Talk us “in” or “out” of our Catherdral Ceiling Rehab Plan?
Aloha,
Been doing a lot of reading on this wonderful site. Our Location: Upcountry Maui – 3000′ elevation where we have high humidity but cool climate. We hit dew point most nights of the year.
Situation: a 52′ square house (think ‘box’) that has 4 cathedral ceilings meeting in a single point at the geometric center of the square. Walls are 11′ high and roof pitch is 4/12. The current layers of the roof/ceiling are: corrugated steel roof, black roofing felt, 1/2 in plywood sheathing attached to 2×10 rafters, fiberglass batting (R-19), 1/2 in gypsum board, spayed on texture, paint. The 4 glu-lam hip beams (4×12) are exposed to the interior of the home. There are no ridge vents along the 4 hips but there are 2 ea 2″ ‘mini-louvers’ in the eave of each rafter bay. Rafters are 2×10 spaced 24″ oc. Ventilation year round is via opening windows. There is no central heat or cooling and in the winter, yes, we do get winter! but NO snow :-)) , we use polartec and a wood stove in the mornings when it is a chilly 56 degrees inside.
We have been noticing areas of mold growth on some of the exposed portions of the ridge beams and some texture bubbling where the gypsum board contacts the beams. The house does not feel ‘damp’ nor uncomfortable nor ‘sick’. The amount of visual mold is small and we are primarily concerned with the detrimental effect on the house structure. We’ve taken out some of the drywall ceiling in a storage space and see that there is some mold on the rafters and on the paper backing of the insulation as well as the inside of the roof decking. Two roofers have said that they do not think that our roof is leaking – one even refused to quote on a new roof saying “it wouldn’t be right”! After reading a lot on this site and others, we believe that our problem is most likely condensation on the underside of the roof sheathing – the moisture most probably coming in from both the eave vents (we know that some spay washing prior to painting exterior made it past the vents) and from the living space (essentially 1 very large room).
Installing ridge vents would be problematic due to the width of the glu-lam beams and because all rafter bays do not die into a hip beam: some die into beams that are horizontal and serve to tie the 4 hip glu-lams together. Thus, the center bays of each of the 4 roof/ceiling planes dead end about 2/3 of the way to the peak of the house.
We are considering the following: remove the drywall ceiling (about 3000 sf!!) and have the mold mitigated professionally; have the roofing guys come back to replace/tighten the 10-year old screws and check roof panel sealing (roof tune-up); then use ccSPF to insulate the underside of the roof sheathing; put up new ceiling. We assume that the SPF will block the eave vents and will ‘seal’ the entire roof structure essentially bringing the newly insulated underside of the roof sheathing into our living space.
The article “How to Build an Insulated Cathedral Ceiling” states:
“If you are building a cathedral ceiling, the biggest air-barrier blunder is to install tongue-and-groove boards as your finish ceiling without first installing taped gypsum drywall. A board ceiling is notoriously leaky, and this type of ceiling is often associated with roof sheathing rot.”
Is this statement true for internally sealed and insulated roofs as described in the article and above? Since the inter-rafter space is now ‘interior’ space, is air movement into this area bad? It seems that if one puts up gypsum board this is isolating the space and any moisture that does make its way into the space will have trouble finding a place to go.
We are considering tongue-and-groove for the new ceilings and if putting up gypsum board first is extraneous or even !!detrimental!! – we’d like to skip this expense. But if it will help the condensation problem from re-occurring, we’ll go for it.
So, my 2 questions:
Does our rehab/insulation plan $eem $ound? I$ it likely to fix our problem?
What $hould the ‘below-the-rafter$” layer$ be in this in$tallation?
Mahalo for your kokua (thank$) for your help!
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Replies
Aloha,
Charlie, get your building permit people to help you. And local builders and spray contractors.
There are firecodes etc....
From the sound of it, your problem is likely condensation on the underside of the roof sheathing, and if windows are open most days of the year, this doesn't sound like an issue involving interior humidity, as is typical in colder climates, so the usual "air-seal the lid and improve the ventilation" approach won't help. I'd say the spray foam approach would solve it... but, I'd look for other examples of the same issue around you, and find out what solutions are used. It seems like this should be a fairly common problem, unless you're the only person on the island with a corrugated metal vented roof. Like AJ says, science-savvy local builders, insulators, energy auditors, home inspectors, and building officials ought to have seen it before and know what to recommend.
And yeah, the wood ceiling + foam with no drywall, that's probably not going to fly.
