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ice/water on whole roof/ open cell spray foam under roof deck
Last Post 27 Sep 2012 05:48 PM by joeboy. 11 Replies. |
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joeb
New Member
Posts:14
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| 23 Sep 2012 08:11 AM |
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New
member here. Still learning he ropes of this site. Sorry if screwing up
the current path of discussion here. I've been reading a ton of
previous posts and thought I would reach out for advice.
I'm looking forward to any insight you can provide. I live in NW
Pennsylvania. We get dumped on with snow in the winter, frozen stiff in
January and February and then a month or so of real humid weather in
summer. I designed my own house and went for it. Its a bit Frank Loyd
Wright meets Asia. It has 6-12 pitch roofs and a couple flat roof/deck
sections.
Basement- It has a daylighted basement with 8" block that i added
extra rebar to and filled with cement. I sprayed 2lb. foam on the above
grade exterior of the basement and below grade sprayed rubber sealer
and covered with 2" foam board. My goal of insulating the exterior of
the basement rather than the interior was to have the floor and solid
cement basement walls act as extra thermal mass for the structure. I
plan on covering the exposed 2lb foam outside with a flexable parge of
some kind that would stick to the foam or maybe tapcon wire mesh on and
then parge as normal. ? The goal was to have the exposed basement look
like rock when I was all done. As you'll be able to tell I had some
theories and finally just went for it. :-) Anyways, the upper portions
of the home steps up in height by floor levels.
Upper levels- My tallest/3rd floor tower room is 14x14 with a walk
around treated cantelevered deck. It gives great veiws and a tower
affect for natural ventilation. I filled all my 2x6 walls with 1/2
lb. foam then the sheeting, tyvec, 1" foam board as a thermal break and
extra r-value and then fiber cement board siding with asics trim to
detail it out. My roof sheeting is treated 3/4 plywood with high temp
ice and water over all of it and then architectual shingles. My 4 foot
overhangs are all treated materials. I used no roof venting and sprayed
1/2 foam to the underside of the roof decks. Where i have flat
roofs/outside decks there is little space of course to the interior
drywall ceiling below. I used 2 x 12's. I'm planniing on putting 1"
foam board on top the flat sealed rubber roof and then my actual decking
materials. The pitched roof portions of the house have limited closed
attic space being 6-12 pitches and the 1/2 lb. foam is probably at least
8" or more thick and sprayed over the truss itself as well as thermal
break. Now, i did not spray a vapor sealer on any of the 1/2 lb foam.
It sounds by reading the posts on the building site that that is
needed. I have no way of getting to the flat roof sections other than
pulling the drywall ceiling down and then spraying it. Not going to do
that. My hope was the 1" foam board on top the rubber flat roof would
act as the moisture barrier plus it would act as skid sleeves so my
decking material does not cut into my rubber roof. My pitched roofs are
a different story as i cant put foam board on the exterior. It might
be possible to cut access holes and spray up in to it to seal it. Do i
really need to do this or will it be fine the way I have it? It would
be a ton of headache and mess I'm sure. I have noticed on some winter
days that I can see the rafter outlines. I'm wondering if that is just
because of the temperature sweeps of the day when there is just a skim
cote of snow sitting on the roof. I thought maybe I should also cut an
access hole in the pitched areas and blow in cellulose to increase
R-value as well. Is this needed and will it create extra moisture
issues up there?
So, my worries are roof rot or moisture trapped because I have high
temp Ice/water on the outside and 1/2lb only on the inside. Even though
the roof sheeting is 3/4 treated i still dont want any issues with it
of course. And should , could I blow in more cellulose for more
R-value.
