or a foil-faced polyiso installed on the underside of the rafters (with the sheets gapped during installation and then the gaps foamed with canned foam after installation) would provide a radiant barrier and an air/vapor barrier and solve your problems. The former meets accepted building code, but there are installation issues to contend with in order for that foam to survive long-term and to not shrink away from the framing. The latter may not neccessarily conform to code, but when done properly it will not and cannot be compromised in terms of the barrier.
Personally I prefer the latter, and I did the same detail (2" sheets of foil-faced polyiso on the rafter faces) in my own house, and did it with the approval of the building department. Worked like a champ. Summer attic temp went from 127 before to 77 degrees the day after the polyiso was installed. My attic is now finished living space, but not heated, and the winter temps (in CT) up there are about 65-67 degrees unoccupied. Once the kids get up there (body heat plus a few lights turned on) the temp increases to the low to mid-70s. Summertime, the attic temp will be in the low to mid-70s or so with no air conditioning running, but I did pull a couple of runs off the HVAC system to recirculate air up there in the summer.
I'm a believer in radiant barriers/foam in the attic, You just need to decide how you want to go about getting it up there, and if you're capable if doing the job correctly...or having someone do it for you...and ensuring that they do it correctly.
I recommend you not do exhaust fans. When you understand the theory behind them, they just won't do what you're trying to get them to do.
My opinion.
Thanks Mongo. That is a great testimonial. I have looked at the 4x8 sheets at the big box, but they ain't cheap. With a 50 degree temperature difference it sounds like theyre worth it though - and it will save me from having to rent scaffolding or a scissor lift to instell vents in the remaining eaves.
Some details - should they be mounted between the rafters or below them? If they are to be ripped to width, should they be mounted flush to the underside of the roof or should I leave an air channel? I re-read your post and you suggest mounting to the underside of the rafters, which would leave a channel. What's the best way to attach the foam to the wood? Pneumatic stapler? Roofing nails?
If I'm to do this the right way, I'll probably rip the rough-looking drywall & insulation off of the knee walls and run the foam the full length of the roof. I will be tempted to use something thinner then 2". Care to talk me out of it?
You could use polyiso thinner than 2". You can also use a different rigid foam board insulation (RFBI) altogether.
Lew, here's the problem. I did it in my house several years ago. Now, my house is a simple colonial style, so it has a soffit to ridge vent, thus I have a vented roof. When I built I wans;t terribly familiar with celulose, so my house had FG batts in the rafter bays. Totally ineffective at stopping radiant.
An alternative for you would be to use cellulose in the rafter bays.
With your house having nothing but hips, you have no sffit-to-ridge venting.
ACCORDING TO CODE (based on the latest, which finally allows hot roofs, you need the foam directly against the underside of the sheathing. While there are reasons for that, there are reasons to go against it as well. Here's the kicker...after my inspector saw what I did, he did the same thing in his house. Go figure...
So, proceed at your own risk, or do more research and see if this fits your house.
If you want to stuff the bays full of foam you can hire out or rent a spray setup and have at it.
If that won't work, you can rip sheets of foam into slightly less than rafter bay width and put them up against the underside of the sheathing, then use canned foam to foam the gap between the sides of the inserted foam panels and the rafters. When the foam cures it'll lock the foam pieces in place.
What I did?
I installed 2" thick foil-faced polyiso to the bottom faces of the rafters. I gapped the sheets about 1/4" to 3/8" during install. Each was held up with just three or four button nails (nails inserted through circular disks, the disks provide holding power to prevent the nail from tearing through the foam) nailed through the foam and into the rafters.
The foam sheets do not have to break over rafters. They can break mid-span.
I then used a sharpie marker and peeked between the gaps to see the rafters, and made a tick mark on the foil face of the polyiso showing the location of the rafters behind the foam.
I did the same on the gable-end walls, but only used 1" foil-faced polyiso foam.
Then use canned foam to foam the gaps between the sheets. This'll seal them tight. I then used Al tape to cover the foamed gaps.
I then ripped scraps (as well as a new sheet or two) of leftover 3/4" CDX into 2" furring strips. Use ready made furring strips should you choose.
I ran the furring strips horizontally across the polyiso on 16" centers. SCREW these with long enough screws...through the furring strip, through the foam, and iinto the rafters...using the tick marks t ensure you hit the rafters. Again, just like the sheets of foam, the furring strips do not have to break over a rafter. They can break mid-span.
Drywall gets screwed to the furring strips with standard-length screws.
What you don't want is any penetrations through your foam. No can lights, no cutouts for junction boxes, etc.
To break up the ceiling plane in my attic (the room is one open space, about 46' by 18' with 5' kneewalls and a ridge height of about 11') I ran a faux ridge beam as well as a few faux beams from the kneewall to the ridge. Wiring is run in these U-shaped beams, and the boxes for the pendent ceiling lights are buried in the faux ridge beam.
