Any problems with unfaced insulation on exterior walls?

[RNeumann]

Hard to find a recommended wall assembly with plastic on the inside of the wall-
Understanding Vapor Barriers | Building Science Corporation

Funny some would say you should “Always” have one!

 

[Tractor Seabee]

I worked on modular buildings going to the pacific islands. Engineered process was vapor barrier on the exterior of the wall to prevent the AC from sucking outside moisture into the walls. The dialog above is right on the point. It depends on the climatic conditions. In Alaska we put the roof insulation on top of the roof membrane not under it. same thing all relation to dew points. HVAC Engineers are supposed to be experts in this arena because of their concerns with heat and moisture transfers. HVAC affects how all this works also. No HVAC, everything equalizes.

Ron

 

[TMGT]

I have a hatred for fiberglass insulation although it is typically the lowest cost per R value. When I was researching to redo my attic I looked at a lot of different options, one thing I found, as has been stated here, is that what you should do is very region specific.
I finally decided to go with Roxul comfort batt, not the cheapest option but very happy with it, also is able to be dried out and reused in the event it gets wet. Also has a higher R-value for inch.

 

[GirlWhoWantsTractor]

Just reinforcing what others said. Houses need to "breathe," especially in humid climates. Moisture will get in regardless; make sure there is a way for it to get back out.

Is this a wooden or concrete block structure, and how old is it? Does it have any kind of attic or crawlspace? Do you run A/C? How is it heated?

Since this is just a hunting shack, are you mainly trying to stay warm in winter, or cool in summer? Since you're in north-central FL winters can be a bit chilly. Generally speaking, to stay warmer insulating the ceiling will do the most good, plus making sure doors/windows are tight; to stay cooler, avoiding solar heat gain should be your first goal in FL.

 

[Storm56]

Rayfromtexas makes a very good point in his text regarding location of op and high humidity levels. My location is NE PA. For what it is worth, I did not want the inside moisture barrier but code forced me into it. But no problems in 11 years now so it must be ok in PA. Florida might be another matter. What is code in Florida?

 

[shooterdon]

Region matters!!! And in your case a vapor barrier is not needed. No cold exterior.

In cold climates, only a fool will not install it. It is important to insure that either no moisture permeates the insulation, or you use some from of vapor and moisture resistant insulation on the cold side of a fiberglass (or any other permeable) stud wall insulation that has a high enough R value to insure the dew point does not occur within the stud wall cavity where a batt type product is installed....or you will have problems.

It is simple to understand. Moisture will condense out of the air when it reaches the dew point. As the temperature drops, humid air will lose its water content. It that happens in a studded wall, the material used to insulate the wall will get wet. Unless the moisture DOES NOT migrate through the warm side of the enclosure. Thus the need for a vapor barrier. In a cold climate, the interior wall will be at say 70* and the exterior at say 0*. There is a temperature gradient between those surfaces where the dew point will be reached and water condenses out.

In my earlier years I worked for an insulation manufacturer in Canada in corporate engineering, R&D and manufacturing, so I have some experience.

A vapor barrier is not needed in every climate in the US so be careful who you listen too. Every region of Canada and the northern US requires it. Check your building codes to be sure.

 

[RayfromTX]

Region matters!!! And in your case a vapor barrier is not needed. No cold exterior.

In cold climates, only a fool will not install it. It is important to insure that either no moisture permeates the insulation, or you use some from of vapor and moisture resistant insulation on the cold side of a fiberglass (or any other permeable) stud wall insulation that has a high enough R value to insure the dew point does not occur within the stud wall cavity where a batt type product is installed....or you will have problems.

It is simple to understand. Moisture will condense out of the air when it reaches the dew point. As the temperature drops, humid air will lose its water content. It that happens in a studded wall, the material used to insulate the wall will get wet. Unless the moisture DOES NOT migrate through the warm side of the enclosure. Thus the need for a vapor barrier. In a cold climate, the interior wall will be at say 70* and the exterior at say 0*. There is a temperature gradient between those surfaces where the dew point will be reached and water condenses out.

In my earlier years I worked for an insulation manufacturer in Canada in corporate engineering, R&D and manufacturing, so I have some experience.

A vapor barrier is not needed in every climate in the US so be careful who you listen too. Every region of Canada and the northern US requires it. Check your building codes to be sure.

A vapor barrier is not required if the first condensing surface is maintained above the dew point of the interior air. This can be accomplished by placing enough or all of the insulation outboard of the wall sheathing. How much insulation can go in the wall cavity is dependent on climate.

There are problems associated with cavity insulation. If you use a poly vapor barrier on the interior face of the wall to control winter moisture then you are at risk in mixed climates with humid summers when the vapor is going the other way. Many buildings in areas like Michigan, Minnesota and Massachusetts have experienced mold and decay from summer driven moisture from the exterior. Many factors play into whether this will occur.

Cavity insulation also creates a condition where moisture moves back and forth with the season which can cause substantial movement in framing members. Headers above windows will will expand in width on the exterior in winter and on the interior in the summer. Those cracks you sometimes see in the drywall at the corners of the windows are not caused by the foundation settling in most cases. There are other issues such as water pipes in exterior walls freezing and many others.