Ditto the above...my additional comment is that this maybe one of those cases where an interior poly vapor barrier in combo with good air sealing may be in order.
Bruce, this seems like a situation where outdoor air is humid enough to condense on the underside of the roof decking, possibly aided by night sky radiation cooling the roof surface... so the fact that the roof is vented is problematic, and air-sealing and/or vapor barrier to control migration of indoor humidity won't be enough. Foaming the roof would mean that the underside of the foam--inside the house, essentially--would have to hit the dewpoint, and it seems likely to be warmer if it's inside.
I'd still like to know more about what's common in this location.
Charlie,
I think your plan will work.
Once you have installed closed-cell spray polyurethane foam, you are good to go with a ceiling of tongue-and-groove boards if that is what you want.
Thank you for your thoughtful comments. Some additional information: we've opened up a couple more bays by pulling off the drywall ceiling and are seeing mold primarily at the top of each bay. Areas lower down seem fairly clean. The protruding roofing screws in the areas having mold are rusty at their tips buy still somewhat shiny where they start to protrude through the roofing deck. So far, everything is consistent with water condensation on the underside of the roofing deck.
We've done some additional reading on ccSPF and based on the experiences of some people with chemical sensitivity to some component(s) of this product we've decided to abandon this approach and explore other options (well, actually I almost killed my wife several years ago by using TV tuner spray in the same room - she started having breathing problems...and now she says, "No way! to ccSPF").
Everyone we've had out to look at the situation (2 roofers, 2 mold mitigators, the original builder, and an additional GC) together with your comments, all seem to agree that our problem is water condensation on the underside of the roof deck and that ccSPF would solve the problem. Instead of ccSPF, could we use rigid foam fitted and glued to the underside of the roof deck and sealed to the rafters with canned spay foam. And, of course, we would seal up the mini-louvers in the eaves along with adding rigid foam to insulate the lower end of each bay.
So far, everyone we've had in looks at it and scratches their heads and say that they have not encountered this situation before. We've got a call out to an engineering firm to help us develop a plan to remedy our problem. We really appreciate the education and guidance we are receiving from you all. Again, thank you!
Rigid foam will work in theory, but you need to be really carefully to get it tight to the bottom of the roof deck, everywhere. Those protruding screw tips will hinder you, as will any stray wood fibers sticking out around them, etc. Electrical or other stuff in the framing cavities really slows things down. You'll need a huge stack of rigid board and many cases of one-part canned foam, and someone really methodical and diligent to do the work. If you hire a contractor who sends over laborers to do the work... forget it.
AJ, that's complete nonsense. Outside air moving under the roof deck is likely part of the problem here. Fiberglass for what?? Cupola???
Better than trying to cut/fit/foam the rigid in from below might be to remove the metal, install foam and furring over the deck, then re-install the metal.
Hi Charlie,
How about posting some photos
Can foam is the same as spray foam. Sensitive to one is sensitive to the other.
Build a venting cupola. Then build vented roof with vent area, fiberglass batt insulation, Membrain next, then T&G.
David, the inside is being removed to get at mold. So close cell foam is best. Second best is rigid foam on the outside like you mention. But two problems, budget and possible sensitivity to the closed cell.
So my idea is go with basic building but this time no drywall to mold and better ridge venting.
In the end, an onsite builder with the right experience is needed.
Use ANY spray foam and there's always the risk of a problem with the job, even if a thousand others have gone well. I tend to feel the one-part canned stuff is not prone to surprises but have no science to prove it.
Charlie, closed cell foam would be great if you used it. You could visit homes that have it with your wife to see how she reacts to it. Also David's exterior idea would do you well with no sensitivity issues though costs for each idea need to be looked into.
David, Northeast Sprayfoam that I use is a successful company doing very good work for people like me, and thousands of others. They told me for an unrelated reason they removed one job. Rigid foam, fiberglass batts, cellulose, all have issues with mold as often as all kinds of other problems happen in building homes.
Sounds like you are in the hater of spray foam contingent. I am not. Done poorly anything we build can fail. That said I prefer to move toward Riversong, Chandler methods....
Don't be a hater....
Aloha All,
Here are some pictures of the house. The first two show the initial framing. What you might note is the horizontal beam cutting off the rafter bays about 2/3 of the way up to the peak of our pyramid. Makes doing a cupola (or for that matter, even ridge vents on the hips) next to impossible. Pics 3-5 show the current state of the ceiling. Number 3 shows the peak of the pyramid and 4-5 are details of the mold on the beams. Pic 6 is an upper rafter bay where it dies into the glu-lam hip beam. Pic 7 is the next bay towards the center but is looking down - seems pretty clean but the other direction looks a lot like Pic 6. All consistent with condensation at the upper end of each bay.
