Building Science – Building Shell Layers – Vapor control layer

Last week we talked about the what makes a house “tight”, the air control layer.  This week we will continue with the building shell layers discussion, number 3 on our list is the vapor control layer or vapor retarder.

The four layers, excluding the structural layer, are:

1. The rain control layer or WRB
2. The air control layer
3. The vapor control layer
4. The thermal control layer

The purpose of the vapor control layer is to limit the amount of moisture that enters the wall cavity and ceiling through diffusion.  This outward moisture drive during a Minnesota winter occurs because of temperature and humidity differences between inside and outside the structure.  So why is it only number 3 on our list?  Because air leaks contribute to much more moisture loading inside wall and ceiling assemblies than diffusion.  I have read some articles  suggesting 98% moisture loading from air leaks and 2% from diffusion.  I don’t think the spread is quite that high for our climate, but the fact remains, air leaks cause more problems.

Polyethylene sheet on warm                        in winter side of wall assembly.

Building codes quantify the rate of diffusion by an assembly or product’s permeance, or perm.

One perm 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.

Building scientist have known about the diffusion of water through building components since the 1940’s, but it wasn’t until the last few decades that we seriously started testing different building products for their permeance. One of the lowest perm ratings for a building material is polyethylene sheeting at .04-.06 perms for 4-6 mil poly.  In comparison, raw drywall has a perm rating of 40.  Here is a list of the perm ratings for some common 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 3-10 perm
House wrap                        5 – 50 perm
XPS 2 inch                          Less than 1 perm
Foil faced polyiso           Less than .1 perm
Poly sheeting                  .05 perm
Raw drywall                      40 perm

Let’s look at what the 2012 IRC, the current building code in Minnesota, has to say about vapor retarders:

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.

The state of Minnesota slightly changed the code requirements regarding class II vapor retarders over the 2012 IRC codes.  The way I’m interpreting this code change is a class II will need to be specified and approved before installation.

There are 3 classes of vapor retarders.

Class I – .1 perm or less. Polyethylene sheeting or unperforated aluminum foil.
Class II – .1 perm to 1 perm. Kraft faced fiberglass batts, vapor retarder primer.
Class III – 1 perm to 10 perms. Latex paint, 5 inches of open cell spray foam.

I think most of us in Minnesota associate the vapor control layer (and air control layer) with polyethylene sheeting on the warm in winter side of our building assemblies.  Minnesota is one of the few areas in the country still using poly as a vapor retarder.  We can get away with its use because of our high heating requirements and our relatively low cooling demand.  Vapor drive during the summer is from outside to the inside.  If a building is air conditioned and there is a high exterior temperature and humidity level compared to temperature and humidity level of the interior, moisture can accumulate on the inner cavity side of the poly.  This is the reason most of the country is no longer using poly.

A hole cut in both the vapor and air control layer by an HVAC contractor.

The code says we can use either a class 1 or class 2 vapor retarders. (Technically, class 1 is vapor impermeable, or a vapor barrier and both class 2 and class 3 are vapor semi-impermeable or vapor retarders. Permeability above class 3, 10 perms or more is considered vapor open.)  Polyethylene, kraft faced fiberglass insulation, smart vapor retarders such as Certainteed’s MemBrain, 2 ½ inches of closed cell spray foam and vapor retarding primer properly installed all qualify and are used throughout the country as class 1 or 2 vapor retarders.  Be sure to check with the authority having jurisdiction (building inspector), to get approval for alternative vapor retarders. Some of the listed products are not common in Minnesota and some building officials might not be aware of their perm ratings and uses.

When is a class 3 vapor retarder allowed? When you move your thermal layer (insulation) to the exterior of the building, as long as the proper ratio of exterior and interior insulations are installed. This ratio is different in wall and roof assemblies.  In my opinion, this will be the way most buildings will be insulated in the future.  I will dedicate a future blog posting to insulating the exterior of a building.  Dr. Joseph Lstiburek, one of the top building science professionals in the country, has the best quote for insulating the exterior of our buildings.  “Would you rather wear the jacket or eat the jacket.”  Of course we living in a cold climate know its best to wear the jacket!

OK, we’ve talked about the first three layers of the building shell, all dealing with moisture and air. The last layer, the thermal control layer, can also affect moisture and the rate at which it dries. Until next week…

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