I do not insulate my buildings in the stud cavities. I place all of my insulation of whatever type outside of the wall which also eliminates thermal bridging of the framing members and protects the entire structure and the air and moisture control layers from thermal degradation. No condensation can occur in the walls of any building built in this manner.

 

[shooterdon]

Ray,
That only works if the exterior wall insulation is also a vapor barrier like a closed cell foam. My experience indicates that mold is rarely caused by summer driven moisture from the exterior due to dew point issues. Most it caused by poor sealing/flashing/etc and rain water entering, or leaks in pipes.

In my area, I can have a maximum temperature gradient of up to 100* in the winter (70 in the house, -30 outside). In the summer, it is about 25* (75 in the house, 100 outside) But I do not use AC, so my house temperature gets over 75 if a heat wave lasts for more than a day. It is the extreme difference in outside vs inside temperatures that drives the effect of dew point condensation when dealing with humid air. Most of the time, at least here in MI, there is a long period in the winter where that difference will be about 40* for months at a time. In the summer, a difference of 15* may last for a few weeks and the small amount of condensation that is formed will evaporate if it can escape. It is one reason, that house wrap is permeable...to allow water vapor that may be in the stud walls to exit the house.

Where I have humidity issues is the basement; in spite of having foam boards on the foundation side of the stud walls and a VP under the flooring. I use a commercial dehumidifier to address it.

In any case, what we think is immaterial when building a house or a structure that must pass inspection. Local building codes must be met. Our opinions can address how we build other structures, and to address the OP I think we both can agree he is fine with unfaced insulation for his project.

 

[PILOON]

When I built 20 years ago I had purposely left out the vapor barrier in a 3-4 ft section for a proposed later event.

Temps in winter go as low as -25 or so around here.
Now my walls were 6" of wool insulation and at one point I accessed that 3-4 ft area only to find 5 inches or so of almost solid ice! Granted there was lots of humidity due to plastering and often wet wood etc and, LOL wet boots.

It however was quite an eye opener.

 

[RayfromTX]

Ray,
That only works if the exterior wall insulation is also a vapor barrier like a closed cell foam. My experience indicates that mold is rarely caused by summer driven moisture from the exterior due to dew point issues. Most it caused by poor sealing/flashing/etc and rain water entering, or leaks in pipes.

In my area, I can have a maximum temperature gradient of up to 100* in the winter (70 in the house, -30 outside). In the summer, it is about 25* (75 in the house, 100 outside) But I do not use AC, so my house temperature gets over 75 if a heat wave lasts for more than a day. It is the extreme difference in outside vs inside temperatures that drives the effect of dew point condensation when dealing with humid air. Most of the time, at least here in MI, there is a long period in the winter where that difference will be about 40* for months at a time. In the summer, a difference of 15* may last for a few weeks and the small amount of condensation that is formed will evaporate if it can escape. It is one reason, that house wrap is permeable...to allow water vapor that may be in the stud walls to exit the house.

Where I have humidity issues is the basement; in spite of having foam boards on the foundation side of the stud walls and a VP under the flooring. I use a commercial dehumidifier to address it.

In any case, what we think is immaterial when building a house or a structure that must pass inspection. Local building codes must be met. Our opinions can address how we build other structures, and to address the OP I think we both can agree he is fine with unfaced insulation for his project.

Exterior insulation does not require a vapor barrier though I prefer to use a mod bit membrane that is also my air barrier and an effective way to seal to windows and other penetrations. Felt paper or house wraps can be used but it is better to use a membrane. I am interested to hear why you think a vapor barrier is required.

The delta T that you are referencing in your second paragraph fails to account for solar driven vapor drives. If you have a cladding that stores rain water then the sun will warm that masonry and drying will occur towards the exterior and the interior. The space behind the masonry cladding can acquire dew points that are literally off the psychrometric charts. Imagine the stone or brick is warmed to 140 degrees f. The vapor pressure produced will be quite high. You can measure dew points in excess of 100 degrees f which is a condition that can lead to accelerated vapor drives. I have taken apart walls with masonry cladding facing west with no overhangs and found substantial mold growth in less than 6 years. One wall in particular was covered in #30 felt and contained fiberglass cavity fill insulation. The interior finish was a faux finish that utilized a poly based glaze as the final coat. Water vapor was driven through the wall by solar heating and convected up through the fiberglass insulation and was drawn through to the cool surface of the drywall. The fiberglass wall formed a convective loop in it's fibers which is a common phenomenon. The mold on the back of the drywall was heaviest at the top of the wall and diminished lower down as it was deposited on the drywall as it sank down the cool inner face of the wall. So much water collected that when I removed a switch plate, water ran out. Solar heating of claddings with hydric storage capacity makes your statement incorrect in certain conditions which was why I said "Many factors play into whether this will occur."

Building codes allow for exterior insulation. The code books are a better reference for this than the statements made by building inspectors. I have educated many of our local code officials who know what is allowed relative to common local practices but rarely know the code book by heart. All of us will keep learning every day. When I finally retire I will still be learning how to improve on how we do things.