Two Vapor Barriers on Roof of Cottage Remodel
Hello all,
New to the forum but long-time Fine Homebuilding subscriber…
I recently completely renovated our “cottage” up on the hill to turn it into a nice woodworking shop and my office in the upstairs. I will be there every day, all day, so it needed to be nice and comfortable.
I live in a far northern climate where winters can be very cold and long.
Unfortunately, I am asking this question after the fact, but here is the deal. I put Titanium UDL on the roof without understanding much about its permeance and what that means. I think put horizontal purlins on top of that and my steel roof on top of the purlins. The purlins were necessary since the roof is a 12/12 and very challenging to work on without.
Underneath that Titanium is the 1/2″ sheathing that the previous builders had installed (40 years old). That is another reason for the purlins (to add a little extra strength).
Inside of the sheathing is open-cell Icynene spray foam. It is 5.5″ thick in the rafter except when it passes above the collar ties and below the knee walls, they went thicker there. Then, everywhere it is only 5.5″, we wrapped it with 2 inch rigid foam and filled the gaps with expandable. Behind the knee walls and above the collar ties it was then “painted” with a vapor barrier paint.
This building should be pretty tight now, but should I be worried about having the rigid foam and painted open-cell foam as a vapor barrier on the inside and the Titanium on the outside? Am I going to rot my sheathing and rafters? Is there anything I can do?
Thank you very much!
Mike
P.S. Let me know if anyone prefers to see images to help understand what was done.
Replies
Mike
What type of meal roof and maybe it's color?
Did you vent the soffits and ridge?
and the more pictures the better, thanks
welcome to Breaktime.
probably need to maintain drying to the inside
Seems me that it doesn't matter that much what type of metal roof you have, or what color it is. It's not your primary vapor barrier or an integral part of your thermal envelope becasue it's fastened atop purlins which creates an air gap. Your roofing paper is another matter.
Your Titanium underlayment is made of polypropylene and has a permeability rating based on ASTM E96 of approximately .06 perms. It's installed directly atop your 1/2 sheathing (typical). So you've installed a well performing air and vapor barrier on the exterior side of your roof envelope. There is very little chance that any moisture within your building envelope will be able to dry to the outside; it will have to dry to the inside. Also, wind wash from any venting would not be an issue with your continuous thermal envelope which is covered with the Titanium.
IMO, you've made this difficult by applying a vapor impermable paint film to the inside of your open cell insulation. That difficulity is based on the assumption that there would be the introduction of high moisture content into your framing or sheathing (19% M.C. or more) that would need drying in order to avoid rot. Seems to me that if you never have conditions of excessive moisture inside your building (40%R.H. or less), then your vapor drive toward the outside would be very low, and thus moisture would not want to be driven through the open cell foam and into the framing.
I don't know what climate zone you're in, but assuming maybe zone 6. I'm not a professional building scientist, but it seems like your thin areas of insulation are a concern. I believe your code required roofing insulation componet is an R-49. If you have rafter bays of 5.5" filled with open cell rated at R-4 per inch (typical), then that gives you about a net R-22. You say you've supplemented this by your 2" of rigid foam (which may or not be vapor permeable) giving you another R-10 (maybe) with possible gapping. Thus, you seem to be under code here, which might bring up dew point issues withing your roof assembly. YOu may want to run a dew point analysis with someone that can factor in possible air barrier advantages of a metal roof or other mitigating factors that make what you currently have acceptable from a thermal and dew point perspective.
Other wise, it seems to me that your best bet is to make sure you have maintained about an R-50 thermal envelope with a vapor permeable type insulation like open cell spray foam. This may mean removing the vapor impermeable paint film.
More Information on the Cottage
Thank you both for your responses. I appreciate your thoughtful and detailed response. I've attached some pictures for your reference.
First off, I looked it up and I am in zone 7 (cold/very cold). One of the tricky things about our area is that it can be -20º in the winter and 95º in the summer (though summer is only 2-3 months long). Heating in the winter and A/C in the summer.
The roof is not vented wide open per se, but I was told (correctly or incorrectly I am not sure) that with a metal roof it would not be an issue, especially because it was up on purlins and enough air would be able to move underneath the purlins and through the ribs in the metal.
I look it up and with the 2" of rigid foam, our system is R32 on the ceiling. This is a little bit less than what is typical up here, but the installer assured me that it would be a great system being that the rigid foam would take care of any thermal bridging through the studs/headers/rafters and it would be a nice tight enveloper. He also said that almost no one goes to R49. I've been using it for years with just 2x4 and 2x6 fiberglass and in this building achieving R49 would be quite challenging.
All that said, I've put my life into this project for the last year and pretty much all the spare change I have. I want to make sure I am not creating problems.
The paint applied does have a perm of less than 1, but the installation installer said that "we recommend the vapor barrier since it will allow some water vapor to diffuse if the insulation ever gets wet, but will still restrict moisture from moving into the foam."
The relative humidity in the building during the winter would almost certainly never reach 40%. It is really dry here during the winter.
Interested to hear your thoughts with this additional information.
Thank you!
Mike
I insulate to min. R-38 by code and I am in zone 4. R-32 is not going to "take care" of it in your zone whether you, I, or your installer thinks so; or not. Your installer is guilty of two things:
1. Wishful thinking. He is not immune to the laws of thermal dynamics, physics...or code.
2. He did not install a "system". He installed open cell spray foam. And he apparently installed it with the framing limitations in place and is choosing to overlook those. While it is a great material, it is not a cure all for creating a well perfoming thermal envelope.
Same goes for your installer's paint vapor barrier rationalization. A vapor barrier is not a "thinking" material. It doesn't decide when it's good to let moisture through...and when its not. You essentially have a double vapor barrier (to a variable degree) as a result of installing the low perm Titanium and the liquid applied paint film. Anyone will tell you that double vapor barriers are only acceptable when the material between the two doesn't "care". Wood cares.
Here's the bottom line:
1.Your thermal envelope is not performing to prescritive code in terms of energy conservation. This only matters to you if you're concerned about the cost of energy and/or your thermal comfort. If neither of those two things matter to you, then your fine with a weak link of R-32.
2. You have installed a double vapor barrier set up with materials between that care. This has the potential for mold, mildew, and/or rot. This depends of many factors which include air leaks in the thermal envelope, thermal bridging, quick and large temperature and R.H. swings, user induced moisture generation within the building envelope (flame point heating could be a periodic one), etc. For mold to propogate, there needs to be an ambient and sustained R.H. of 70% or more (can be within a cavity; not necessarily the whole space) and/or a wood moisture content above 20%. You may never get these conditions. Yet you might. Who knows for sure? My suggestion is to get a remote sensoring moisture metering system to monitor the conditions near the bottom of roof the sheathing/top of rafters. This is the system I use to check with:
http://www.lignomatusa.com/MoistureMeter/MLMoisture-Log.htm
It may cost you $500 or so to get set up to monitor it properly. But it will be the best way to put your mind at ease and end a very technical debate that involves many variables that are case specific.
Best of luck with your project!