Black vapor permeable air barrier?
corkydork
| Posted in General Questions on
I’m looking for a black air barrier for my ceiling that is also vapor permeable. Tongue and groove with eventually contract leaving gaps, and I would rather not glue it. All vapor permeable membranes I’ve seen are white or light colored. Anyone have experience with a darker membrane? Thanks!
-Courtney
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Courtney, this product is made for exterior use but it's airtight and vapor-permeable, so I think it would work for you: https://foursevenfive.com/fronta-quattro/
Hey Michael,
Thanks! I planned on getting intello from them if I couldn't find a black barrier. I actually saw your comments on a different thread regarding moisture content in wood. See my reply to Malclom below- trying to get the right EMC for the right wood and interior space seems to be pretty difficult to predict. Is this also your experience?
-Courtney
Drywall is usually a good idea behind T&G ceilings for fire protection of the structure. You could paint that drywall a dark color and detail it as an air barrier.
Thanks Patrick. We are installing dense pack cellulose which is fire resistant, but that's not a bad idea. In reality, there's so much wood in this house, that if there's a fire we are F***ed. S I'm not sure how much a layer of drywall will help or if it's worth the effort, but that's good to know.
Courtney,
Something is going wrong if you have to anticipate your wood ceiling shrinking until you can see through the gaps in the T&G. New wood floors can be installed so that they stay tight enough that a poly finish doesn't crack,. The same should be true of your ceilings.
Make sure any wood is at the appropriate moisture level for the anticipated range in your house before installing it. You can find charts online.
Hey Malcom,
Very interesting. This was my instinct as well-to install at optimal moisture content- but our insulation guy insisted that it would shrink. This is a good article explaining EMC (equilibrium moisture content) in wood:
https://www.wagnermeters.com/moisture-meters/wood-info/acceptable-moisture-levels-wood/
I also found a couple charts on exterior EMC for various regions, but found nothing species specific:
https://www.woodworkerssource.com/wood-moisture-content.html
https://research.cnr.ncsu.edu/blogs/wpe/files/2016/08/Equilibrium-Moisture-Content-of-Exterior-Wood-in-the-US.pdf
What is confusing is that the recommended EMC depends not only on region, but on application and wood species and expected RH (relative humidity). The charts only give exterior EMC based on a region's RH. This T&G will be for an interior application, but our RH will be somewhat unpredictable in our off-grid house because the only heating or cooling system we will use in the house is a wood stove- with an occasional propane backup when we leave. So although the general exterior wood EMC of our region varies from 12.5-15.7%, the recommended EMC for interior flooring when installed is 6-9%. In summer we will open windows, when the humidity is super high (close to 15.7%), and in winter I assume the house will be fairly dry. Since the T&G will be on the vaulted ceiling I'm going to assume that it's essentially going to be like a kiln. I get that interior RH of houses in winter is supposed to be higher than outside, but the last wood heated house we lived in (very crappy insulation lots of air leaks 2x4 walls) we monitored RH in winter down to 25%, much lower than for our region according to the charts. This new house will be 11.25" thick dense pack cellulose in walls, 20" in ceiling, so maybe moisture content will be higher?
In summary, I have not yet found a table that shows species and region and application, and this is new construction so we don't know what the RH levels will be. So I'm a little lost as to the proper EMC we should aim for before installation. Essentially we would need to know the lowest RH expected in the house, assume that the highest EMC is 15.7%, and install when we are halfway in between. Impossible to know.
Is there something else that can be done besides trying to bring the wood to equilibrium with the current humidity levels of the living space? Does the wood need to be pre-finished before installation or possibly just the back covered in shellac? I'm thinking that given the unpredictability of the RH inside the house, if all else fails, a black air barrier might not be that bad of an idea.
Thanks everyone!!
Hey Malcolm,
I spent a long time responding to this comment and somehow it was not posted even though my email says it was.... So sorry if there is eventually a double response. Essentially EMC (equilibrium moisture content) charts for wood show the expected moisture content for the relative humidity (RH) range of the exterior. For my region it's 12.5%-15.7%. However, for interior wood flooring applications the recommended EMC is 6-9%, mostly I believe because it's assumed these spaces are conditioned.
The RH of our new off-grid construction will be unpredictable. The only heating we will use is a central wood stove, and no cooling. Last wood heated house I lived in got down to 20% RH in winter, much lower than exterior RH for our region. Leaky 2x4 construction. New house is 11.25" dense pack cellulose in walls and 20" in ceiling so maybe better moisture content, but since T&G will be on the vaulted ceiling with wood heat only, I assume it will be super dry. Basically I believe you have to know max and min RH of your interior space, and bring the wood to equilibrium with the average value before installation. Impossible to know! Not to mention species of wood.
I'm wondering if there is something else we can do before installing to help shrinkage/expansion? Pre-finishing every piece? Shellac on the backs of all pieces? Wondering if you have any experience with this. Thank you!!
-Courtney
Courtney, this is the best resource I know of for learning about wood shrinkage, EMC etc: https://www.goodreads.com/book/show/156605.Understanding_Wood.
