# Dual vapor barriers?



## Nealtw (Jun 22, 2017)

This is what I would do. 

Tyvek
Sheathing
fiberglass to fit no paper

2" foam sealed and taped.
Cedar.


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## tdisss (Aug 22, 2019)

What about?

Siding
Foam
Tyvek 
Sheathing 
Fiberglass
Foil reflected vapor Barrier
Cedar


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## Bud9051 (Nov 11, 2015)

You need to avoid the double vb even though 2" of rigid foam is actually a vapor retarder. It will limit any drying to the outside sufficiently so moisture problems are likely. See Neal's suggestion.

There are applications that shift the rigid to the exterior, but you are building a sauna so it must be vapor sealed on the inside and allowed to dry to the outside.

Another product to consider would be Roxul mineral wool.

How much of this shed is already built, limiting what can be suggested?

Bud


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## Nealtw (Jun 22, 2017)

tdisss said:


> What about?
> 
> Siding
> Foam
> ...


He already has siding tyvek and sheeting, he has other threads about the shed.


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## Photobug (Jun 25, 2017)

Bud9051 said:


> You need to avoid the double vb even though 2" of rigid foam is actually a vapor retarder. It will limit any drying to the outside sufficiently so moisture problems are likely. See Neal's suggestion.
> 
> There are applications that shift the rigid to the exterior, but you are building a sauna so it must be vapor sealed on the inside and allowed to dry to the outside.
> 
> ...


The shed it built. I have siding on the rest of the shed which is 20 feet long. The last 5 feet of the shed is the sauna. I am still waiting on the window and door for the sauna to finish this and put the siding on. Right now there is just sheeting and Tyvek on the sauna. The rough in wiring is done and i have started to insulate it. I have already cut up a couple sheets of the foam for and installed it on three bays. I wanted to super insulate it but not if it will cause a moisture problem.

Only the sauna portion of shed will be insulated. I could pull out the foam I already installed and just fill the bays with fiberglass.

It will be:

Cedar siding
Tyvek
Sheeting
R-19 fiberglass bats with paper face
Foil Vapor Barrier liner
Cedar tongue and groove lining the sauna.


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## Bud9051 (Nov 11, 2015)

Thinking about this some more, being a sauna you are dealing with an extreme amount of moisture and you probably need a better vapor barrier than listed. Anything stapled up will leak air.

Adding a layer of rigid to the inside could serve a dual purpose, better moisture barrier and better insulation.

I don't see the rigid listed in your last post and if you go without it the foil vapor barrier needs to have every staple and seam taped with a top rated tape. Neal would know the brands. Not all tapes will last over time.

Bud


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## carpdad (Oct 11, 2010)

Is insulation necessary? One that I built for my parents was electric element type and the heat was intense enough without the insulation. Small interior size also kept the heat.


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## Nealtw (Jun 22, 2017)

Bud9051 said:


> Thinking about this some more, being a sauna you are dealing with an extreme amount of moisture and you probably need a better vapor barrier than listed. Anything stapled up will leak air.
> 
> Adding a layer of rigid to the inside could serve a dual purpose, better moisture barrier and better insulation.
> 
> ...


 I can't imagine Tuck tape failing. Up here all the edges would be sealed be sealed with acoustic sealer but the one we use is not available down there so i don't know what you would use. Some one told me Quad, but I have not seen it used. 



Are we talking ceiling venting and insulation too, or will that be common with the rest of the shed.


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## Bud9051 (Nov 11, 2015)

Tuck tape would be fine just not packing tape or similar. Not an application for a no-name brand.

Bud


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## Nealtw (Jun 22, 2017)

When they build these in a basement they add a bath fan to run for a time when you are done to clear out the moisture.


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## Marson (Jan 26, 2018)

I'm a Finn from Minnesota, and I've been taking saunas for 60 years. I've built 6 of them.



It's a common misperception that saunas have high humidity. They are actually quite dry environments. That's because when you heat the air up to 150 to 180 degrees F, it has tremendous moisture holding capacity, and the amount of moisture you add by throwing water on the rocks is actually pretty inconsequential. 



We used to spread wet wash clothes out on the bench after a sauna, and they were always dry in the morning. I have a single pane window in my sauna, and even in the coldest weather there isn't condensation on it. I also have mounted speakers in my saunas, and they have lasted for years with no moisture damage. Saunas are dry environments!



I actually doubt a double vapor barrier is a concern here, but why risk it at all? Either go with rigid foam or go with batts and vapor barrier. No point in mixing it. 



I really question the value of super insulating a sauna. The best sauna IMO is one with a tolerable air temp and very hot rocks, so the blast of heat you get from throwing rocks is what gives you that pleasurable sauna feeling (loyly in Finnish). Now you can and should open your window to vent that heat of your supeinsulated sauna out, but then why bother with the superinsulation in the first place? Put another way, the sauna is ready when the rocks are hot. The amount of insulation in your walls is not really going to affect how quickly the rocks get hot.


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## Bud9051 (Nov 11, 2015)

Hi Marson,
Even a low RH number at 150° is not really dry. Cool that air to 70° and you will have condensation. *Reference*. 

I agree that super insulation is probably overkill and unnecessary but air sealing is a high priority. Even a 1" layer of rigid over the inside would make it much easier to seal that room.

Bud


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