I have radiant floor heat on all levels of the home as well. I am
installing an ERV whole home system in a month or so. I put 2 pvc
chases in my walls, before I foamed, on opposite sides of my home and
they go from basement to cieling of the highest floors. I will tap into
those torun the ERV. I planned on using them for future solar/wind
power cable chases as well if needed. Dana, I really appreciate any
advice you can give me on this crazy house I've built. Thanks!, Joeboy.... |
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cmkavala
Veteran Member
Posts:4327
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| 24 Sep 2012 09:40 AM |
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joeboy; where in NW Pa? |
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| Chris Kavala<br>[email protected]<br>1-877-321-SIPS<br /> |
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joeb
New Member
Posts:14
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| 24 Sep 2012 09:44 AM |
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Hey Chris. I'm located not too far from the city of Erie. |
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joeb
New Member
Posts:14
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| 25 Sep 2012 12:54 PM |
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Update on my research thus far. After everything I've read so far I think my plan of attack is to open up an access hole in the ceiling areas where I have pitched roofs. First off i hope it looks o.k. !!!!!!!!!!!!!!!!!! Ei., no moist foam, mold, etc. Next, i will can spray foam around any fart fans, can lights, etc that would be letting indoor humidity up into the space. After that i would like to blow cellulose into the space to further increase my R-value. My open cell foam has not been sealed to the heated space and is at least 10" thick so i think the roof deck saturation because of the high temp ice and water shield over the entire roof deck is not a problem because the deck is unable to get cold enough. Also if there was some moisture transfer it could dry inward and not be a problem because the open cell is not sealed to the inside and the small attic space is an air tight space (drywall ceiling below). I will then seal up the access holes I created. In addition, the ERV I'm installing will correct the pressure in the home to further help. |
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sharter
New Member
Posts:77
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| 25 Sep 2012 01:23 PM |
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Assuming you can stop the leaks around cans, etc. I suggest to re-paint the drywall ceilings with a high-quality vapor barrier paint. Look for the Perm rating - lower is better. A quick search turned up Zinsser BIN, ICI Ultra-Hide, and Columbia Vapor Shield Latex Barrier.
This will however slow down drying on the interior side through the drywall. You have to choose which way to go. You definately want some interior drying as it is not going to dry through the roof. I think slowing the rate of humidity into the attic through a vapor barrier paint would be beneficial. Although you no longer have the option (unless you add another layer of drywall), using poly on the ceiling would be too much of a vapor barrier IMHO and cause rot. |
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Dana1
Senior Member
Posts:6991
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| 25 Sep 2012 02:25 PM |
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The roof deck get's plenty cold enough. Interior conditioned-space air moisture at the low end of a "healthy" range for humans has a dew point of ~37F. The mean binned hourly outdoor temp in January for any NW-PA location is well below that. And if you don't control the interior humidity to ~30% RH @ 70F the dew point will be higher than that. In very tight houses the interior moisture of an occupied climbs to much higher levels than that unless you adjust the ventilation rates or mechanically dehumidify to keep it down. When there isn't a snow cover the overnight roof temps will be even lower than the outdoor ambient due to radiational cooling, and the north side that gets no wintertime sun never rises above the daytime ambient, so the roof deck on the north side will usually average a few degrees below the outdoor temperature average in winter. It's the north side that is most at risk to incurring moisture problems from interior moisture drives, but the rest is not immune.
Unless that ceiling is truly air tight (blower-door verified, not just eyeballed), putting vapor barrier paint on the ceiling under an unvented attic may make things worse, not better. And putting vb paints on foam has been shown not to work (as attractive as that approach may seem initially.)
Even if air-tight, going ever-lower perm at the ceiling wouldn't be better- the roof deck has a vapor-impermeable underlayment, giving the attic about a half-perm of drying path to the interior would be more resilient, and still prevent high moisture content in the roof deck from wintertime moisture drives.
A better approach than paint (albeit more expensive) would be to put 1-2" of closed cell foam sprayed directly on the underside of the open cell goods to form a lower permeance layer on the interior, bringing the total between interior & roof deck down to ~0.5-1 perms. That would give it an adequate drying path for the roof deck that isn't so vapor-open as to have high moisture build-up in from winter moisture drives. Adding 2" of cc foam would add another ~R12 to the stackup, bringing the whole-assembly R at least into the mid-40s if not quite R50.
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jonr
Senior Member
Posts:5341
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| 25 Sep 2012 02:41 PM |
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my worries are roof rot or moisture trapped Whatever you do, monitor the humidity and if it gets high, install a dehumidifier. |
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sharter
New Member
Posts:77
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| 25 Sep 2012 03:38 PM |
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Yes definately dehumidify when needed. If you have enough room in the sealed attic, consider installing automatic dehumdifier(s) just in that area with drainage tied into the plumbing system somehow. Sounds odd and I don't know anyone that has done this, but I think it is an alternative over keep the whole house dehumidified to an uncomfortable level (say 40-50% in house, 30% attic).
