Insulation Question

[fishspike57]

I have a pole shed and plan to insulate this summer. I wrapped the shed in Tyvek. So here is my question. If I spray one inch of foam and finish off with unfaced batts, cover that with a vapor barrier. Where does the moisture go that might make its way into the wall? Trying to figure this out.

Thanks

 

[Copymutt]

It releases out thru the Tyvek.
No, DuPont™ Tyvek® is not a vapor barrier. It is made with unique material science to keep air and bulk water out while allowing moisture vapor inside walls to escape.
The foam is a vapor barrier so I would not trap moisture between it and another vapor barrier.
There are different schools of thought on which side to place the V.B. Mostly depending on typical relative humidity. High humidity your foam is best.

 

[stm317]

Tyvek is a moisture barrier, but not a vapor barrier. It's designed to keep the bulk of the water from entering your wall cavity, but it's still permeable enough for moisture vapor to pass through. It's great that you have it, but it doesn't really have an impact on a discussion about vapor barriers.

If your spray foam is closed cell, and 2" thick or more, then it will act as your vapor barrier and no other vapor barrier should be used. In this case, the wall would dry to the interior of your building. If the foam is open cell, or less than 2" thick, then a vapor barrier to the inside may be necessary. In this case, the water vapor should be able to dry to the outside by passing through the foam and tyvek. I'm not sure I'd trust the moisture to effectively pass through ~ 1" of spray foam very well, so I'd either go with at least 2" of thickness, or skip the spray foam all together.

Here's a decent report on the different types of foam and when a vapor barrier should be used:
https://buildingscience.com/documents/reports/rr-0912-spray-polyurethane-foam-need-for-vapor-retarders-in-above-grade-walls/view

 

[yeldogt]

The Tyvek is on the other side of the wall? Outside on the sheathing ...

The foam will be inside ....you have to use closed cell for flash and batt. Also 1" is not going to be enough ..... you need enough to be over the dew point .. that's typically 2" or a little more.

The fiberglass goes over that -- no VB. The foam creates the air barrier and enough stops condensation ... foam walls don't work the same. you still have to manage inside air.

without enough foam -- you will have problems.

 

[fishspike57]

Thanks was getting hung up on visqueen on the inside. I will just use unfaced insulation on the inside

 

[yeldogt]

Yes -- don't use plastic with Foam.. you must, must, must have the foam thick enough. If the foam is not thick enough you will get condensation on the foam under the fiberglass.

In some areas you can now do performance ratings -- in reality -- the batts are not doing very much .. and its just to meet the current R code. Some places understand this.

Spend a bit more for the proper foam and you will have a lifetime of savings w/o any problems

 

[DC73]

Also 1" is not going to be enough ..... you need enough to be over the dew point .. that's typically 2" or a little more.

fishspike57 - pay particular attention to what yeldogt is telling you here if you want to avoid condensation issues.

You are near the border of climate zones 6 & 7. For proper condensation control, the closed cell spray foam amount needs to be 35% of the total wall r-value for Zone 6 and 45% for Zone 7. At a minimum, you are going to need somewhere between R7.5 and R15 of closed cell foam depending on the thickness of the walls and whether you decide to use Zone 6 or Zone 7 recommendations.

Here's a link to one article that explains it: BuildingScience.com

See the wall chart (Table 2) near the end of the article. Table 1 is for roof assemblies. See Figure 8 for a description of the flash and batt method you are proposing. Also, note that in Table 2, the top line for each climate zone is for a 2x4 wall and the bottom line is for a 2x6 wall.

DC

 

[Voi]

You are near the border of climate zones 6 & 7. For proper condensation control, the closed cell spray foam amount needs to be 35% of the total wall r-value for Zone 6 and 45% for Zone 7. At a minimum, you are going to need somewhere between R7.5 and R15 of closed cell foam depending on the thickness of the walls and whether you decide to use Zone 6 or Zone 7 recommendations.

I have also heard the argument that in a shop that will be generally be kept cooler & have more moisture than a home that the ratio should increase even more in favor of foam.

 

[fishspike57]

Wow you guys are good thank you��

 

[anythingyoucanimagine]

Lots of really good info here. I believe the holy grail right now is still 2-3" of spray foam with mineral wool inside that. So would be exterior siding, moisture barrier (tyvek, zip, etc.), sheathing, 2-3" of foam (look, there are charts for your climate zone) then mineral wool/rock wool/roxul, etc. and finally drywall. The combo of drywall primer and latex paint on the inside is the barrier. Nothing else needed.


When I did my garage I did the walls first. It did very little (heat rises). I didn't do any foam, just mostly blown-in and some of whatever I had laying around (roxul, pink r-13, etc.). Just walls only being insulated was a little noticeable but honestly very little. Ceiling/roof was where the big gains were.

This is a great link for the underside of a roof deck:

https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling

I'm new, if it won't let me post links then just Google:
"green building advisor insulating a cathedral ceiling" (from November 8, 2011) It'll come right up.

 

[fishspike57]

Got it thanks