Sometimes I blog about things I see that bother me and sometimes I pick topics that people ask me about all the time.  Today, it’s both.  The use of hybrid foam-fiberglass insulation in walls is something that people ask me a lot.  Other people don’t ask and just do things that scare me.

When I talk to people about residential construction, I try to make things as simple as possible.  Simple is easy to remember, easy to get right, easy to explain.  Sadly, not everything is as simple as we want it to be.  So, I’m going to try to make this as simple as I can and put the real geek-out material in footnotes.  I apologize in advance that I can’t make it as simple as “Should I use foam in walls? Yes or no:”

When I talk about hybrid foam-fiberglass insulation in walls, it’s helpful to first define what I mean.  Really I’m talking about two different walls systems which have minor differences, but for the points I’m going to make here, aren’t that different.  The first is the “foam sheathing” scenario, where you build a typical stud frame wall, put insulated foam sheathing on the outside (with or without an additional layer of OSB sheathing), and insulate with fiberglass or cellulose between the studs.  The second is the “flash and batt” scenario, where you build a typical stud wall, “flash” with a layer of closed-cell1 foam, and then add a batt on the inside of it.

foam sheathing wall section
flash and batt wall section

These wall section sketches have a critical similarity.  Most of their components are relatively permeable to water vapor, but they contain a foam layer that is much less permeable.  Permeability is a continuum – materials aren’t totally permeable or totally impermeable, but for the sake of making this discussion simple I’m going to treat these less permeable components as impermeable.2

Something else that we should remember is that there are actually two ways that moisture gets into walls – the first is through vapor diffusion and the second is when it gets carried along with airflow that is infiltrating or exfiltrating through the wall.  When we air seal walls, we drastically reduce the latter.  When we talk about how permeable to water vapor our building materials are, we’re talking about the former.

So, what’s the potential problem?  In the winter, it’s warmer and more humid indoors than it is outdoors. Why?  Because we breathe, shower, and cook.  The dog pants, and some of us do stupid things with our humidifiers.  And because warm air can hold more moisture than cold air can, and it’s warmer inside.  When you have a cold surface exposed to warmer, more humid conditions, you can get condensation on that surface.  If you want to see this in action, check the outside of your iced tea glass in mid-July.3

With the two wall sections above, if conditions include some inappropriate combination of too humid inside, too cold outside, or too thin a layer of foam, then the temperature where I’ve labeled a “possible condensation plane?” with a red star gets too cold and you can have condensation.  So, this is the point where the fans of keeping it simple usually stop me and say, “OK, I’m never using foam in my wall.”  Which is kind of a shame, and not really my intended point at all.  It’s just not that simple.

Imagine what happens in a wall without foam.  If it’s 10 degrees outside and 70 degrees inside, then somewhere inside your wall (which is made up entirely of components that are permeable to water vapor), you will have temperatures that are cold enough for condensation to occur.  The only difference is that the wall without foam can dry to both the inside and the outside.  And, perhaps there’s a perceived safety in doing things the same way that we’ve been doing them for a long time.

If we do use foam, the thicker the foam layer, the warmer its inside surface is.  And, if the temperature at the potential condensation plane (where I’ve drawn the red star) is high enough, then we won’t have condensation at all.  That sounds like a more moisture-robust wall to me.  How warm does it have to be?  Warmer than the dew point of the indoor air.4

If we were going to geek out about this, here’s where we would do a calculation, and that calculation would tell us whether condensation will happen in the wall.  To do that calculation we would need to know several things – one of which is a total guess:

  • How humid is it inside?  (total guess)
  • What’s the temperature outside? (judgment call)
  • What’s the temperature inside? (probably around 70 degrees)
  • What are the R-values of the foam, the fiberglass, and the other components of the wall?

The question of how humid it is inside is a total guess.  Most of the time I don’t even know who will be living in the house, whether they will ultimately go against my advice and install a humidifier, and if so what setting they will use.  If it’s a new, tight house the indoor relative humidity will result from a complex interaction between occupant habits, bath and kitchen fan functionality, ventilation strategy, climate and house airtightness.  My own house maintains a relative humidity in the low 40s in the winter with no humidifier.  That’s too high – we’re working on it.

