One question I get asked a lot is how to deal with vapor, what is the best vapor control product, etc… Before I get into vapor control, lets look at the four control layers and their order of importance:

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

The first three all deal with moisture. (There is always some moisture in air.)  The order of importance is based on how much moisture the control layer sees.  Of course rain and snow will produce the most moisture, and present the highest risk to a structure.  This is the reason the water control layer will always be on the exterior of the building, we want to keep as much water as possible out of the buildings assemblies.

The air and vapor layers can be either inside or outside, or maybe both.  In the case of the vapor control layer, the location will be determined based on your climate.  In a heating dominated region such as mine, the vapor control layer will always be on the warm in winter side of an assembly.  The air control can be either or both.  Air transportation of moisture is almost always more important to control.  This is the reason you want to build a “tight” structure.

So, what is the best way to control issues with interior vapor, either moving in air or through building materials?  These are in no particular order and the importance will most likely be based on the design of the structure. First, control the amount of moisture in the structure.  Second, control the amount of air moving through the building assemblies.  Third, use a product that will limit vapor movement through building materials, I recommend a smart vapor control product.  But probably the best way to control vapor is to not give it a surface to condense on.  In other words, keep the first condensing surface above the dew point temperature.  It’s funny how the least important control layer, thermal, can be used to solve for vapor.  Use exterior insulation.  If enough is used, there is no longer the need for a smart vapor control product.  We can switch to a simple and inexpensive vapor control, painted drywall.

Part of the reason for this blog post, besides the fact that I’m asked about vapor often, is I’ve been involved in several recent building diagnostic investigations where water and ice had formed on the exterior cladding during the heating season.  There are several reasons for this accumulation of wintertime moisture, all could have been prevented by the use of exterior insulation.

In my opinion, these are the best ways to control interior vapor concerns, at least in my climate.  That being said, I would like to hear your thoughts on the subject.  Please leave me a comment!

4 Replies to “Shorts-Vapor”

  1. You are right about the exterior foam insulation being the best solution to the migration of vapor from the inside to the outside, with some caveats – some being regional:
    That there isn’t an existing vapor barrier on the interior or that there isn’t a barrier to the small amount of drying that may need to happen to the interior. For instance my research is showing an interior smart vapor barrier would be okay or perhaps some types of paint.
    For design consideration I have noted that XPS sheets for instance expand and contract as much as 5/8th an inch per 4 X 8 sheet. This means lapping the layers and in the case of complex roofs, it means considering if foam will fill the junctions of angles adequately or whether poly can be used in conjunction with insulation layers.
    In my area, fire danger has been increasingly a design factor. Most insulation boards burn with harmful vapors and with vigor not seen in other insulation types, except for mineral wool board, which is expensive.
    While screws are made to penetrate thick insulation board. They come at a price and they take skill and labor time to apply.
    For walls, insulation board appears to make the design grade more often than roofs. With my home having early nineties construction – inner poly vapor barrier, another choice is soft mineral wool batts over the sheathing, instead of foam board and since this a non-structural type of accessory the codes allow some leeway in technique and detailing. How the cladding (metal shingles in my case) attached to outer nailing surface becomes an interesting detail worthy of another discussion.
    Thanks for the blogs and design insights. I have found them both interesting and educational.

  2. I know for soft insulation batts to go over the exterior wall sheathing that this is called a rainscreen and from what I get from your blog and my recent researching is that there are two ways of keeping the sheathing above condensation temperatures, one is with foamboard and the other is with an outer truss system, the actual rainscreen part of the system is the detail that keeps the cladding or nail base for the cladding dry when raw water happens to penetrate the outer most weather barrier. The detail that has been interesting me is the barrier that touches the exterior cold side of the insulation.
    In my area – high desert, temperature swings in late summer can be more than 60 degrees and what I have never seen written about or discussed is that condensation happens on the first cold surface that occurs outside of the insulation. If condensation were to occur in a wall or roof, I think it would be better for it to happen on a barrier of WRB, where it can hopefully remain until dries than an organic surface like flakeboard, where it might remain and absorb. What do you think?

    1. Hi Chris, Thanks for the comments! I’m involved in a monthly webinar sponsored by Rockwool that I think you might find interesting. Here’s the link:
      Last month was dedicated to fire, especially wild fire concerns. Unfortunately I was having technical difficulties and missed being on that episode. Rockwool’s ComfortBoard 80 is a great product in fire prone areas.

      Personally, I avoid specifying foam products as exterior insulation if I can help it. I know cost is often the biggest factor for energy upgrades, but I feel it’s easier to sell durability, comfort and health than energy cost savings. I think you hit the nail on the head when you mentioned “regional” being a big factor. My buddy Ben Bogie, who is in the New England area is a big fan of double wall construction, I’m an exterior insulation fan and for others, the Larson Truss is an option. All work well when designed and installed correctly. You are absolutely correct in the assessment of your home, exterior foam insulation can be a problem with low perm interior vapor retarders like poly. This exterior foam insulation with interior vapor retarder can work, it’s used all the time in Canada, but I worry enough that I won’t design this assembly for a project I’m involved in.

      Something you might not be aware of. Some manufacturer’s of building products will do analysis of assemblies, like WUFI, to make sure the assembly is safe in a specific climate. Some manufacturers have their own building scientists on staff just for this reason. I don’t know how much residential work they end up with, my guess is they are utilized more for commercial projects, but it’s a service they provide if using their products. I know both Rockwool and Siga have done analysis for my projects.

      Your question on condensation and the first condensing surface. It’s climate based and time of year based. In the typical home in Minnesota during the heating season, the first condensing surface is usually the inside facing of the exterior wall or roof sheeting. In hot and humid climates during the cooling season, it might be the backside of the drywall on the inside of the wall. Time is the other factor, if the assembly becomes wet and dries completely before the next wetting cycle, no issues. It’s when it gets wet and stays wet where we see problems. Some of the data I’ve seen for my climate is late in the heating season, the wall sheeting may become damp, wetting was occurring over the long winter and has not dried, but as the temps rise during the spring and summer, the walls dry and the cycle begins again the following winter. Ben Bogie likes to monitor his projects using sensors placed in the walls during the build. He has a blog post on that subject on the Green Building Advisor website, might be behind a pay wall if you do not subscribe.

      Good luck on your project!

      1. Thanks for the information and links. I am now joining GBA again, and I will look at the rockwool link shortly. I will be using mineral wool batts in my upcoming projects. I found the Comfortboard 80 to be pricey but the batts should fit my budget.
        Going back over our discussion, if those cases where you found damage due to condensation would have had a weather wrap or barrier over the insulation and a air gap or even better a ventilating space between that wrap and the sheathing I’m guessing the damage would have been non-existent or lessened?

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