Building Science-Continuing Education

In a few weeks, I will be teaching my first building science class during a continuing education contractor expo in my area.  We are expecting around 100 contractors to attend, my class is a one hour energy credit titled Air Barrier, Vapor Retarder, and Blower Door Testing.  This weeks blog posting is a preview of that class.  This blog is taken directly from my power point.

Building and energy codes are becoming more “science” based.  Having a basic understanding of building science is going to become more important in the future. Air barriers, vapor barriers and blower door testing are closely related in Minnesota.  It is common for the air barrier and vapor barriers to be combined.  There are some construction assemblies where they should be separated.  This can create a challenge in passing the blower door test.


  1. Rain Control Layer
  2. Air Control Layer
  3. Vapor Control Layer
  4. Thermal Control Layer

There are four building layers building scientist often talk about. They are listed in their order of importance. If we can’t stop exterior moisture from entering our structures, the rest of the barriers don’t mater. If we can’t stop interior moisture from entering our assemblies, insulation doesn’t matter. Today we are going to talk specifically about two of them, the air barrier and vapor barrier. Notice that the air barrier and vapor barrier are two different building shell layers-in Minnesota, they are often combined! There can also be other combinations, such as the thermal barrier can also act as both the vapor retarder and the air barrier…closed cell spray foam.


The purpose of the air control layer or air barrier is to control the movement of air both inward and outward through the building shell.

2012 IRC-R402.4-R4.2.5

“The building thermal envelope shall be constructed to limit air leakage…the building or dwelling unit shall be tested and verified as having an air leakage rate of not exceeding…3 air changes per hour in zones 3 through 8. Testing shall be conducted with a blower door at a pressure of 0.2 inches w.g. (50 Pascals).”

The air barrier is the second largest potential source of moisture within the building assemblies, the rain control layer or weather restive barrier (WRB) is the largest. The blower door test has been code required for all new residential construction in Minnesota since 2015.
What materials act as air barriers?

My blower door testing equipment.


Polyethylene sheeting-commonly used as both the air barrier and vapor barrier. Careful attention must be made to assure proper sealing (many insulation contractors in Minnesota have become very good at using poly as the air barrier).
Closed cell spray foam-Closed cell spray foam-minimum of 2 ½ inches to meet perm rating. We will discuss this in further detail when we get to the vapor barrier.
House wrap or weather resistive barrier (WRB)-Tyvek or Typar are common trade names. Similar to poly, careful attention must be made to assure proper sealing. 
Self adhered membranes (WRB)-great products for air sealing but hard to work with and more expensive than WRB’s. Similar to working with ice and water roof protection. Proper perm rating is critical.
Fluid applied membranes (WRB)-can be used as the WRB or just used in part to seal transitions and corners. Some builders are using fluid applied for window detailing.

Fluid applied membrane on Zip Sheeting.

Wood sheeting (OSB or plywood)-using tapes or fluid applied systems to seal seams in the exterior sheeting. Most builders are using some sort of wood sheeting – OSB or plywood. Adding a sealant to the framing before installation is a cheap way to increase the performance of the air barrier. A system called Zip Sheeting has the WRB manufactured into the OSB.)
Drywall-similar to wood sheeting, adding caulking or a gasket material to the framing before installing drywall is a common method, called the air-tight drywall method.
Aerobarrier system-a new comer to air sealing, an vaporized caulking finding air leaks while under a positive pressure within the building. A good option for production builders, multi-family units and high-performance houses.
Gaskets, seals, caulking and canned spray foam-some of the cheapest options, sometimes used while a blower door is running.


Air leakage-# 2 source of moisture problems in the building shell, a larger problem than vapor diffusion. #1 is exterior rain and snow events.

-Air leakage damage can sometimes be confused with exterior bulk water damage. Attic by-pass may cause frost to form in roofs. Melting in the spring is often misidentified as an issue with the roofing.

-Leaky houses are often uncomfortable houses. I’ve tested many houses where the occupants complain of being too hot or cold in areas of the home. An example is I conducted an energy audit at a home several years ago where the occupants would store cases of fresh fruit along the outside wall of their home because temperatures where maintained at 35-40 degrees when the outside temperature was below 0.

-Air leaks in and out of poorly sealed homes. Were is the air leaking into the home coming from? Through insulations, building materials such as OSB, carpets and manufactured flooring, glues and adhesives, this is the “fresh air” we are breathing.

-Notice that the cost of air leakage in increased heating and cooling is last on the list. It is very hard to sell air sealing to a customer, something that they will never see. Granite countertops are a much easier sell. Countertops are often changed every few decades, air sealing is typically for the life of the structure. It’s easiest to accomplish good air sealing while building the home, much harder after the fact.

The air control layer and “tightness”. What is tight?

