Insulating in Hot Humid geo areas - Be AWARE

Installing fiberglass rolls/ batts, blown into ceiling cellulose, whatever....Most folks think they need a Low Permeability Vapor Barrier facing into their building.

Often, that is like 4 mil Poly.

In Steel/Wood pole barn buildings many of us get extra wide rolls of various thickness fiberglass that is clad from factory w "Plastic" or "Vinyl" facing to inside.

In recent years, a change has been discovered by Scientists that study "Building Envelope" design.

I'ts all about preventing condensation and other problems.

Relatively few know about this change at this time.. those that study and are knowledgeable know.

Bottom line in rough terms... about 80% of the 48 state area is exempt from the following info...

BUT... the approx 20% of the 48 state area considered "Hot/ Humid" have found to need different installs.

You can find geo maps that relate to this on the Interweb.

Below is a very very basic bottom line Tech sheet From Lamtec, the largest company in US to make facings for wide fiberglass batt rolls, typically used in
Steel and Wood Post frame buildings, like the Shops most of us build.

It also applies to living structures, just plain homes, in this Hot/ Humid area.

So here's the deal - And PLEASE - Do not take my word for this - Do YOUR due diligence and Google "Vapor Barrier Hot Humid"

Like go to ORNL, the scientific study lab for the US government.

Look at "Building Envelope Vapor Barrier"" at that site and others.

Read Engineering type articles, not some fluff **** like at Houzz or whatever.

You will find - In the coldest areas, Vapor barrier is definitely in toward interior as done for years.

In the approx 20% of country Hot Humid, Vapor barrier is to the OUTSIDE of wall.

In the middle like 50% ballpark of the country, it's still being studied.

READ and LEARN.

Do NOT expect your insulation supplier to know this., they are usually behind the times .

In the approx 20% in the "hot humid" areas, you need to order "Perforated High Perm" facing on your rolls.. and if you are blowing.. make sure whatever is between your shop/ home interior and the insul ALLOWS Water vapor and Air to freely pass through.

Again, this is not "my" idea - It is common knowledge, but only w those that stay current.

https://www.lamtec.com/technical-bulletins/vapor-retarder-location/

Thanks, I am in the SE Tx hot humid zone so it is relevant to my building interests.

The use of plastic should always raise a red flag -- plastic prevents drying. If you use a plastic VB and one of the new roof underlayments -- you now have a closed space w/o any way for that are to breath. Same with some of the newest rain screens.

The key is allowing a space to dry --- That's why spray foam works so well. With traditional building products.

Vapor pressure makes moist air seek out dry air.
In the north, warm moist air is inside, cool dry air is outside. A vapor barrier on the inside of the wall keeps the moisture in. If some moisture gets in the wall, it dries out.
In the south, warm moist air is outside and cool dry air is inside. A vapor barrier inside will make moist outside air condense in the wall. It will not dry out with high moisture levels outside. Without a vapor barrier on the inside, it will dry in.

I recently moved from pretty much desert climate to the south. Dealing with moisture and mold has a very eye opening experience. Frankly a lot of times I just do not understand because I lived my whole life in a desert.

Around here making sure things either stay dry or can be aired out is vital.

yeldogt said:

That's why spray foam works so well. With traditional building products.

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Closed cell, specifically, for roof decks, right?

Tullugeon said:

I recently moved from pretty much desert climate to the south. Dealing with moisture and mold has a very eye opening experience. Frankly a lot of times I just do not understand because I lived my whole life in a desert.

Around here making sure things either stay dry or can be aired out is vital.

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This is why we suggest that you put a general location in your profile. It helps your fellow members give you the best advice based on where you live. What works in one part of the country may not or will not work in another...not to mention, that all GJ members don't live in North America.

Nothing new here. They were drilling proper VB location into our heads in energy management school back in the seventies.

readhead said:

Nothing new here. They were drilling proper VB location into our heads in energy management school back in the seventies.

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What guidance is there for locations in the transition areas? Areas where it is some times high humidity hot on the outside but other times it is high humidity on the inside? My comfort and health seems to go for about 50% year round in the dwelling. Seems like closed cell foam pretty much covers it but I am just now looking into building a barndominium in central KY.

Some years in central KY are 70-90 percent relative humidity all summer long but last winter was brutally subfreezing for several months and below zero for several weeks.

readhead said:

Nothing new here. They were drilling proper VB location into our heads in energy management school back in the seventies.

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That could be, but most still are not aware of this.

Even in the Building Trades.

Marc

RaisedonaDeere - US Maps re - Vapor barrier


https://www.google.com/search?q=vap...9IPbAhVknuAKHRKfDlIQ_AUICigB&biw=1229&bih=608

aunsafe2015 said:

Closed cell, specifically, for roof decks, right?

Click to expand...

I live in Coastal NC and I"m a big fan of open cell spray foam especially under a roof deck. A leak will drip right thru the foam and show up as a water stain or a wet spot as soon as it happens.

With closed cell foam the roof could leak for years before it is noticed. Since it is not permeable water can run on the outside of the foam and keep the structure damp, causing rot and mold.

The other problem with it comes from poor installation. If the foam is not perfectly bonded to the framing, condensation can happen in the voids in the summer. At least with open cell this condensation can pass thru the insulation instead of being trapped.

Build it tight and ventilate it right!

So I ordered this for my 24x36 non a/c detached garage in Central Florida. Did I order the wrong thing? How should I of insulated it? Building not built yet. Spray foam is a no-go. Will void warranty.

https://www.lamtec.com/our-products/wmp-vr-r-plus/

Astross - It is unclear to me, I don't know how it's spec'd other than - just call them and MAKE SURE it is "Perforated" High permeable to use in FL.

If they give you **** like "No, but too late Charlie, you already ordered it"... Tell Them "BS - YOU should have had the brains to advise me when you saw it was going to FL"

Marc

The vapor barrier goes on the warm, wet side of the wall. Always has. That was being taught in engineering school in the '80s, and it was old news then.