Today, we are leaning towards AJ's and David's suggestions in post #8/#9 to move the rigid foam to the outside and re-install the roof, as it does seem that cutting, fitting, and sealing rigid foam inside would only be as good as the individual installer at each and every instant that they are working. And excluding air between the rigid foam and the underside of the deck might be impossible as again pointed out by David in post #7). A lot to count on for a roof this size!
So, we now "let the rice cook" for a while as some say here on the island!
Again, thanks for the help!
Aloha Charlie,
Interesting photos...
Especially where there is mold showing on the exposed portions of the beams...
Have you ever checked the relative humidity in your home with a hygrometer?
Charlie, in the past two solutions to blocked vent runs. The horizontal beam should be not be full height which then let's vent continue. Or drill holes. Hips and valley beams are a bear to deal with as to venting. Same problem happens with skylights. I always drill pathways.
Now that you have mold, those areas may want to return. Treat with pro mold solution. If you put up just T&G(with exterior insulation), air can circulate in rafter bays to keep it conditioned the same as your interior.
Lucas, Yes, the RH is ALWAYS high - we are in the middle of the Pacific and at 3000 ft elevation. Example: this morning the kitchen was 70 deg F with RH at 65% and up in the loft area it is just about the same. We have no AC/heat - just open windows that allow unconditioned air entry to our home. It really seems that our problems are primarily in the winter nighttime when the hot trapped air in our partially vented rafter bays (see initial post) condenses on the underside of the roof decking. We are now committed to taking down the ceiling, removing the fiberglass batts, and cleaning/treating the mold. We'll tighten up the rafter wells (e.g. plug the eave vents and insulate the fascia with solid cc foam sealed in with canned foam, and have an externally insulated roof installed, either with our existing roofing material or with Arch80 style shingles. This will bring the interior of the roof decking "into" the interior of our home and condensation due to cooling on "winter" nights should be a thing of the past. We are now exploring the appropriate layers for the new roof-ceiling structure.
AJ- everyone that has looked at this roof agrees that venting would be a good thing but nobody can come up with a design to do this with invoking a bulldozer! So exterior insulation is our choice since ccSPF is out of consideration due to sensitivity issues.
Can any of you provide what you would consider to be the appropriate layers? e.g. from exterior to interior: something like: metal roofing, peel/stick membrane, plywood or OSB decking, two 1" layers of rigid polyurethane, roof felting, original decking, air space, fiberglass batts or other air-perm insulation, drywall(is this needed?), T&G. Are weI missing anything? (roofer guys haven't gotten back to us if they are experienced with installing externally insulated roofs here in Maui). Again, thanks to all for your valuable help and insights!
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BTW, your discussions with Don J. on his cathedral ceiling problems have been really enlightening! We all appreciate your different points of view and they really help us all in our education and understanding. After all, as a former colleague was fond of saying: "When you don't get what you want, what you get is EXPERIENCE!" Seems many of us are "Experienced"! Mahalo!
Regarding layers, I think you're going to want to vent the underside of the metal... something like (top down): metal roofing, purlins (possibly something like Eldorado battens that will drain), felt or other roofing underlayment, rigid foam (installed in taped layers), original roof deck, rafter bays with NO insulation, drywall (taped), T&G. I think you want several inches of foam, but this is a really important factor and should be spec'd by someone who can run an accurate simulation.
Keep us posted.
Aloha Charlie,
1- Some night time reading info- Building Science Paper; http://www.buildingscience.com/documents/reports/rr-1006-ba-high-r-roofs-case-study-analysis
2- Quoting myself since you are outsulating, T&G only so that the rafter space is conditioned space. "Now that you have mold, those areas may want to return. Treat with pro mold solution. If you put up just T&G(with exterior insulation), air can circulate in rafter bays to keep it conditioned the same as your interior."
3- So, the BSC paper is to do with a more extreme climate and lots of R. For you I would do rigid foam over Zip taped, Zip sheathing perhaps, Then 2 offset layers of 1.5" foam taped, then 2x4s for vent space, more Zip sheathing, Zip taped, then roof that baby. NO batts on the inside, your climate is too mild, and the space should be just left connected and at the same condition as your interior.
