A challenging cathedral ceiling in a tiny house
I have a tricky question I’d like to pose to the experts…
I am building a tiny house (30′ long on a triple axle trailer). I recently tore out all of my blown in wool insulation in the ceiling due to condensation issues. (I found it just before I sealed up the ceiling…whew!) I mistakenly did not vent the ceiling – I am new to house building and this was an admitted newbie mistake. I have read all of the cathedral ceiling articles on this site but I need to make sure I do this right this time. As I mentioned I am hoping to use blown in wool insulation (it has most of the same properties as cellulose) The roofing is metal, with Grace Water and Ice shield on the deck. The rafters are 2×8’s. The roof is a shed roof style, pitch is 3/12. I am using T+G pine board panels on the ceiling. I have been consulting with an engineer about this roof and he recommends using permeable rafter vents but after hours of research I can find no such product on the market and actually no such mention of anyone doing this. The most common recommendation is to vapor barrier the ceiling, and use rafter vents, but I keep thinking that rafter vents could be a problem because they could still trap house humidity below, and a vapor barrier on the ceiling might actually cause some interior moisture issues (all of the living activity and cooking are in one space in a tiny house, plenty of humidity migrates upward).
I need additional advice in this situation. What do you think of the permeable rafter venting with NO interior vapor barrier? Or should I vapor barrier the ceiling, and put in a continuous (back wall to front wall) rafter venting such as Accuvent or Smart Baffles…with screened holes drilled into the rafter blocking?
Or site build the baffles as I have seen on this site with foam (more expense but I want to do the right thing)
If I did permeable baffles, the best I can come up with is using window screen tacked onto 1×2″s. Any other ideas? Or not a good idea?
If I did a ceiling vapor barrier – What product do you recommend?
Tricky tricky! Hope someone can help. We are currently living in this structure with no ceiling insulation and its chilly! Locality is Western WA state – cold and wet.
Appreciate any advice,
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Am I correct in reading that you currently live in this house and that you installed insulation in the cathedral ceiling but did not yet install an air barrier directly below the insulation and that you found condensation in the insulation and/or on the underside of the roof sheathing? The activities of daily living create quite a bit of interior water vapor which has easy access to the cold roof sheathing since there is no air barrier in place, and this may be the source of your condensation problem. If so, the solution is to install an air barrier as soon as possible after the insulation is installed. The tongue and groove pine boards that you are planning for your ceiling will not act as an air barrier, but you can install drywall first, which will act as an air barrier. It needs to be taped and spackled in order to seal all seams, and then install the t&g boards on top of that if you like. Avoid adding can lights to the ceiling. If you have any junction boxes for lighting or ceiling fans, be sure to air seal them with spray foam. Be sure to tape the drywall where it meets the walls, otherwise you will have a gap around the entire perimeter of the house where water vapor can enter your ceiling. Here is an article that describes the phenomenon that you may be experiencing: https://www.greenbuildingadvisor.com/article/condensation-wall-sheathing-construction
Here is an article that explains what an air barrier is and isn't (it's not the same as a vapor barrier): https://www.greenbuildingadvisor.com/green-basics/air-barriers What you need is an air barrier, not a vapor barrier.
It is possible to build a cathedral ceiling that is either vented or unvented. It's important to get the details right. According to this article https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling if you are going to use air permeable insulation (which you are) then venting is required. If you are going to vent, you should use ventilation baffles that are air tight because air permeable insulation should have an air barrier on all 6 sides (that includes the top, which would be the baffle.)) This article will tell you everything that you need to know in order to build your cathedral ceiling correctly.
Ironically (perhaps,) this article currently appears on the home page of GBA, as does your question: https://www.greenbuildingadvisor.com/article/still-fighting-the-same-battles-20-years-later
+1 on creating a very good air barrier on the interior side. Also monitor humidity (living space and inside the cavity).
Best to use baffles from something that is moisture resistant (because it is a condensing surface of interest) and has 5x+ more perms than the interior side (because you want more drying that wetting). One foam vent I just looked at is rated at 1.3 perms (so use < .26 perms on the interior side). DIY with unfaced EPS would be more permeable.
Thanks for the replies and the advice Adrienne and Jon,
As I suspected it seems the consensus is to air barrier the ceiling behind the pine boards - however drywall is not recommended in tiny houses partially because of the weight, but also because it can crack during transport. The air barrier needs to be a film of some sort - what do you recommend?
Also it seems I need to use baffles above the wool and drill holes into the rafter blocking to gain ventilation. I also have recessed can lights - do I really need to remove them?
Does this sound like the best solution? Any additional advice?
Re- posting this question to the community and still hoping for a bit of input - thanks to all.
Recently tore out my ceiling insulation due to this same issue - and then checked the walls - no good! I've got some work to do!
I used tongue and groove pine planks on the interior of my tiny house without air sealing the walls or ceiling- yes a mistake. The Tyvek against the exterior wall has a bit of condensation on it. I'm looking for recommendations on air sealing behind the T&G pine. I've seen info here on a product called Intello - is this the one?
Other mentions were about something called Membrain... how does this compare with Intello?
Remember - Can't use sheet rock in a tiny house... Locality is Western WA - cold and wet.
Thanks for the help,
I would use membrain, or intello (which I’m told is more durable, but I’ve never used it myself). These are “smart” vapor retarders, so no issues like you'd have if you used polyethylene. In a small space it won’t be too difficult to do a good job of detailing the membrane as an air barrier with tape.
I think I will use Intello since I have also heard it is more durable and I am using blown in wool insulation.
Thanks all for the replies:
I think I will go with Intello since I hear it is more durable than Membrain and I'm blowing in wool insulation. Walls are 2x4's : R14 - should be warm enough to prevent interior condensation.
Sheet rock in a tiny house is in general too heavy and tends to crack when the house is transported. (Tiny house on wheels)
Ceiling will be air barriered and vented: Im looking at SmartBaffle.
Thanks again for the excellent advice on this forum. I have to cancel out of my subscription as money is tight right now and I've got to save up to buy building materials!