-The IRC requires no more than 3 ACH50 in Minnesota. IRC R402.4-402.5. This requirement is while the blower door is running. 3ACH50 is the equivalent of approximately .2 air changes naturally, or the entire volume of air within the structure is exchanged with outside air every 5 hours or almost 5 times per day. This is considered “tight”.

-The average home I test is 2.25 ACH50.

-The Canadian R-2000 standard is 1.5 ACH50.

-Passive House standard is .6 ACH50. Passive House is a German based construction standard adopted in the United States. A very low energy structure built to a rigorous building standard.

-Tightest house I’ve seen is .35 ACH50-framing stage during Aerobarrier application.  Aerobarrier is a new air sealing technology that was introduced at the beginning of 2018. The Center for the Energy and the Environment in Minnesota (CEE) was one of two areas in the United States where this technology was tested.

-Tightest verified house in the world .05ACH50 or 5 CFM50.  Tightest house leaks 5 CFM50, average house I test leaks 600-1200 CFM50, depending on volume of the home. It’s rare I see anything other than code minimum construction techniques with regards to the air barrier.

Vapor control layer

The purpose of the vapor control layer or vapor retarder is to limit the amount of moisture that can enter a wall or ceiling by way of water diffusion.

Vapor diffusion is the third highest source of moisture in our building assemblies.  Common in Minnesota for polyethylene sheeting to be used as the vapor retarder. Other products may also satisfy code requirements, but require approval from the AHJ.

2012 IRC R702.7 Vapor Retarders.

A Class I or II vapor retarder is required on the interior side of frame walls in Climate Zones 6 and 7. Class II vapor retarders are permitted only when specified on the construction documents.

Code will allow either a class I or II vapor retarder on the warm in winter side of the framed walls. Minnesota code requirements for a class II vapor retarders have been amended by the state. The change requires class II vapor retarders to be permitted only when specified on the construction documents. Basically, you need approval from the AHJ before a Class II can be used.

2012 IRC R702.7.1 Class III Vapor Retarders.

Class III vapor retarders shall be permitted where any one of the conditions in Table R702.7.1 is met.
Insulated sheathing with R-value greater than 10 over 2 x 4 wall.
Insulated sheathing with R-value greater than 15 over 2 x 6 wall.

Class three vapor retarders are allowed when exterior insulation is used. R-10 or more using a 2 x 4 wall, or R-15 or more with 2 x 6 wall framing.
There are a few exceptions for vapor retarders, such as in R806.5 unvented attic and unvented enclosed rafter assemblies, this code deals with insulating unvented roof assemblies.

Class I, II, III vapor retarders

I’m not a fan of how the code is listing product types to illustrate the class of vapor retarder (the 2012 IRC does not list the perm ratings identifying with the class of retarder). There are many products that can be used as a vapor retarder, not just the ones listed in the code. Be sure to discuss any choices that are not listed in the code book with your building official. Most building inspectors I have talked with are open to other products, but may need more information to approve their use.

Class I vapor retarder.1 perm or less
Polyethylene sheeting or unperforated aluminum foil

A class I vapor retarder is considered vapor impermeable. Polyethylene has a perm rating of .04-.06 perms.

Class II vapor retarder.1 perm to 1 perm
Kraft faced batts

Class II vapor retarders-In my opinion, are what we should be using in our wall assemblies. Slightly more vapor open, drying potential to the interior with enough retarding properties during the winter. Some of the best class II vapor retarders can change their permeance based on moisture within the assembly. Kraft faced batts are the original, but new option such as Certainteed’s Membrane, Pro Clima’s Intello, or Siga Majrex are also great variable perm vapor retarders. Again, get approval from the AHJ before using some of the newer products.

CertainTeed’s Membrain

Class III vapor retarder1 perm to 10 perm
Latex or enamel paint

Class III vapor retarders can only be using in specific conditions, such as when insulation is added on the exterior of the building.

Class II and Class III are considered vapor retarders, but usually not particularly good air barriers.

Perm ratings

According to Wikipedia, permeance is the degree to which a material admits a flow of matter or energy. With regards to building materials, permeance is moisture’s ability to move through these materials.

Perm rating is defined as 1 grain of water passing through one square foot of material in 1 hour with a vapor pressure difference of 1 inch of mercury between the warm and cold side of a material.

Perms-7000 grains is equal to 1 pound. Inch of mercury is the pressure exerted by a column of mercury at the standard acceleration of gravity, simply, a unit of measurement for pressure. One inch is equal to around 3380 Pascals, depending on temperature. As a comparison, a blower door test is conducted at 50 Pascals. Perm ratings of materials are tested using either the wet-cup or dry-cup test. All testing is performed by manufacturers or independent testing facilities under controlled conditions.

The perm rating of a material will not only affect how an assembly might become wet, it also affects the ability of the material to dry. It’s important that our wall and roof assemblies have the ability to dry in at least one direction, better if they dry both in and out. When we choose to use polyethylene sheeting, the drying potential is only outward. This becomes a problem if the exterior of the building also has a higher perm rating. Vapor drive in the winter is from inside the building envelope to the outside, and in the summer, the drive is from outside to the inside.