Roof membrane is overkill and has too low a perm. It works sometimes and sometimes horror stories from having such low perm when used on the entire roof. For standard 30 year shingles and even metal roof I have had no problems with 30# felt, but of course I build in one climate zone. Felt is forgiving or at least some of us believe so.
Note- I say no drywall. Two reasons. The wood ceiling will let air through keeping mold conditions away. And no drywall for mold to grow on. Also two layers of 1.5 to 2" foam is plenty of insulation for your climate. I think David missed the details in your post as to temps their.
Hi Charlie,
thanks for posting the photos...very interesting
I don't have any suggestions
I am curious if you spend much money to heat the house?
And....do you use the loft very often (the high stack chamber)?
Charlie,
I wasn't trying to suggest that there wasn't a condensation issue at the underside of your roof deck, but it seems unusual to me that the mold growth is not limited to the roof sheathing/rafter framing -photos 3,4 & 5 seem to show mold on the exposed posts and beams...
Is the framing so wet from condensation that it has wicked its way down to the posts?
Is there enough condensation at the roof deck that liquid water is running down?
I wonder if maybe the humidity is high enough that the ceiling area is on average damp enough to support mold growth?
I think insulating over the roof deck should lessen the risk of condensation under the sheathing but you will still need to establish the pressure boundary (ie: install an air barrier) - rigid insulation will give you r value but I'm not sure it will do a very good job as an air barrier even if the seams are taped.
Lucas, with the windows open daily, I don't know if the pressure boundary does much. Interior air and exterior air are the same. The issue has been condensation on the underside of the roof deck--or at least that's how I read it--and with that condensing surface inside the house, it's been causing a problem. The idea is to keep the dewpoint where it can do no harm, inside the foam somewhere. There is still going to be condensation on the bottom of the metal, that is inescapable in my opinion... that same thing happens here on the underside of metal roofs in carports and barns, and if they are flat enough it drips straight down. I think Charlie wants to be able to rely on whatever heat is in the building to keep his ceiling above the dewpoint, and he wants to have the condensation drain out of his roof assembly easily, on top of the felt or roofing underlayment.
I had the same question about the interior mold on the post/beams, and assume that enough water has been collecting to get those areas wet by wicking.
Lucas and David - David has the situation correctly described - interior and exterior air are the same during the day with the main floor slider open and the window in the loft open - we get a lot of air exchange. In the evening, we close the first floor slider and in the winter (sorry guys, we do call it 'winter' but....) may close the upper window to just a crack. So the warm air admitted in the day is what keeps us cozy in the evening. The condensation that occurs on the metal roof (not standing seam but corrugated - screwed through the 'hill's') does drip into the exterior gutters each morning. Not too worried about that moisture 'cause it stays outside. But the moisture that condenses on the inside is what appears to be giving the problem. And the mold on the beams is consistent with this coming from the upper reaches of the rafter bays. We see no condensation on the walls or ceiling surfaces of the loft. So I think David has it right - we want to keep the inside of the roof decking as warm as the interior of the house during the early morning cool (can be as low as 45 deg F ) and it seems that exterior rigid insulation should do the trick.
So, felt or peel and stick? The article that AJ pointed to uses peel/stick for both exterior decks. Do I need to be concerned about 'tightness' since I let air into the interior anyway? Oh, and the mold guys want to spray the rafters/decking with an 'encapsulating' paint product to prevent return of mold. Should this affect my choice of decking membranes?
And could you discuss drywall ceiling + T&G versus just T&G? I see the advantage of no interior insulation - it allows the underside of the decking to temperature equilibrate faster than if insulation were installed. T&G alone would allow more air to ventilate the inter-rafter space keeping things moving. It seems that taped drywall would decrease this ventilation much in the same way as inter-rafter insulation.
And John, since our only source of heat (other than outside air) is our smallish wood stove, our heating costs are essentially nil. We use the stove maybe for a total of 10-20 times in a cool season. And yes, we keep our books up there and it is nice and warm in the 'winter'
So, I guess my remaining questions are those above: felt or peel/stick; drywall or not; whether mildicide paint on underside of deck and rafters affects the decision on the first two.
Now to find a roofer here that can do the job!
Thanks and have a wonderful holiday!
Charlie said:
"It really seems that our problems are primarily in the winter nighttime when the hot trapped air in our partially vented rafter bays.... "
If Charlie is correct ....
Wouldn't the "problem" be warm moist interior air ... finding it's way to cold surfaces?