I have over-dried wood floors before installation and had them buckle in summer humidity, so wood can be too dry. I have also had many projects where the woodwork was installed very tightly but shrunk enough to have major gaps in winter.
Where I am, in Maine, we have some of the widest swings in interior RH anywhere; wood moisture content can be 6% in winter and 14% in summer. If you only need to plan for 12.5% to 15.7% that's a pretty tight range. Store-bought, kiln-dried softwood will likely be in the high teens so to prevent or reduce dimensional changes, plan on getting it down to the 12% range before installation.
Pre-finishing will slow dimensional changes but won't prevent them, so I wouldn't bother trying to prevent movement. Certainly don't glue the boards together or you will get cracking or buckling. The narrower each board is, the less it will shrink or expand.
Corky,
I agree with Malcolm. There's few reasons one should be building new and planning for T&G to open up completely. That said, you still want a good air barrier there, you may just not need it to be colored for exposure.
"Essentially EMC (equilibrium moisture content) charts for wood show the expected moisture content for the relative humidity (RH) range of the exterior."
It's not the 'exterior' RH, its the 'experienced' RH (experienced by the wood). It may be impossible to know these exact min and max RH values, but you're not flying completely blind. One can approximate a rough range.
The nice thing (I suppose) about wood is that the highest rate of EMC change is at the upper limits of RH values (80+). So unless your expecting to experience very high RH, the EMC change is somewhat close to linear (see the graph here: https://www.buildingscience.com/documents/insights/bsi-023-wood-is-good-but-strange).
How wide are your boards, and do you know the species yet?
It's important to remember that the shrinkage is distributed among all the boards, so unless your boards are very wide, your tongues exceedingly small, or the wood you install is fairly green still, your simply not likely to experience shrinkage that will open up past the tongue.
Tyler, I don't know where you live but I know Malcolm lives where the RH is pretty constant. Here in New England I don't think I've ever seen a wood floor without gaps in winter, and I have seen many examples of wider T+G boards that have shrunk more than the 1/4" tongue. We have shiplap pine boards with 3/4" tongues and even those, if put up wet (28% WMC) will shrink more than the tongue. Fortunately it sounds like the OP's RH range is pretty narrow.
Michael,
I live very close to you (relatively speaking).
Central Maine.
I may have made it sound too easy. You are right. Though I would push back slightly and say there is a big difference between putting up 'green' wood vs putting up kiln dried wood or even properly air-dried wood.
For very wide boards, I think you are right. That's why I asked the dimension of Corky's T&G. And it's why my list of 'exceptions' was 1) very wide boards, 2) small tongues, and 3) very wet wood.
As an example, using eastern white pine, if we assume the MC swing (difference between min and max) to be around 10% (say, 6% low, 16% high), and we use an 8" wide board, we can use a formula to calculate the difference in width (using Understanding Wood here):
8(.061)(.1/.28)= 0.174 inches
where 8 is the initial dimension (at 16% MC), .061 is total tangential shrinkage percent (green to oven dry pulled from table 6.2 in the book) .1 is for 10% MC change, and .28 is fiber saturation MC.
There is a catch that .174 inches is total dimension change, and so if the wood is put up at the upper MC limit (or especially beyond) without being completely tight, then the effective tongue size is reduced. Which implies one would need to have some idea of where the wood is in the RH equilibrium swings. Because on the flip side, if you put the wood up tight and it is NOT at it's upper experienced EC, it will buckle.
In the end, experience trumps maths, and if you say you see many examples of opened up T&G, I believe you. I have not personally seen it, and while I have seen a fair amount of installed T&G, I'm not a designer or builder and therefore not as exposed to the plethora of cases.
For sure, wide boards and wet wood is far more likely to lead to opened up joints.
Tyler, I think I remember you mentioning before that you live nearby. I looked you up online and see that you're just across the river from me. Howdy, neighbor! I don't mean to be argumentative, it just comes naturally...
I can't argue with your math. People often think that white pine moves a lot but it's actually one of the more stable species.
One minor quibble--wood doesn't expand above its fiber saturation point (above ±28%, as you note) so I'm curious why you say, "or especially beyond [the upper MC limit]."
One of many examples--about 30 years ago I installed T+G wainscot at my now-mother-in-law's house. I installed the 4" pine boards with 1/8" tongues tightly. She's two miles from my house and every time I'm there I push a few boards back into place because many of the tongues no longer engage the grooves.
Haha, hi neighbor indeed! (I'm waving across the river :))
Believe it or not, being 'argumentative,' in some sense of the word, comes naturally to me as well. haha. I've maybe even been accused of being pedantic (what!? no way). I think it is a way certain detail oriented people learn and express ideas; we're certainly not the only ones on these boards with that tendency. I don't mind it and I think most of us understand we're all working through interesting problems, not trying to start fights (98% of the time anyways).
'wood doesn't expand above its fiber saturation point (above ±28%, as you note) so I'm curious why you say, "or especially beyond [the upper MC limit]."'
Good point, what I meant was above the EMC experienced by the lived conditions, not the fsp. Meaning if the wood get's installed with higher EMC than it will otherwise experience over the duration of it's life in the assembly. As would certainly be the case if it was installed green (above or close to fsp).
