Air, Thermal, Vapor & Water Control Layers in Wall Assembly

On the Mastering Building Science discussion board…

Allison A. Bailes III, Ph.D. answers student questions about air, thermal, vapor and water control layers in wall assembly.

Student 1:

I’m going to repeat what’s been said about the control layers in my own words, and ask a few questions for clarity:

There’s rain control, air control, vapor control, and thermal control. In a typical wall assembly we primarily use house wrap for the rain control layer (along with the finish), plywood/OSB for air control, and insulation for thermal control. Vapor is not typically stopped except under the slab, but it’s controlled in the sense that the wall assembly “breathes”. Does that sum it up?

The “perfect wall” assembly shows a “vapor barrier” as part of the control layers to the inside of the insulation for all climate zones. However, if it’s hot inside and cold outside then a dew point would occur within the thermal layer, regardless of whether or not there’s a vapor barrier (unless the insulation is closed cell) no? Elsewhere in the readings it says that vapor barriers aren’t used in walls except in extremely cold temperatures. I don’t see how a vapor barrier is ever effective anywhere if a dew point will occur in open cell insulation anyway. Do we just expect it to dry out since the insulation is outside the vapor barrier? If so then why are we bothering to stop vapor at all unless we’re controlling indoor humidity? Also, I don’t really see how sheathing stops air unless the joints are taped. And what exactly is “stopping air” anyway? It seems to me that a vapor barrier would stop air, not sheathing. Where am I going wrong here? This seems like it should only take a few sentences to explain clearly, yet I’m either reading mixed information or misinterpreting it.

Student 2:

This is exactly what has lead me to building with SIPs, because anywhere you have a mixed use climate the wall is created wrong for part of the year or temperature swings. But in a SIP the wall has bi-lateral symmetry or it works in both directions because it does not have a cavity… rather the whole wall is the vapor barrier. This brings to light the 2012 IECC code going into effect in states around us. It seems that 1 inch of foam on the outside of most wall make-ups just pushes the dewpoint into the middle of the wall. Am I doing something wrong? I would appreciate feedback on this.

Student 3:

I think you make some good points and have a good grasp of the concept . In the “perfect wall”, it is showing the vapor barrier between the rigid insulation and the cavity. A rule of thumb I hear is, for this to work in a cold climate the r-value of the outside insulation should be a minimum of 40% of the cavity. This helps to assure that a dew point can only be met on the outside of the vapor barrier and where moisture can’t occur in the rigid foam. A riskier version of this wall in a cold climate would have less insulation on the outside.

When you talk about dew point forming in the insulation, I would agree with you. Moisture can appear from dew point being met in the insulation unless it is closed cell foam or something similar like XPS. Does that have to do with the perm rating of the product?

It is also interesting that you talk about the sheathing as the air barrier. The majority of wall assemblies I see are 2 x6, R-19 batts, with a poly vapor barrier (plastic) on the inside. For these walls we talk about 2 different air barriers, the primary and secondary. The primary air barrier becomes the poly sheeting and the osb/plywood becomes the secondary. When the poly is sealed and backed with drywall it becomes almost air tight and if dew point is met it is supposed to be able to dry to the outside. Adding any more insulation to the exterior of this wall can be a risky situation.

If 1″ of rigid exterior insulation is added to this wall, the dew point can still be met in the cavity. If the dew point is met, moisture is then trapped by the interior poly sheething and the 1″ of exterior foam. The 1″ doesn’t have enough r-value to warm the cavity up enough to move the dew point to the exterior insulation.

Allison A. Bailes, III:

Student 1, you’ve hit on what is often the most confusing part about designing and building assemblies and enclosures that work. The control layers are the heart of the matter, and you’ve almost got it, I think.

Let’s start with the easy ones. The air control layer has to limit the amount of air crossing the building enclosure. It can be house wrap, taped sheathing, a liquid-applied membrane, spray foam, or something else. You have to choose materials that qualify as air barriers and put them in assemblies sealed well enough to prevent the air from moving through. Then the assemblies are put together as the enclosure, and all the connections and penetrations have to be sealed there, too.

The thermal control layer, insulation, needs to be as complete and continuous as you can make it. It needs to be installed well and in sufficient quantity to either meet code or reduce heat flow to the level you want it.

Water is where things get tricky. Liquid isn’t too hard, though. Keep it out by moving it down and away from the building. Sloped roofs, kickout flashing, gutters, downspouts, drainage planes, and grade sloped away from the foundation are all important for rain. Capillary breaks are necessary to keep liquid water from wicking up from the ground.

Water vapor is the tricky one, but the main rule here is not to trap it or let it condense inside an assembly. Remember water wants to move from wet areas to dry areas, so as long as you don’t put a vapor barrier in a place that stops it in a bad place, you’re OK. You also have to consider the ability of the materials in an assembly to store moisture.

Chapter 3 in Chris Timusk’s PhD thesis has one of the best explanations of what goes on with water in wood that I’ve seen anywhere. Bill Rose’s book, Water In Buildings, is also really good.

Joe Lstiburek’s paper on the “Perfect Wall” does indeed show vapor barriers in the middle of walls, but the walls are designed so that those vapor barriers are not condensing surfaces. They won’t be cool enough to start condensing water and causing problems. On the outside, the vented rain screen helps allow the wall to dry out.

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About Instructor Allison A. Bailes III, PhD – President, Energy Vanguard

Allison A. Bailes III, PhD, is founder and owner of Energy Vanguard in Decatur, Georgia. Like many in the field of building science and green building, he is multi-faceted: His academic credentials in physics (BS, MS, MST, and PhD all in that field) give him a solid foundation in the science that underlies buildings. Having taught physics at the high school and college levels, he’s adept at explaining technical concepts in a way that people new to green building can understand. In addition, he has practical, hands-on experience. He built a high-performance home out of structural insulated panels, doing much of the work himself, and ran a home performance contracting business. Numerous homes in the Atlanta area had their ducts sealed and crawl spaces encapsulated by Dr. Bailes himself. Between his first and second businesses in this field, he gained more green building experience by working as the regional manager for the EarthCraft House program in the Southeast. What Dr. Bailes has become most known for in recent years, though, is writing the Energy Vanguard blog. In it he covers everything from building science fundamentals to HVAC particulars to big-picture topics like energy security and peak oil. The blog has gained a wide readership in a short time and is often cited and linked to from other websites. In fact, it is the Energy Vanguard Blog that garnered Dr. Bailes an invitation to become a Green Building Advisor. As a result of his varied experiences and abilities, Dr. Bailes is a highly respected teacher, speaker, and writer, praised for his ability to turn what often comes across as dry and technical into something fun and understandable. As one of his readers commented, “Your blog is off the chain! Your writing is technical yet easily readable, such a rare combination.” He travels across North America, speaking, teaching, and leading workshops.