In detailing where the kneewall meets the rafters, in each rafter bay I slid a piece of foam between the rafters to sit between the rafters and on top of the cap plate of the kneewall. Thus the bottom of that slid in piece mates with the tops of the pieces that are mounted on the rafter faces on either side of the knee, giving a stepped, but continuous, run of 2" thick polyiso from one end of the rafter to the other.
Now, will my foil face eventually become dirty and be less of a radiant barrier? I'm sure it will, but the polyiso will block the radiant gain nonetheless.
Added benfit. Killing the radiant gain will cool the attic. A cooler attic will be kinder to your HVAC equipment up in the attic.
A cooler attic will reduce the heat load on the second floor of your house. More comfort, and again, a lesser load on your HVAC system.
Sealing the attic will minimize air infiltration throughout your house. Less air infiltration wil effectively increase the R-value of the existing insulation in your house.
Yadda, yadda, yadda.
Some will naysay the process, but it worked for me.
So, if I understand this right, you only put up a 2" RFBI (about R10) under the rafters and then sheetrock over this and you got an acceptable reduction in heating / cooling loads? I was planning on doing the same thing except also putting dense pack cellulose between the rafters, which now I'm wondering if it isn't overkill.
Steve in NC.
When I finished off my attic space there were already unfaced FG batts friction-fit between the rafters, so they are still there as well as the 2"of polyiso on the bottom face of the rafters.
If I didn't already have the FG in the bays, I would have densepacked cells in the rafter bays behind the polyiso, but only AFTER strapping the foam. Thge furring strips will help the sheets of RFBI resist blowout and deflection.
The HVAC load comparisons before and after were somewhat interesting (I'm in CT). While I forget the specifics, finishing off the attic added about another 850 sqft to the living space of our house.
Once the attic was finished, however, the fuel usage to heat the house (including the additional attic space) decreased. I'll credit that to the tighter lid on the house and the elimination of air infiltration up and out through the previously unfinished attic.
For what it's worth, my roof also holds snow in the winter longer than any of the neighbors, and I have a dark laminated shingle roof facing SSW.
Realize that I don't have any supplemental heat up in the attic. There is an open door from a second floor hallway up to the attic, so the attic simply gets residual heat from the house as well as heat from the incandescent bulbs and bodies up in the attic. It's quite comfortable up there.
As far as the cooling loads, One of the reasons for finishing off the attic was to bring it into the thermal envelope of the house, as the soon-to-be-installed air handler and ducting for the second floor and attic air conditioning was going to be placed behind the attic kneewalls. I wanted the entire HVAC system within the envelope.
Doing the polyiso created an immediate difference in the summer comfort levels in the house, especially on the second floor. Sleeping on the second floor was somewhat miserable due to the hot attic radiating residual heat into the upstairs bedrooms via the bedroom ceilings/attic floor. And that was with FG batts in the rafter bays and FG batts in the attic floor platform (second floor ceiling). But again, radiant passes right through FG.
Just by putting the polyiso on the attic rafters, I noted a 50-degree drop in attic temp from the day of installation to the day after, and sleeping conditions in the upstairs bedrooms improved equally well.
So again, while I didn't have air conditioning prior to the polyiso going up, I do know that the polyiso reduced the radiant gain in the attic and subsequently on the second floor, so it would have had an impact on cooling loads as well.
I hope all that makes sense!
Mongo -
Dug up this old thread since I'm trying to figure out what to do with an attic that sounds similar to yours:
I'm also in CT, and have a 1920's colonial with a walk-up attic. It has 2x6 rafters currently filled with kraft-paper-faced R19 FG batts (edit: unvented). Winter isn't that bad up there (there is one original cast-iron radiator, and snow stays on the roof as long as any of the neighbors' houses). But it's h*ll in the summer, so I've been wondering about replacing the FG with spray-in foam or rigid insulation, and started reading the varying opinions about radiant barriers...
...but then I found this thread, and I'm wondering if based on your experience here in CT, I ought to leave the FG batts and just add 2" of foil-faced polyiso beneath the rafters.
In your case, did the polyiso end up tight to the FG, or did you have (even a small) air space between the two? Any reason to think that my faced batts will cause a different outcome than your unfaced batts?
Edited 4/17/2008 11:51 pm ET by steeeve
Yes, the FG batts were flush to the bottom edge of the rafters, so the polyiso was tight to the FG.As far as faced versus unfaced batts, the only thing I'd do prior to hanging your polyiso would be to take a utility knife to the kraft facing. Slice and dice it a bit so that the foil face on the polyiso will be the only true barrier in the roof.Mongo
Thanks for the info, Mongo - I hope to try to get to installing it over the next few months.
Hope it goes smoothly for you.
I used 2" on the ceiling and 1" on the gable end walls. I wish I had taken more pictures of this project, the one below is the only one set I have.
You can see the nail caps I used, maybe 3 or 4 per sheet of polyiso. The next pic shows the false beams going up, they hide the electrical and help break up the plane of the ceiling. The last pic shows the beams and woodwork painted, although the bookcase surrounding the stairway is still only primed.
