While making a presentation recently at the Oakland Space and Science Center in California, the topic of fiberglass versus spray foam insulation came up and was quickly followed by questions about hybrid insulation systems. It seems like a lot of people have hybrids on their mind these days.
One of the positives about spray foam is that the insulation itself is air resistant. The gas or air which does the majority of actual insulating is trapped in bubbles inside the plastic foam matrix and can’t be washed out by air flow. Now that we recognize the influence of air leakage on a home’s energy consumption, many builders and homeowners are trying to get their houses as “tight” as they can. Installing spray foams where air is leaking through the building envelope can reduce that flow rate. One of the issues people struggle with when considering foams as a solution has been their cost- they can have a significantly higher installed cost than fibrous insulations.
For this reason, in some markets, contractors are turning to hybrid insulation systems often referred to as flash and batt insulation. To fill a 2 x 6 inch empty wall cavity, first add a flash coat of closed cell foam on the exterior wall to a thickness of 1 -2 inches then take a low density batt to fill in the space before the drywall is installed. What you get is the best of both worlds – the inexpensive high R-value of the fiberglass batt combined with a smaller amount of foam which gives you the air tightening effects desired.
An even more effective option is using loose fill insulation such as OPTIMA® in the hybrid system instead of fiberglass batts. This is a cost efficient, high R-value, well performing system. A challenge for the hybrid approach lies in very cold climates. Fiberglass is a very efficient insulating material so if too much is used with the foam it will make the foam cold and moisture can form in the wall assembly. That needs to be avoided at all costs. For colder climates there are very specific recommendations for the amount of foam you need to use in the wall constructions before you can eliminate the need for a vapor retarder on the warm-in-winter side of the insulation. We have recommendations for hybrid insulation systems, as does the Blow-in Blanket Contractors Association (BIBCA), designed to create the appropriate foam to fiberglass ratios which will prevent this from occurring. If you choose to install less foam and more fiberglass or if you have any lingering concerns about moisture in these assemblies, I recommend installing a Smart Vapor Retarder such as the MemBrain™ product. As an example, the First LEED Platinum Home in Colorado applied the hybrid system using CertainTeed products.
If you are using a hybrid insulation system in your construction projects I would love to hear your experiences.
Lucas Hamilton is Manager, Building Science Applications for CertainTeed Corporation