Listed perm ratings of building materials

Vinyl siding                           40 perm-due to the air leakage of the joints
Wood siding                          10 perm-due to the air leakage of the joints
OSB sheathing                      Approximately 2 perm
Plywood sheathing             Approximately 10 perm
House wrap                          5 – 50 perm
XPS 2 inch                             Less than 1 perm
Foil faced polyiso                Less than .1 perm
Polyethylene sheeting       .05 perm
Raw drywall                        40 perm

Source of perm ratings-Building Science Corporation.

Most building materials will have a published perm rating. According to Joseph Lstiburek of Building Science Corporation, the “sweet spot” for the permeance on the exterior of the assembly is between 10 and 20 perms. This perm rating is outward of the wall sheathing.

Passing a blower door test without poly

Code requires 3 or less air changes per hour at 50 Pascals (with the blower door running). We’ve chosen an assembly that requires us not to use poly because of the perm rating or insulation location. This changes the air sealing strategy. Other methods used to air seal a structure…

Exterior air sealing
-Sealing the sheathing to the framing using caulking, foam and gaskets.
-Taping sheathing seams
-Choosing a house wrap or WRB that makes a good air barrier.
-Fluid applied WRB’s

All air sealing should include the exterior of the building, even when using poly!

Air-tight drywall method
-Using caulking and gaskets to seal the drywall to the framing.
-Requires the use of air-tight electrical boxes.
-Careful attention must be made at all penetrations.

Air tight drywall method changes the air barrier duties from the insulator to the drywall installers. There will be a learning curve!

Smart vapor retarders
-There are three vapor retarders with class II perm ratings.
-Class II vapor retarders may require approval.

Smart vapor retarders can also be used as an air barrier. Three products I am aware of: Certainteed’s Membrain, Pro Clima’s Intello and Siga’s Marjex. Be sure to talk to the building official before using a class II vapor retarder.

Multiple blower door tests, air sealing while the blower door is running
-Traditional techniques of air sealing.
-Typically used by weatherization crews and high performance builders.

Multiple blower door testing is typically used when building high-performance houses.

Closed cell spray foam
-Insulation and air sealing in one application.
-Dependent on insulation contractors knowledge and experience.

Closed cell spray foam applied to at least 2.5 inches qualifies as both the air barrier and vapor retarder, but 2.5 inches only has an R-value of around 17. 3 inches of closed cell supplies the required R-21 for wall insulation in our climate.

Aerobarrier system
-An air sealing method launched in 2018.
-Works great with multi-family construction and production builders.
-Aerosol caulking mist released in a structure under positive pressure.

The Aerobarrier system is very similar to the Aeroseal duct sealing process. Aeroseal has been around for over 20 years. Disadvantage is the cost. Approximately $1 per square foot. I believe this process will become more common in the future.

Misters used to disperse caulking fog inside structure during Aerobarrier application.

Critical air sealing locations

-Basement slab-basement wall intersection
-Basement wall-mudsill connection
-Mudsill-rim joist connection
-Rim joist-subfloor connection
-Floor/subfloor-wall connection
-Top plate-vertical interior finish connection (typically drywall)
-Top plate-ceiling connection (typically drywall)                                                   -Windows and doors                                                                                                      -Penetrations through building plates and assemblies

There are many different techniques and methods for air sealing a building. For the best performance and durability, multiple air sealing applications at each critical location will work best. An example is using both sill seal and acoustical caulking at the floor/wall connection.
There can be air leakage in below-grade assemblies.  Both concrete and soil can be porous.  Drain tiles and basement sumps that are daylighted are common basement leak points.

My advice for air and vapor control layers

Air control
-All air sealing should begin on the exterior of the structure.
-Plan all air sealing strategies before construction begins.
-Set a goal for ACH50. Try to better yourself on each building you build.

Air sealing should begin during planning the building and executed during the whole construction process. All subs need to be involved.
The “breathe” argument for assembly drying is a poor choice and building codes now require building tightness levels to be achieved. A better statement is all buildings need the ability to dry.
Code requires blower door testing on all new homes constructed after 2015. Working as an energy auditor, I have been called to new homes because of comfort or high bill complaint issues and asked for the blower door test result. Testing was never completed.

Vapor control
-Know how the structure and assemblies are going to dry. “All buildings need to breathe” is a poor drying potential choice.
-Know the perm ratings of the materials you are using.

Vapor retarders are needed in our climate. The class of retarder is dependent on the assembly.                                                                           Always consult with the AHJ before using a new or different vapor retarding product.
It’s our responsibility as builders to understand the drying potential of the assemblies we are using. It’s also important for the homeowner to understand how they live in the structure can affect the structure itself. Knowing the humidity level of the home, use HRV, bath and vented kitchen fans, and maintaining the equipment are just as important as how the building is built. 

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