What's a good airtight ceiling material?

I am trying to figure out what to use for the ceiling in my small house. I am planning to attach a layer of rock wool board (Roxul Comfortboard) under the rafters, both to hold in cellulose insulation and as a thermal break. But obviously that won't be the finished ceiling so what goes under that?

I don't really want an all wood ceiling, like T+G boards. Lots of wood, not airtight, and I just don't really like the look.

One idea was to have 1x2s or 1x3s running vertically, which would hold up the Roxul and a layer of hardware cloth which I could plaster over. A good plaster job is a good air barrier. But as I thought through it, it seemed to get tricky. There are issues with both running the plaster over the strips and having it just butt up to the sides. The first would get pretty thick. The second would compromise the air-tightness, or I'd have to deal with finicky air fins. Also I'd worry about it getting bumped and cracked and dirty, since it's a low ceiling over a loft. And I was dreading all that upside-down plaster work and wrestling with hardware cloth, trying to get it flat.

So now I'm thinking about keeping the furring strips, with something like canvas stretched tight behind them. That would be really simple to install and would look nice. Does a heavy, tight fabric like canvas act as an air barrier? What if it is oiled, or painted? I'm not looking for a vapor barrier, just something to stop air leakage. Has anyone seen something like this done?

Any other ideas?

Anything that has breathability will leak air. Currently the only ways I know of make use of manmade materials (tyvek house wrap and spray foam insulation).

Painted, oiled or waxed canvas is going to need at least yearly maintenance to do any good at all.

For a Ceiling plaster can work very well but you don't use hardware cloth you use expanded metal lathe to provide the plaster something to cling too.

If you were to go into just about any Timber Framed house, you would notice wood between the rafters, this is because it has always worked.
You can paint wood, Wood can be caulked, it can be used in many different forms and it is pretty easy to use a router to make great designs that add to the beauty of a ceiling.

If you use wood lathe strips, you can plaster on top of them but the plaster will not adhere as well as if you used EML.

Plastering a ceiling is not super hard, it just takes time and if you don't do a lot of it, it will make your shoulders and back ache for a few days once your finished.
Thin ply wood works well for ceilings, doesn't take a lot of work to install, just trim with quarter round and apply caulk for an air-tight ceiling. It comes smooth, beaded or ribbed. Goes up like drywall. There are also plastic panels which make cleaning super easy.

Thanks Bryant,

I am trying to avoid plywood, plastic, foam, caulk, etc. I guess "man-made" materials in general, wherever possible. Otherwise plain old drywall would be an easy option.

Why do you say that oiled or waxed canvas will need yearly maintenance? It wouldn't be exposed to any sunlight, or rain, or... I would think if canvas was coated with linseed oil, or beeswax, or something, and installed inside, it would be pretty durable for what it was being asked to do.

And now I'm thinking about the danger of oil soaked rags. I wonder if canvas, brushed with oil and spread out flat on the ceiling with plenty of ventilation is a fire risk. Just curious, I probably wouldn't use oil anyway since I doubt I'd like the color. I'd have to experiment to see what colors and textures looked good. I wonder how shellac would work...

Hau Ben, I just got through checking with the Nicwax folks and they say "Canvas treated with Nicwax will holdup indoors where the temperatures do not get above 85 f".
Apparently temps above that and there could be some melting and slipping of the wax in the treated fabric.

Paint will dry out and begin to flake off canvas even indoors. I previously thought that Nicwax would need a similar touch up scenario.

I have a hat and slicker that are treated with Nicwax and they do pretty well for a season before I have to re-treat by washing them in new wax.

Oil cloth, is not pure oil on fabric, the oil is heated and paraffin wax is melted into it before this mixture is applied to the fabric you want water repellant.

By the way, any of the two treatments tend to make the fabric color darken some. Nice white canvas will turn a light gray or slightly yellowish with Nicwax.

Shellac should work really well and be quite durable for an interior ceiling, you could even put a colored dye on the canvas first then brush on a few coats for a different look.
If you did use dye, you have a whole new pallet of ideas for designs at your beck and call. I like that idea.

Good point on the heat. The ceiling might get quite warm being above a woodstove. I don't know anything about Nicwax, looks like it's a spray on product? I was thinking more about beeswax, or similar, thinned out in some kind of solvent, and brushed on. Kind of like what's used to seal the ends of logs.

Interesting idea with the dye under clear shellac. I'll probably aim to keep it fairly light colored since there won't be a ton of daylight upstairs. One nice things about the shellac is that I could avoid too much yellowing or darkening. But some colored accents could be fun. My girlfriend would enjoy playing with that.