John,
That's correct. We want to remove the cold surfaces or at least prevent the surfaces from getting cold. Or we could try to exclude this warm moist air from getting to the cold surfaces, or both. But I think we have a better chance at successfully achieving the former using the exterior insulation and the fact that barriers are rarely leak free.
David and Charlie,
I agree that there is probably no energy performance benefit to having an air barrier in this case.
But I'm not-so-sure it isn't still a good idea from a durability point of view...
I think John's comment (#24) is salient.
Without an air barrier I imagine there could still be a condensation issue within the "3D network" - see the qucik and dirty drawing that I attached (the gaps are exaggerated for clarity).
Condensation in this case would be trapped between the rigid foam and the top of the roof deck.
A drywall ceiling could possibly be detailed to act as an air barrier, with T&G applied afterwards - but if drywall is to go back up why not just finish the drywall without T&G?
Maybe a sprayable caulk inside the rafter bays could be used instead of drywall as an air barrier?
Then the T&G could be applied without bothering with drywall...
Remember Dr. Joe's barn...
Another option for an air barrier might be to apply peel&stick to roof deck before laying down the rigid foam - which I think is what Dr. Joe did when re-renovating his barn.
http://www.buildingscience.com/documents/insights/bsi-036-complex-three-dimensional-air-flow-networks/
Aloha Charlie.
Lucas did not draw Zip sheathing taped that I propose. That tape will stop 3D at each layer. The 3D problems stem from thick foam set ups that were not taped foil to foil.
No drywall, let the ceiling be at the interior condition. Yes to Microban, or your choice of mold conditioner.
Felt in your non heating climate is the most forgiving. This is NOT Alaska. Use 30# felt and if concerned then doulbe overlap your laps.
Thats it Charlie. Get at it.
Yes AJ,
Zip sheathing could also be used as an air barrier - if Charlie is willing to rip up his existing sheathing...
Charlie,
This is a very interesting dilema.
You are currently spending "next to nothing" to heat/cool/ventilate your home.
Your roof is in good condition.
it is corrugated (self-ventilating) and draining condensate below
the structure is "real wood" and the deck is plywood
not a lot of wuz-wood (OSB or ZIP)
it has been getting wet...but does not appear to be too "rotten"
You do not seem to be sensitive to mold.
You believe the problem is mostly a wintertime problem
you have an interesting "truth window" in your storage closet
The proposals so far times 3,000 sf sound very $$$
the idea of peel & stick and or any vapor barrier (rigid foam) sounds risky
Don't be hasty....(DON"T Get at It) you may make things worse
maybe you just need to provide for some drying and...
reduce the absolute humidity at certain times??
just thinking out loud ... I am not an expert...just an Interested Onlooker
Charlie,
besides windows...where are the intentional openings?
do you have a photo of the "eave" vents?
of the Chimney?
of the "spot"( bathroom and kitchen)ventilation termination?
Brooks, I second your idea that the climate may not merit an expensive exterior roof insulation redo. I think like I posted before, Charlie could rip out drywall, remediate mold, and put up T&G.
That is my final answer in fact.
1-Remove drywall, and insulation, save clean insulation only.
2-Remidiate mold
3-Re insulate with fiberglass insulation batts, low density batts (better for air circulation)
4-Pre-finish T&G at least one coat backside, finish front side and install.
5-Done, invite aj to New Years 2013 Luau and surfing party.
Aloha,
aj ajbuilderny at gmail (for Luau dates and location)
Just thinking, Charlie, I am a hang glider pilot, 3,000' nice roof... thinking launch spot! Pics pics pics...
Really like $ugge$tion #32 from AJ based on John's observations. I was talking with my brother (a building rehabber in coastal Los Angeles area), and he suggested that we solve this like a moldy closet. Give the rafter bay area circulation from below and then vent above. Because our inside/outside air are essentially the same (we ventilate using doors and windows!) the vents in the rafter bays could be in the drywall ceiling. So our proposal is similar to AJ's #32:
1-Remove moldy drywall, and insulation, save clean insulation only.
2-Remidiate mold
3-Re insulate with fiberglass insulation batts, low density batts (better for air circulation)
4-Re-install new drywall where old was removed
5-Cut out a 2" horizontal strip of drywall all along the perimeter of each ceiling panel and along the edges of the hip beams
6-Add furring strips to drywall to establish attachment points for T&G ceiling (about 2" lower than drywall ceiling - allows us to easily cover exposed beams)
7-Install T&G ceiling in a removable manner (in case we need to come back in 5 years and re-do this all over again!
8-Have New Year's Eve Luau!