If the RH of the assembly often reaches close to 100%, this would be a moot distinction, but in most cases I am assuming 100% RH conditions are not experienced for interior wood finishes (or only for extremely short bursts, and wood does have a lag reaction, so it likely wouldn't come into equilibrium with the RH in such a short duration. This also explains why finishes can help with wood movement problems—by providing further lag—even though the tendency towards equilibrium is only slowed, not stopped.). Reminiscent of thermal mass discussions (where's DCCon at? :))
That's interesting that even 4" pine boards would show gaps. Certainly getting this right involves some detail and planning. I would wonder 2 things about that T&G:
1) was it installed wetter than average (or even maximum) interior conditions, making the effective engagement of the tongue smaller than 1/8" to begin with? and;
2) are they floating or is each individual board attached? Because if floating, gaps could migrate to local spots (by closing up in other spots) and become sizable.
Also if Courtney comes back and says the T&G is oak, this is a whole nother can of river eels.
In my experience when you have an area of T&G the gaps aren't even. When they expand they push on each other, and the boards aren't all attached equally well, so the less well-attached ones move more. So if the average dimension change is 0.174 -- about 1/6 of an inch -- you'll get some boards that have no gap and some that have a quarter inch gap or more.
Micheal,
Maybe I should have put my response another way: If the building you are constructing will experience swings in humidity sufficient to open the t&g such the the underlayment shows, it probably isn't a the right material to be using in that structure.
Malcolm, it's hard to argue with that logic!
Until your post here I had never thought of our high but fairly consistent humidity levels as having any benefit, but you are right, it really helps wood stay stable.
If T&G opens up completely, wouldn't it allow one side to sort of 'hang loose' and get wonky due to warping? Even possibly making it so the tongue doesn't reinsert into the groove when it re-expands with humidity increases?
Hey all,
Thanks so much for the thoughtful responses. Glad you found each other neighbors! We've been doing a big push to get insulation in so I haven't had much time to respond. The boards are 1x6 T&G (3/4"x5.5") doug fir. Although I understand the concept of finding the "right" boards for a building, I can't afford to. $0.85/linear ft right now is a great deal, which I got for these boards. And I got to pick through the stack and reject the baddies. Micheal and Tyler, thanks for the very useful links. Based on past houses I've lived in, I can guess that the RH of the house will be between 25 and 85%. More or less the linear range. My plan is to sticker up the boards inside the house and let them dry for a few weeks before installation. I'm lucky that right now we are in between seasons, so the RH should be somewhere in the middle. I will test the boards with a moisture meter, I just don't know how I'd change the MC if it's not what I'm aiming for (11-12.5% ish). Worst case scenario is buckling, so I'd rather install it in a slightly higher average MC range rather than lower. I could not find a black air barrier that functions how I want it to. I asked about the Fronta Quattro and the company said not to install it inside because it relies on the differential air pressure of the exterior to move air through. We used Membraine. If there are gaps, we plan to go through and paint each gap with a paintbrush haha! It's only about 700 sq ft.
Corky,
There is no way you should be planning for a new house anywhere to fluctuate seasonally between 25% and 80%. You need to come up with a strategy (ventilation or dehumidification) to modulate the swings. Not just for the finishes, but for the longevity of the whole structure and the quality of the inside air.
Courtney, Doug fir doesn't seem bad. Compared to many hardwoods, its closer to eastern white pine in terms of green to oven dry shrinkage percentage (7.6-7.8).
At 5.5" wide and even using 12% EMC change, which is admittedly extreme though not unheard of, you're looking at about 0.18" change. Even with some uncertainty about where the limits are, I think you can make that work. Of course if you don't, you don't, and life goes on.
I doubt that wood will come to EMC with those RH extremes, even if they are met in your dwelling (more likely on the low end perhaps with a steady fire). With a tighter structure and as long as you don't open windows on all the really humid days, you will probably avoid the higher limits of RH, no less EMC.
If you want some insurance, you could seal the face of the boards, which does make a difference (shaves the peak EMC's; there's a great graph of this in Understand Wood, but I can't find it online).
The key is that you measure MC of the wood and install accordingly. If the MC is hovering around 14%, you install it pretty tight. There's not necessarily the need to dry it to the 'middle point', you just adjust your installation.
Is this wood kiln dried? It takes some time for wood to fully come to equilibrium, but it probably doesn't need to. If this is 'green wood' you should rethink the whole thing.
It seems like a non-issue at this point for you, but I'm pretty confused and interested why Fronta Quattra isn't advised for indoor use? That explanation seems a bit jumbled. It's an air barrier, therefore air should not pass through it at all, in either case. Perhaps a vapor thing? But even then it seems a bit odd.
Hey Tyler,
Awesome. I’ll just leave it inside the house for a while, measure moisture content, and install accordingly. Yes it is kiln dried thank goodness.
Regarding Fronta Quattro, no idea. Maybe it’s a sales thing? I also assumed that it would allow moisture to pass like tar paper, but their tech support person said it wasn’t a good idea.
Thanks so much for your help!
-Courtney