Energsmart Insulation Thoughts on Flash and Batt Hybrid Systems

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By: John Bartlo

Energsmart and our partners often field questions regarding flash and batt (FAB) or so-called hybrid insulation systems.  The general concept with FAB is to install a thin layer of closed cell spray polyurethane foam (CCSPF) to the exterior plywood or sheathing to act as an air barrier and then install a fiberglass batt in the cavity to reach the required R-value.  The purpose of this paper is to explain why we’re seeing this so much lately and what you should know before using it in your home. 

The reason we see this method used is because fiberglass is cheap, but it’s a terrible air barrier.  The best of both worlds is to combine spray foam and fiberglass, right?  The best answer is “sometimes”.  Before using such a method, we urge you to understand some of the science involved. 

Western New York is a mixed climate, but has many more heating days than cooling days.  As such, for a majority of the year, moisture will want to move from the inside of the home to the outside.  Because of this, a poly vapor barrier is typically installed on the inside of the insulation when fiberglass is used.  Since a FAB system has a vapor barrier on the outside of the insulation layer (the CCSPF), its concept is backward to begin with for our relatively cold climate.

We started seeing these FAB systems from competitors around 2008.  In 2009 we started to see failures.  A thin layer of foam, sometimes only a half inch thick, was installed on the exterior side along with a fiberglass batt to the interior side of the wall cavity.  The point at which the foam and fiberglass met (interface point) was so cold that moisture vapor condensed on the foam and completely soaked the cavity. 

To lessen the risk associated with the FAB system, enough closed cell foam must be installed to prevent the temperature on the interior facing surface of the foam from reaching the dew point (the temperature at which moisture vapor condenses into water).  The temperature of the inside surface of the foam must be above the dew point temperature, or liquid water will form.  Without getting into a big confusing mathematical calculation, the magic number for walls in the Buffalo area is 2” of CCSPF (R-13) and an R-13 fiberglass batt. The interface temp in this situation is halfway between the interior and exterior temperatures.  For those of us in WNY, that’s about 50 degrees in the middle of winter.  With typical humidity, the dewpoint is in the mid-40s, so it’s not cold enough for water to condense.  If, instead, only an inch (R-6.8) of foam was installed with an R-19 batt, then the interface temp would be more like 35, which would make it rain in the cavity.  Note that this is all because moisture vapor easily moves throughout the fiberglass, but once it hits the foam it has nowhere to go.     

So now we get to what Energsmart calls R-value roulette.  2” of CCSPF and R-13 performs fine under normal circumstances, but what if things change?  What if it’s zero degrees for a month straight? Or you start heating your house to 80 degrees?  Or your wife/husband decide to turn your house into a greenhouse?  All of these things change the interface temp and the humidity in your home.  If you instead install 3” of CCSPF with no fiberglass, all of these potential issues would go away, and it doesn’t cost much more.  You could also install open cell foam with no fiberglass, which would be cheaper, and you wouldn’t have this moisture risk.    

At the end of the day, putting in less than 2” of CCSPF carries a risk that is not worth the money you’ll save on insulating your home.  Some contractors who install a thin coat of CCSPF claim there is no moisture risk because they install a second vapor barrier (plastic sheeting) on the interior facing side of the fiberglass, thus sandwiching the fiberglass with vapor retarders on both sides.  This is allowed by most building inspectors, but is a disaster waiting to happen.  In theory, it would work fine if the barrier was perfect, but in reality, that’s not possible.  Plastic sheeting isn’t made to fit your wall so it has to be cut to size and pieced together.  Unless every one of these joints is taped, it provides a path for moisture.  The plastic is stapled to the framing of the house creating more gaps.  Finally, plugs and switches on the wall provide gaping holes in the plastic.  It may not seem like much, but a simple 1”diameter hole has the ability to let 50 cups of water into a wall system per year!  Once in the wall it will stay there because the laws of physics will drive toward the relatively cool outer wall.  If the foam is thick enough in the FAB system you will have no problem, but if it’s too thin, you could be in for a lot of costly headaches.  We have firsthand knowledge of this in our region (see the “Problems We Fix” area of our picture gallery).     

Let’s turn to attics.  This is actually a good place to use a hybrid system since Building Code R-values for attics have gotten so high.  An attic in Zone 5 (Buffalo) typically has R-49 as a requirement.  Sometimes R-38 can be used.  With either number, using all foam can get expensive.  Using a hybrid system is safe, less expensive, and almost as efficient. 

For insulation install to an attic floor, it’s simple because the vapor barrier is on the proper side.  Spray foam (open or closed) is used as a base layer and can be installed to any depth, even just a thin air seal.  The additional Rs can be made up with inexpensive fiberglass or cellulose. 

When installing to a roofline, we still need to be cognizant of dewpoint and moisture concerns.  Generally, a little more than half of the Rs should be spray foam, with the remaining Rs taking the form of a kraft faced fiberglass batt.  For example, 3” closed cell foam (R-21) along with an R-19 fiberglass batt, works well.  A minimum 2×10 rafter would be required under this scenario to allow enough space.  Exterior floors would work in a similar way (i.e. a living space above a garage).

In conclusion, FAB systems are better than no foam at all, but make sure you hire someone that knows what they’re doing, or you’ll regret it.  Use the foam depth guidelines we discuss above.  There are some companies out there that put a VERY thin coat of foam on because they focus on the air-sealing benefit of foam and disregard the conduction and moisture dimensions of the products in the FAB system.  Bottom line…don’t save nickels just to spend dimes later. 

Although we at Energsmart feel that the FAB system is inferior to foam alone, we would be happy to quote that system for you; however, our quote will reflect adequate foam depths we discuss above such that we will not expose ourselves or our customers to foolish risks.

This is big time energy nerd stuff and is complicated, so don’t hesitate to ask the professionals at Energsmart if you have any questions!

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Code Reports and Technical Data Sheets for Energsmart Foam Insulation Products:

Open vs. closed cell foam: Know your options, make the right decision.

Why doesn’t my builder use foam? 6 rules about builders, insulation, and your home.

Flash and batt (FAB) hybrid systems: What you need to know before you install.

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