I know it is probably out of scope, but the best option IMO is to remove the shingles and ice\water shield, add 2x4 furring strips and another layer of plywood\icewater shield and then re-shingle. Exterior fascia may need to be extended. Then you can get the cold roof and exterior air flow and drying. Same process as for bullet-proof SIPs. |
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Dana1
Senior Member
Posts:6991
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| 25 Sep 2012 03:42 PM |
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BTW: If you put 2" of 2lb foam over the half pound foam, you could then put as much as R50 of cellulose at the ceiling without much moisture risk (not that going to an R90+ total at the roof would make a lot of economic sense.) |
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joeb
New Member
Posts:14
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| 27 Sep 2012 12:33 PM |
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Thanks to everyone for the information especially Dana (off line) for your detailed input. I now have a much better understanding of the variables and principles involved with wall and roof foam insulation and it's installation. I also contacted the leading foam companies where my residence is. According to them the high amount of open cell i have on the underside of the roof deck (at least 10" thick = R38) along with the entire roof covered in high temp ice and water is o.k. the way it is. Adding extra blown cellulose insulation to the ceiling would possibly trap moisture and the rate of return (cost vs gained r value) is not worth it. Again this is my area in NW Pennsylvania and each area would have its own parameters to follow. These are reputable companies with many years of operating experience locally who are saying the same thing. This is my current plan of attack. I will install the whole home ERV for ventilation and some humidity control, put all by bathroom fart fans on timers, and seal up the can lights or fart fans, etc. going into my conditioned attics spaces. The key of course is to pressurize and condition the house correctly with the ERV, if needed also install a whole house dehumidifier, and put 2 layers of 1" xps on top my exterior flat roofs that will also act as skid pads for the walkable floating decks there. The ultimate goal is to seal any leaks to the conditioned space/attics and keep the humidity in the home between 30-55%. I will continue to monitor humidity in the spaces as well throughout the seasons and see how things progress. Thanks again to all!...... |
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Dana1
Senior Member
Posts:6991
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| 27 Sep 2012 03:29 PM |
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Ten inches of open cell foam is too vapor-open to use without interior vapor retarders in a NW PA climate- you will have high seasonal moisture cycling in the roof deck, with a risk of rot over the long term. Yes, people (even "reputable companies") do it and get away with it, but it's not risk-free. Where you're most likely to run into trouble is on north facing pitches or fully shaded pitches that get little or no sun, especially if the roofing material is a lighter color. OSB roof decking would develop issues sooner than plywood, plywood sooner than solid planks, but they're all at-risk looking out 2-3 decades or more. Don't just take a foam vendor's word for it. WUFI (a well-vetted climate-specific moisture modeling tool) simulations tell a different story:
http://www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems
See table 3, p.12. Even drier warmer climate zones like 4A (Boston, Kansas City) would see undesirably high moisture levels in the roof deck with an all o.c. foam fill (the fourth insulation-option column from the left labeled "Full Depth ocSPF"). In NW PA you're in the cool edge of zone 5A, which is at much higher risk of wintertime moisture accumulation with an all o.c. foam fill- high enough that they didn't even bother to run the simulation for any zone 5 location.
Adding cellulose wouldn't trap more water in the attic, and would have zero effect on the moisture issues. It'll make the attic run cooler, but will have zero affect on the average temperature & moisture content of the susceptible roof deck. At R38 you're at code-min with the o.c. foam which is fine, but going higher-R with cheaper goods isn't always a bad idea. But R-value is the least of it.
Adding 2" of XPS to the top side of the roof deck is definitely moving in the right direction, since it raises the average min-winter temp of the roof deck, reducing moisture absorption.
A whole house dehumidifer is a waste of money in this situation- it just converts a latent load into a sensible load. ERV/HRV active ventilation purges unwanted humidity at low cost during the winter season, and air conditioning would handle it during the cooling season. It's OK to let the humidity rise to 55-60% during the shoulder seasons or summer, since the average temp at the roof deck would be above the dew point of the interior air and drying out, not absorbing moisture. The risky period is during the mid-winter, when controlling moisture with ventilation rates is dead-cheap, when you want to keep the interior RH under 35% (to protect the roof deck and sheathing), but above 25% (for comfort and human health.) Keeping it at 55% in winter would potentially put HUGE amounts of moisture in the roof deck, but from April-November it's not such a big deal. The dew point of 55% RH 70F air is 53F. When the average daily outdoor temp drops below 53F it's time to peel back on indoor humidity. But that's easy to adjust via ventilation rates, since the ventilation air dew point is reliably below the outdoor temp. (The outdoor dew point only approaches the outdoor temp in an thick pea-soup fog, which isn't the standard all-day weather anywhere in NW PA.) |
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joeb
New Member
Posts:14
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| 27 Sep 2012 05:48 PM |
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Dana, I have read the info and graphs at the link you sent. I get that the outside temps affects the dew point and Mid Winter on the North facing roof would be the worst case scenario. If my house is between 25-35% humidity during that time then I should be o.k. ? I believe it is around those levels during that time.
Thanks, Joeboy.... |
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