The kneewalls are about 5' tall, the doorways provide access to storage behind them. The cathedral ridge is about 11' up, the footprint of the room is about 18' or 19' wide by by 45' long.
It was a project easy on the pocketbook. I hung the sheetrock behind the kneewalls, I subbed out the hanging and taping of the rest of the rock. Other than the sheetrock I did the rest of the labor myself. I think I spent a total of $7000 to $8000 on everything, roughly $8 to $9 a square foot.
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hello again, you've helped me alot in the past with direct replies along with your many other posts... I guess after seeing your pics I had a question.... I just wanna be sure... your polyiso on the slanted wall/ceiling is attached directly to the face of the rafters correct?? and if so, was the next step just strapping over that, followed by sheetrock?? finally, there was no other vapor barrier needed over the polyiso correct??thanks again in advance for all your help....
Todd, here's what I did:Attached 2" thick foil-faced polyiso to the faces of the rafters. Used 3 or 4 nails with 3" button caps per each 4' by 8' sheet to hold each sheet in place. I gapped each sheet about 3/8" from it's neighbor. The sheets can break over a rafter bay, the seams don't need to break over a rafter.With all the polyiso hung, I then took a sharpie marker and by peeking through the gaps between the sheets of polyiso, I located and marked the centers of the rafter edges on the foil face of the polyiso with a little tick mark.I then used canned foam to seal the gaps between the sheets.I then used aluminum tape to tape the seams.The foil is now my vapor barrier, the only one in the roof assembly.I then ripped 2-3" wide strips of 3/4" cdx ply and installed those horizontally, across the ceiling, 16" on center. I used the previously made tick marks on the face of the foil to locate the rafters, and used long screws to screw through the CDX strips, through the 2" polyiso, and into the edges of the rafters. The furring strips don't have to break over a rafter either. I'm pretty certain I used a PL adhesive to stick the CDX rips to the polyiso as well.You can see I attached faux beams to the ceiling. Where they went I'd make "blocking" out of a larger piece of 3/4" cdx ply, say a 12" by 24" piece, and place that on the polyiso and screw that into the rafters as well. The beams then got screwed into that.Drywall got screwed to the 3/4" strips of CDX with regular length drywall screws.Trying to screw drywall directly over foam can be tedious, the initial compression of the foam with later bounce-back can result in a lot of popped drywall.
thanks alot, always great information....
Can you post a picture of how the doors to the kneewall storage look? I'm redoing an attic, want to have storage in the eaves as you do, but I don't know a nice way to have access to the eaves.
Foam, whether it be foam sprayed into the rafter bays, or a foil-faced polyiso installed on the underside of the rafters (with the sheets gapped during installation and then the gaps foamed with canned foam after installation) would provide a radiant barrier and an air/vapor barrier and solve your problems. The former meets accepted building code, but there are installation issues to contend with in order for that foam to survive long-term and to not shrink away from the framing.
Mongo, can you explain what you mean by installation issues? Does SPF have long term problems? I was thinking of insulating the underside of my roof with Demilec but I'm concerned over how it will hold up over time.
Thanks.
It's more of an installer thang.
Like compacting earth.
When going for depth, it's often better to spray foam in several thinner applications than to try to go full-depth in one shot.
It results in better long-term conformity and adhesion to the framing. The last thing you want is foam pulling away or shrinking away from the framing. Once you get gaps between the foam and framing and lose the air infiltration barrier, your assembly can be considered toast.
It can just create the possibility for unwanted problems.
Hmm. It is for this reason, which I was starting to suspect, that I'm becoming unsure of foaming my own attic. I assume sooner or later it will develope cracks and it might be nice to have venting for just that reason. So I'm thinking of foaming some hard to reach areas under a vent baffle, instead of doing an unvented attic assembly.Thanks for the info. Have you seen any studies on this?
No, I haven't seen any formal studies. But I've seen failed foam installations in trade publications, we've had a few posted here over the years as well.
Now, foam is a great thing, so those few cases shouldn't scare you off. Like everything else, though, it's the details that count. Especially in something like a roof assembly that's going to be closed up and you hope you never ever have to see it again.
If you do have to do a hot roof, RFBI on the underside of the rafters and cellulose blown into the rafter bays is a very nice setup.
No air infiltration.
No radiant gain.
An excellent sound attenuator.
A very effective R-value.
And if there is any wetting within the bays, for whatever bizarre reason, the cells can absorb and hold the moisture and then release it in a drying cycle, with no damage to the framing. They'll do a much better job than FG (batts or blown) where the water could flow through the FG and puddle, possibly damaging the framing, the drywall, or setting up localized mold colonies.
All that said, I'm sure a reputable foam installer would do a fine installation for you, especially if you let them know your concerns up front.
I don't think you mean to sound as extreme as you came across. If the foam pulls away from the framing in a few spots in a typical roof assembly, the problems are minimal if at all. A comp roof is fairly air tight on it's own.