Nicwax is a wash in product, if you buy waxed cotton items, it is most likely the product used in the treatment of the cloth.

When I was in the Navy I had to use slickers that were waxed, I don't know what the Navy used but it was pretty dang good stuff, didn't need retreatment unless you went through a few hurricanes weather wise.
I found the Nicwax product later on and I really like it, it is much easier to use (no brushing then rubbing).

You can get "blonde" shellac and it is nearly clear. (all shellac will darken with age, I know that from the guitars I build and French polish)

check here if you want to source some top quality shellac shellac shack

I recently built a passive house and dealt with extreme air sealing. I used zip system on the ceilings, it's osb with a green air barrier on one side that would go on the room side. You then use their special tape to seal the seams. I ran 2x2 furring strips on top of zip system to attach either t&g aspen or drywall. Electrical goes in the 2x2 gap, ceiling electrical boxes fit in this gap. Any penetration was sealed with prosoco joint and seam filler. I did blower door test and got to 0.7 ACH on the first pass. This method has been used many times on passive houses and really works. Certainly not the cheapest, but mine with 21+" of insulation gave me around R96, and no air leaks.

Wow Kevin, that must be a warm house! Is that in Durango? Sounds like a good system for you (I'm familiar with Zip) but not really the aesthetic I'm shooting for. Really trying to eliminate manufactured materials and anything off-gassy like OSB. The Roxul is one concession right now, since it's both a thermal break and forms the cavity for the roof insulation, and is non-toxic. But even that I would like to find a more natural solution for. Air sealing in general is tough. Hopefully my carpentry around windows and doors will be tight enough that I can avoid spray foam and get by with just a little caulk.

Ben, yes it's in Durango. The house is the most comfortable I have ever been in! I did the passive house training a couple of years ago and have been studying methods and materials since. It's tough to find materials that do the job that are not either offgassing or environmentally detrimental. The passive house group does not like spray foam because of the nasty blowing agents. Air sealing tapes or wet flashing products are standard. Foam, by the way, is not considered an effective air seal. Plywood and osb are not air tight also, I talked to a guy who used unfaced osb and couldn't get good blower door results, he found air was traveling through the osb, I used foam backer rod and prosoco air dam to seal around Windows. I installed Viega radiant heat system myself as well as the Zhender erv. 14" walls, 8" geofoam under slab, 6" geofoam under footers, 4" external wall foam to move dew point out of wall cavity to eliminate chance of condensation in cellulose. I have hydro shark 10 heating house till I get solar thermal going, boiler core is the size of a beer can. This is the first winter and the house performed better than I ever expected.

Sealing a Passivhaus (Germany) building's is entirely different than US (PHIUS) like apples and oranges different you can see in the WUFI models with regard to air flow & material mold. That is why PHIUS now exist, they disagreed and got a divorce in 2013. If one does not understand the difference it is dangerous, deadly, sealing in a sick home that an HRV/ERV won't cure. Just when you thought everything was comfortable and fine, well think again!!

Hi terry, I did my training with phius in 2013 and am familiar with the "divorce". I understood it to be about treating different climate zones differently, not just using northern Germany as the base model. Your point is well taken, I modeled my design using climate data for my city and ran 10 year simulations on walls and ceiling. That is when I had my "aha" moment in regards to dew point. With no foam on the exterior I was showing a good deal of moisture accumulation in the dense pack cellulose, which means mold and rot! 4" of external foam moved the dew point out of the wall cavity into the foam and the simulation showed no moisture accumulation over the 10 years.

In the training, we looked at several passive house failures, and I have since studied more of the building science. There are some critical things that most people miss or don't give enough attention to, like interior wall finishings. There was a single sentence in the training material that stated you could have only 3 coats of pain, ever! So 1 coat of primer and 2 finish coats is all you get, not very realistic in my opinion. That, surprisingly enough, didn't bother anyone but me! If the wall dries to the inside, and you seal the surface with too much paint, you get a vapor sandwich, which again means mold and rot. I decided to use American clay on most of the walls because it is very permiable and moisture regulating. Primers and paint were a different story, I spent months talking to all of the paint manufacturers about permeability of their paints and primers. I found nearly all had perm rating of 4 or less, compared to around 45 for drywall. (It was interesting that the paint mfgs were not asked about perm rating much). I found a primer (Roma) that could be tinted and used as final coat, and with sand added, was acceptable for primer for the clay. I think at this point, I think I have done a pretty decent job with addressing the building science part of the project. As they say, the devil is in the details, and time will tell.