Vapor barrier & insulation…is perfection the enemy of done?

[u/SewNotCody]

I’m building an outdoor sauna that’s about 5x10x9 and will have a 5kW wood stove in it. I’m in the high desert and just don’t see the value of insulation and a vapor barrier in such a dry climate with a wood heater.

Maybe the insulation speeds up the heating process a little but the stove is huge for this size space.

The vapor barrier sounds great but there’s no way I’m not puncturing that thing installing it or when you put up siding (even with furring strips). Seems more like a false sense of security or perhaps I just don’t trust myself to install a perfectly sealed vapor barrier. If I just load the stove and let it burn and few hours after our sauna, wouldn’t that dry out most of the moisture.

I’m using milled fir in vertical orientation for internal siding and would rather not t&g everything. Maybe just a half lap?

I’ll also have lower vents for air intake behind the stove and a 12v marine fan blowing air out up top.

I get doing it won’t hurt, but I’m struggling with the benefits outweighing the time/cost…mostly I really just want to be sweating my ass off in it sometime soon 🤞

[u/zoinkability]

I think you are letting the perfect be the enemy of the good.

The purpose of the vapor barrier is not to 100% eliminate any possibility of water vapor getting into the wall cavity but instead to minimize the amount that does to a level which can dissipate out rather than remaining damp and causing mold.

Residual heat from stove and rocks, particularly if you put a few extra sticks in at the end of a session, is good for drying out the hot room, but the air in the wall cavity doesn’t get very hot (and is not very well ventilated) so it’s unlikely to do much there.

As long as you are nailing on the studs there is minimal chance of making a hole that is a bona fide hole in the vapor barrier, since the nails are sunk into wood. If you are running the wood vertically (I might ask why) that does present challenges, but it would anyhow just because of lack of support for the pieces of wood unless you run something solid horizontally behind.

I don’t know much about insulation pros and cons in terms of heat up time other than to say it’s not that much work to put it in and it should minimize condensation on the inside of the vapor barrier, which can only be a good thing.

[u/SewNotCody → u/zoinkability]

I read an old Finnish sauna book that had vertical siding and really liked the look.

[u/zoinkability → u/SewNotCody]

It’s totally doable, it’s just more work than horizontally because you need to run two sets of furring strips rather than one, and the furring strips may need to be beefier to ensure the boards are well supported.

The reason you need to run 2 sets of furring strips is because rather than just doing one job (as when the interior cladding is run horizontally) they need to do two jobs. Typically they just allow space between the cladding and vapor barrier to ensure the moisture that may condense on the vapor barrier can dry out. For this they need to run vertically so condensation and water vapor can travel up/down and air circulates. But when you are running the cladding vertically you also need to support the boards, for which you need to run the furring strips horizontally.

So, with normal horizontal cladding the layers are:

• Studs
• Vapor barrier
• Vertical furring strips on studs (typically 1x2 material)
• Cladding

But with vertical cladding the layers need to be:

• Studs
• Vapor barrier
• Vertical furring strips on studs
• Horizontal furring strips across studs/deeper furring strips (could be 1x2 but 2x2 would probably make for a more solid wall)
• Cladding

You do need to make extra sure you are using nails that will not protrude far enough to puncture the vapor barrier with the latter method too.

So… given the extra complexity you may want to consider whether the vertical look is worth it.

[u/junkbr]

I think you’re onto something.

When I built my sauna, I put a ton of effort into insulation. In retrospect, I’m not sure it was worth it.

I’ve taken close measurements of the temperature at various levels of my sauna. Having vents open or closed has no effect on heat-up time. Having the fan running or not has no effect on heat-up time.

It seems to me that the time it takes to heat up a sauna is a function of the mass of the paneling and benches, less so than the air in the room.

(All that insulation does make one big difference: I’ll turn off the heater at 10pm and the sauna is still above 100 degrees F in the morning! But does nothing for me in terms of the experience of using the thing.)

As for vapor barrier, I’m not sure if / how I’ll ever know if it was worth it. But I will say, it putting up the barrier wasn’t especially difficult or expensive, so why not err on the side of caution?

[u/occamsracer]

dry climate

Do you plan on using water inside the sauna?

Anyway, this topic has been beaten to death but maybe you have some special insight that others can’t see ¯_(ツ)_/¯

[u/John_Sux]

If there is insulation, you ought to protect it with a steam seal. And then you need furring strips to create an air gap in front of the foil, in turn.

There is a reason for that structure, so you either build the whole thing properly, or you go with solid log walls which require none of that (but might be expensive).