Who knows, this might even make our place warmer in the winter with sun heated air in the rafter space actually making it's way into the house.
So, that's where the "rice is cooking" today. Mahalo again to you all!
Oh, and John, I haven't posted any pics of the eave vents, but they are simply the little (~3" diameter) metal round 'mini-vents' - 2 in each rafter bay. The chimney is in an 'air tight' stove that is kept closed and neither bathroom nor kitchen have exhaust fans (a bathroom fan will be installed as part of this project). No other 'intentional' openings other than plumbing vents that are well flashed.
Charlie,
putting aside whether it is a "good" solution....
please consult with the Building Code (fire safety)
Your proposal is likely a violation
I also believe that AJ's suggestion of tongue&groove Sans Drywall would not fly either unless the T&G is nominal 2" thick
but you should verify
and look back at David's Post #2
And getting back to David's post #2
IF the problem is mostly a winter problem....
maybe you could use the "typical" approach
airtight drywall, spot ventilation and dilution ventilation (plus a little heat) and
find a way to vent the tops of the rafter chambers to the outside
again...I am Not an Expert....just offering ideas for discussion
In Texas we say "chew on it for a while"
I'm having a hard time seeing ventilation as the solution here. It sounds like current ventilation may be less than ideal, but how do we know that more will solve it? My belief is still that the underside of the roof deck has to be kept warm, as close to interior temperature as possible. There isn't condensation forming on the rest of the interior of the house, is there? If not, then night sky radiation is the culprit, it's causing the roof deck to be colder than ambient temp and thus become a condensing surface when other interior surfaces do not. I recognize that it co$t$ a lot of dollar$ to remove and rein$tall the roof, but if $pray foam i$ not acceptable then I think it'$ the an$wer. Either that or a local expert (none of us are) has a different, proven solution. Charlie, you got anyone from the local A-list on this?
Lucas's point about an air barrier at the sheathing plane is very good. If the layers of foam above are not enough, then I would consider taping the roof sheathing from above before laying the foam. Spray caulk from below might also do it, although who needs another specialty sub on the job?
please check my math
the source I googled showed a 3" dia vent to have only 1.34 sq in net free area
if you have around 204 vents = 273 sq in = less than 2 square feet
2 sf per 3,000 sf = 1 /1,500
code says 1/150 if soffit vents only
???
John, I don't know if you're addressing my comment, but I still wonder, if the issue is night sky radiation cooling the roof and causing condensation on the bottom of the roof deck (the air being some combination of interior and exterior air, which are approximately equal in MC, temps unknown)... and at the moment we have more wetting potential than drying potential... will more ventilation change the equation enough to solve the problem? You are correct in the math re code required vent areas, but I can't tell whether simply following that requirement will alter the dynamic, let it continue approximately as it has been, or even make it worse. How do we determine this?
I have had to solve problems here involving strictly outdoor air condensing on the bottom of open metal roofs, which is why I wonder and worry about the venting solution....
David, I think the not-so-code roof venting is just one piece of th problem.
gotta run
will get back later
Good
David, I am waitng for permission to quote an expert
you are on a good track
Yes, I too say vents should be closed. Make the rafter space conditioned space though, using NO drywall. And as to Microban, I think it is rather low in human toxicity. I use it and sense absolutely nothing.
Drywall us bad. Sealing it tight is impossible too.
IMO
John,
Thanks for bringing Robert Riversong into the conversation.
I think that it is fair to say that we all agree that exterior insulation and re-roofing would probably solve the problem. But is there an easier/cheaper solution? And, of course, if the problem can be addressed stepwise without one solution precluding another, so much the better!
My reasoning on the "interior" vents at the base and top of each rafter bay is that in order to get condensation we need 2 things: moist air and a surface that is at a temperature lower than the dew point for that moist air. In exposing the rafter bays in the storage space, it was evident that the top portions of the bays were much hotter than the lower portions (this was done in the day!), no surprise to anyone. And, noting that the bays are 'blocked' at the top, humid air continues to accumulate at the top of the bays - probably with an absolute moisture content far above that of the ambient air. [my reasoning: moisture will migrate from higher RH areas to lower RH areas: the 'hot' top portion initially having lower RH due to high temperature than the 'cooler' lower bay areas; moisture then migrates to the hotter areas]. I propose that the moisture that eventually condenses on the inside of the roof deck (so far observed ONLY in the upper, blocked portion of the rafter bays) is coming from this 'overly saturated' air that collected during the sunny winter day.
