Does interior ceiling strapping cause a moisture issue?
I am in climate zone 6 and am currently concerned about my insulation strategy on
a remodel of a 100 year old kitchen.The customer wants cathedral ceilings so I am trying to do my best
with all the advice here to insulate it properly. i have insulated the 24″ o.c. rafter bays with r-23 mineral wool, will be stripping the roof and applying Henry vp160 and then 2 layers of polyiso staggered and taped. i have cut off the soffit as well so I can have the peel and stick Henry wrap onto exterior 2′ zip r sheathing. I will likely also do a cold roof assembly on top of this with 2×4’s fastened through to the rafters and zip system sheathing on top of the 2×4’s.for a roofing deck.
My main question is the ceiling will be taped and finished drywall, but we strapped the ceiling because of the 24′ o.c. rafter spacing. Will the 3/4′ space between the mineral wool and back of drywall cause
any kind of moisture problem?Any other criticism of anything else I have listed in this assembly would be greatly appreciated! I am pretty confused about properly insulating an unvented ceiling like this.
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Replies
Are there any penetrations in the drywall?
Also, how thick is the polyiso?
Hi Steve,
I accidentally omitted that the polyiso would be 4"at least: 2" staggered, but i could add more.
The drywall has a few round boxes for pendant lights, but i had been planning on sealing them in tight with caulking and the backs are sealed with stretchy acoustic tile.We have been trying to be careful with air sealing on this project.
Thanks..Eric
I'm curious - what in the code prevents this from being considered a vented roof (without rigid foam R value requirements)?
Thanks Steve,
I knew I wasn't quite hitting the mark on the foam r-value considering having already read the link you gave me, so i will add more to be safe.I also did not know the formula, so that is very helpful for sure!I am on the edge, of 6a and zone 5 and on an island miles offshore, so we never get the low temps seen on the mainland; so I was hoping I could get by with 4 inches total polyiso.
Back to my other question: do you or anyone know about the minor void on the backside of the drywall caused by strapping a ceiling? I think its a Maine thing, but strapping is very common here, especially with the 24" o.c. rafters I am dealing with. I would have skipped it otherwise in this case. We have been working hard to air seal everything everywhere.
To Jon's question; I am thinking about venting the top roof deck mostly because I am concerned about the shingles degrading faster from heat over time. It seems like a safer bet to me.Someone else could address your question i am sure. I guess this assembly still seems like its unvented in some way too me.
Eric, strapping ceilings is standard practice in New England and I can't think of any reason it could cause moisture problems--is there something specific you think might happen? As long as you paint the drywall with standard latex paint to retard moisture diffusion from interior to exterior, the exterior foam will keep the sheathing above the dewpoint temperature.
Every situation is different; the moderated temperatures at your location may mean that skimping on foam is ok, but the elevated humidity levels that come with island living may work against you. Suffice to say that the code requirements, both for minimum R-value and proportion of interior to exterior, should provide you with a safe assembly.
Steve gave you excellent advice. Polyiso's R-value does decline with temperature, but a couple of inches into the assembly it will be warmer. I work in zone 6/Maine and use R-5.6/in for relatively thick exterior polyiso, like yours, derating the exterior 2" to R-5. In your situation, 6" of exterior polyiso would provide a safe assembly. 5" would be cutting it close but might be fine, and would meet code (although you are probably in a place that doesn't need to follow MUBEC).
(Edited to balance Steve's edit... nothing to see here.)
@Michael. Thanks for catching that error. I must have lost the line while scanning the table and read the requirement for Zone 7. That or my banana fingers got the better of me while typing. I will fix my earlier post.
Eric,
You need a minimum of R-25 of rigid foam and it has to account for 51% (corrected) of the total R-value. (See https://www.greenbuildingadvisor.com/articles/dept/musings/combining-exterior-rigid-foam-fluffy-insulation)
You have to assume that the polyiso has an R-value of 4.5 to 5 per inch. (See https://www.greenbuildingadvisor.com/articles/dept/musings/thermal-drift-polyiso-and-xps) So you are a little short, but it may be enough to perform adequately. Maybe Dana will do some math to verify my WAG.
Michael, I had honestly not given the strapping issue a thought as it is so common. I frequently see moisture issues on the upper side of the insulation under the roof sheathing during demo. My fairly experienced HVAC contractor thought that it could cause a problem and caused me to wonder if it could actually be. I will be using standard latex paint, and sticking to all the suggested assemblies and practices i am learning here on this site for sure.
I will also go with the 6 inches of polyiso per your recommendation to be safe. Humidity is certainly higher out here at times per your suggestion. I am in no position to argue with your advice . I had originally been using the r-value on the polyiso panel, which i now see is somewhat misleading. Even then i was slightly below code.
I know this type of question is old hat for you guys on here, so i truly appreciate you all taking the time to answer my questions!!
Happy to help, Eric. Upon further thought, there are situations where three dimensional air flow due to strapping can cause problems, but that's most commonly an issue with vented attics with batt insulation on the attic floor. As long as you make sure you have a continuous airtight layer you should be all set.
Some polyiso manufacturers are now listing "LTTR," or long-term thermal resistance, where they admit that the R-value declines to around R-5.6 or R-5.7 over time. Thermax claims higher values but doesn't share what provides those values.