Architects routinely recommend 40% relative humidity indoors for the sake of wood work.  I would personally advocate for a lower number.  What that number should be depends on how cold your climate is and what your walls are made of, and is a great topic of discussion for another time.5  All I know is that if you try to keep your house at 50% in a climate that experiences actual winter, you’re crazy and if you have problems it’s your fault.  I’d feel a lot better if you kept it at 30% or below.  And I think it’s more likely that a lot of people are going to use 40% no matter what I say.  So I tend to assume 40% relative humidity, not because I like it or know that’s what it will be, but it’s my best guess.

The outdoor temperature to use is also a little more complicated than it seems.  You could use the winter design temperature, the average daily temperature in January, the average low temperature in January, or some other number.  The winter design temperature (14ºF in Asheville) is conservative – it only gets colder than that 1% of the time.  The average January daily temperature (about 37ºF) or low temperature (about 28ºF) are both higher and using those will result in less foam being necessary.  These numbers are somewhat defensible, since it takes a while for walls to change temperature.  It really all just depends how conservative you want to be.  When it comes to moisture in other people’s walls, I like to be conservative.

Once you decide all of this, you would do the math.6  Instead, to keep it simple, I’ll just provide a few of examples.  Or, if you really hate numbers, skip the bullets and scroll down to my super-conservative rule of thumb (unless you live in a really cold climate, in which case you should do your own math – sorry).

  • Flash and batt #1:  2×6 wall with 2 inches of foam (R-14) and an R-13 fiberglass batt.  Assuming 70 degrees and 40% relative humidity inside, 14 degrees outside, you need to keep the potential condensing plane above 44 degrees to avoid condensation in the wall.  If you do the math, you get about 42 degrees at the potential condensing plane, which means that it’s practically never going to be a problem in Asheville.  This wall can handle just about anything Asheville weather can throw at it.
  • Flash and batt #2:  2×6 wall with ½ inch of foam (R-3.5) and an R-19 batt.  If it’s 14 degrees outside and 70 degrees inside, the temperature of the condensing plane is now 23 degrees.  We would need to keep the indoors at 17% relative humidity to avoid condensation.  If we wanted to keep the indoors at 40% relative humidity, condensation could occur when the outdoor temperature fell below about 40 degrees.  In Asheville, we sometimes spend long periods of time below 40 degrees, so this wall scares me.
  • Foam sheathing #1:  2×4 wall with two-inch (R-10) exterior sheathing and R-13 cavity insulation.  If it’s 14 degrees outside and 70 degrees inside, the temperature of the condensing plane is about 38 degrees, and you get no condensation if the indoors is below 31% relative humidity.  If we have 40% relative humidity indoors, condensation can happen below 26 degrees outdoors.  Not as conservative as the first example, but I can live with it.
  • Foam sheathing #2:  2×6 wall with one-inch (R-5) exterior sheathing and R-19 cavity insulation.  At 14 degrees outside and 70 degrees inside, the potential condensing plane is going to be about 26 degrees.  To keep the indoor dew point above 26 degrees, the indoor relative humidity would need to be 19% or lower.  How likely is that?  And if the homeowner instead keeps the indoor relative humidity at 40%, the inside surface of the foam will be below the dew point of the indoor air whenever it’s less than about 38 degrees outside.  That’s OK if we’re designing for the average daily temperature in January, but you need to have good indoor humidity control to make this work.

All this leads me to what I like to call my “super-conservative rule of thumb”.  If you really don’t want to do the math and want something easy to remember, it’s hard to go wrong if you make the R-values of the foam and the fiberglass equal (or close to equal).  Flash and batt with R-14 (2 inch) foam and an R-13 batt works great in a 2×6 wall and accomplishes this.  Exterior 2 inch foam board (R-10) with an R-13 batt is also close enough.  Sure, a crazy person can still cause problems with a humidifier, but in that case a standard wall section is toast too.

1 Closed cell and open cell foam are more accurately referred to by their density, but the guy who sells it to you will call them “closed” or “open” cell.  Read more if you want.

2 There’s a lot to read about the relative permeability of various building materials, and data on where materials fall on the continuum.  Google it, or here are some links:

3 You can geek out about condensation and the psychrometric chart work if you want to.

4 Check out this cool calculator:

5 What should indoor relative humidity be in the winter?  I’d start with this article.

6 This book will walk you through the calculation and gives examples.  They use the monthly average outdoor temperature.