I'm reasoning that if this 'overly saturated' air was allowed to escape from the top of the bay into the interior of the house (dilution ventilation?) and then out the upstairs window, the air that is in the rafter bay at night will not reach its dew point because it is just like the air that resides in the lower portion of the bays. And, that the stack effect of a doubly vented rafter bay would also serve to keep the underside of the roof decking at a slightly warmer temperature than in the now-blocked/blind rafter bay. So the venting attacks the problem on two fronts: slight warming of the underside of the roof deck, and removal of moisture laden air.
Charlie,
I visualize the air in the rafter cavity to flow more like the air in this wall Illustration.
I adapted this sketch from an illustration in John Straube's textbook.
only there would be more condensation near the top
Charlie,
about your reasoning that moisture will migrate from higher RH areas to lower RH areas....
I used to reason the same thing.......
Actually it does something that is not so intuitive.
see the attached snipit from BSC insight 036
http://www.buildingscience.com/documents/insights/bsi-036-complex-three-dimensional-air-flow-networks/files/BSI-036_Complex_3D_Airflow_Networks.pdf
I do understand that moisture-laden air is buoyant - and it is this buoyancy that I think might be driving the condensation issue because of the blocked rafter bays. My imagining of the air flow was for the daytime heating cycle - and I understand that the flow will reverse as things cool down in the evening as is shown in your posted diagram.
If the condensation is coming from 'ambient' air, my venting solution will not solve the problem and in fact would make it worse. If condensation is coming from daytime accumulated/trapped moisture because of the blocked bays, then venting the top of each bay and letting it escape rather than build up might solve the problem.
So, I am seeing 3 solutions:
1) use ccSPF on inside of roof rafter bays; block mini-louvers
2) remove current roof, install rigid foam above current roofing sheathing, re-install roof, block mini-louvers
3) block mini-louvers and do 'internal' venting as in post #34
It seems as if these are listed in order of workability with ccSPF being the most probable to solve my issue without inadvertently causing other problems.
We've talked with a ccSPF installer and he will bring by a piece of foam for us to live with for a while to see if we are sensitive ("Here, honey, take a snort of this!") ;-)
We are currently in the process of vacating the house prior to drywall ceiling removal. Getting a look at the whole thing should clarify the extent of the problem. I'm hoping that the 'foam' sensitivity issue will not be a problem and we'll proceed as in post #1.
Thank you all for suggesting/exploring alternative options with us. I'll keep you posted on what we find next month when the drywall ceiling comes down.
Charlie, I asked an Expert for his thoughts.
This is what Robert Riversong had to say:
"....Clearly, the problem is ambient moist air getting into rafter bays (from button vents and interior leaks around penetrating beams, condensing at night and saturating the sheathing (the plywood may have delaminated as well). It's always far worse to have vent inlets with no clear vent path and no outlets than to have no venting at all.
If wife is chemically-sensitive, then I'd urge caution about encapsulating the roof assembly in some kind of anti-fungal (particularly if an air-open ceiling is used) and it may be better to allow the sheathing and rafters to breathe naturally.
The exterior insulation is the best option, since SPF is ruled out, and it may be time to replace the roof and screw it down properly in the flats and not in the ribs. I would recommend replacing any damaged sheathing, install roofing underlayment, rigid foam board with taped seams, horizontal battens and new metal roofing. Remove drywall and fiberglass below, remove mold with with detergent and stiff brush, allow to dry thoroughly, paint all wood with BIN shellac-based sealer, and refinish ceiling with drywall with vinyl j-channel and caulk where it meets wood beams.
- Robert"
Charlie, my nose stuffed in foam tests... open cell Icynene no smell at all, closed cell definitely could smell the blowing agent. Icynene makes a in between foam, or they did, and the in between foam would work for you and it is like the open cell in that it is water blown. (the water turns to steam, which is how the foam rises into foam).
Hello Charlie, you have had a lot of suggestions. I will make an observation first. I assume when talking cost that replacing the roof structure would be a large cost, hence the very reason you are trying to fix this. I have no experience in your climate but see from comments that with such high humidity condensation can easily occur on a SLIGHTLY cooler surface. You may have no problems with the spray foam from below but it still leaves the top side of your structure and the sheathing exposed to moisture since there is an airspace between the sheathing and the roofing, the felt may deflect most of the water but..... if any of the wood gets wet.... If you insulate on top I would (after replacing any sheathing that might be needed) cover the whole roof with a peel and stick. I would want an air barrier but with having all the windows open not so important but I would want a waterproof barrier above my roof structure. The foam can then be installed with no worry about taping or sealing, horizontal strapping and metal. I would detail the peel and stick as tight as possible regarding water from above and make sure it extends out at the eaves and that water could get out at the eaves. Your structural elements will all be protected. The foam doesn't mind water but the strapping could be something that would better tolerate moisture (pressure treated wood or a rot resistant wood?) but even if not it would be cheaper to replace down the road than the structure.
Inside I think all the wood (sheathing, rafters and beams) should be able to breath, no sealing. If your code requires drywall do all possible to have it breath.
It's too bad the roof structure isn't all timbers and could be left exposed, would look nice. T&G will look nice as well.
It's tempting to offer to come over to do the work for you!
Hope it goes well and lasts a long time whatever you end up doing.
Cheers from the Yukon.
Aloha Charlie, be great to here how you make out with your rebuild.
And maybe martin or Yost will chime in or blog this.
Charlie,
Aloha.
One solution to consider is covering the roof with solar panels. This will shade the metal roofing and reduce the heat gain. Call for a solar quote.
More seriously, I would speak with Green Building Hawaii. They are based on-island and have the building science to advise you.
Good luck!
Aloha Everyone,
Thought I'd give an update on this project. We have moved out of the house and our GC has had his guys in to do unit protection (kraft paper, 1/8" masonite, all taped, 2x4s protecting the bullnose corners, etc). A couple of weeks ago the drywall guys ripped out the entire ceiling. Two of the four sides looked pretty good while the north and west sides showed considerable staining with mold especially near the hip beams and the cross-tie beam where the caulk between the beam and drywall had failed (probably after our earthquake a couple of years ago). All surfaces were dry as was the fiberglass batting. The batting had many mold colonies on the upper batting surfaces at the top of the bays. On the west side where the rafters hit the cross-beam, the Simpson ties at the top of the bays were corroded while those directly on the upper side of beam were still shiny. We replaced the corroded ties. So the conclusion of Robert R. (see #43) that ambient moist air (from multiple sources) was the problem seems correct. Because of a couple unexplainable stains on the inside of the roof deck, we decided to replace the roof with a new standing-seam metal roof laid over Titanium felt. On the inside, we have removed all the fiberglass batting (the clean stuff will get re-used by one of the drywall guys), have cleaned the surfaces with an industrial degreaser detergent with bleach, and then have treated the surfaces with a light spray of Concrobium, a mold inhibitor. We taped over the mini-louver vents in the eaves and have caulked the wall-roof joints. The surfaces have been drying for a week. Today, we are prepping for the closed cell spray foam installation. Our SPF contractor is using a JM product called CoreBond III. He'll put in 3" (R-19) tomorrow and the next day. After that, our GC will install additional furring strips perpendicular to the rafters in order to lower the ceiling about 2". By doing this, the drywall will not butt up against the beams but will meet itself and get taped at the diagonals of the ceiling. After taping, our GC will install a new ceiling of T&G cedar in a vertical direction (the reason for putting the furring strips perpendicular to the rafters). Then we will re-install the lights and fans, re-paint, and this part of the remodel will be complete!
Thanks to all of you for your interest and discussions. I've really learned a lot from you all!
Mahalo for your kokua (thanks for your help!)
Charlie
Aloha All, I'm back here because my neighbors are having almost the same problem. I realized that I never gave a final 'how did it turn out' post. Well, it's been seven years since starting this discussion and I wanted to again thank all of you for your input. The solution of insulating on the inside of the roof deck with ccSPF has worked like a charm. Not a speck of condensation or mold has ever appeared. And the house maintains a much more even temperature both on hot and cool days - an added bonus. We had the same problem in our ohana cottage (we designed it ourselves, too!). The moldy beams in the cottage were even more pronounced. Again, we removed the drywall ceiling (seams butting up to the exposed beams had split), cleaned everything, and had ccSPF installed on the interior side of the roof deck after plugging the round soffit vents. Drywall was re-installed and instead of butting up to the exposed beams, we wrapped them in drywall. So no dissimilar materials joining together. That project was completed four years ago - again, with perfect results. No condensation, no mold, and more even interior temperature. I'm including two pics - a before (Problem) and after (Solved) of the main house project. Again, mahalo for your kokua (thanks for your help!) - the discussion was very enlightening and our problem clearly has been solved in a lovely way!
Charlie,
As one of the participants in the original 2012 discussion, I'm glad to hear of your success. Thanks for informing us of the outcome.