Substitution Isn’t Just for Teaching – Spray Foam Insulation

by Erik North on June 25, 2012

Spray foam


Hey, and I get to use the same foam image again.

People don’t like abrupt change and in business it’s a formula for chaos. Excelling is hard if you constantly change personnel, products or ways of business. Do one thing superbly. In construction, this means sticking with the framing techniques you know rather than re-training for SIPs or external board insulation. But what happens when this butts against thermal bridging, air leakage standards and updated energy codes?

What happens is that builders adapt newer techniques and insulation to standard framing. Flash and batt, and sprayed caulk allow continued use of fiberglass and cellulose, while open or closed cell spray foam can be used with standard framing. And spray foam addresses most of fiberglass’s shortcomings at only triple the price! Cherry solution.

Spray Foam Substitution

Here’s your standard wall (totally not copied from the standard wall framing post). It is 2×4 vertical studs (later 2×6 studs) spaced 16 inches on center (for non-nailing folks, 16 inches on center means 16 inches from the center of one stud to the center of the next one). There’s a single stud bottom plate and a double stud top plate for strength. In the corners, a third stud is added to hang dry wall.

The cavity is normally insulated with either fiberglass or cellulose. The exterior is sheathed in plywood or OSB, then wrapped with a water resistant barrier (#15 building felt in older homes, Tyvek in newer construction), then finished with whatever siding/cladding is desired. As construction goes, pretty straight-forward and definitely time tested.

So how do we greatly improve this wall assembly R-value? Just add foam.


Substituting spray foam for fiberglass is one of those classic ‘many benefits, one doozy negative’ things. First, the positives (because we’re upbeat that way).

R-Value – Closed cell spray foam enjoys a very large R-value edge over fiberglass batts, while open cell foam is roughly equal. This varies baseed on many factors like the exact product and application method, aged off-gassing (meaning the R-value foam insulation settles on after some of the gas foaming agent dissipates) and the employees.

The aged R-value of closed cell spray foam is around R-6/inch and open cell in around R-3.5/inch versus R-19 for a standard 6-inch fiberglass batt. Closed cell spray foam nearly doubles the R-value and open cell has a definite edge.

Air Leakage – Both open and closed cell spray foam can act as an air barrier. Neither fiberglass or cellulose is sufficiently impermeable to be an air barrier. Spray foam generally produces a tighter house, better effective R-value and prevents air flow within the wall cavity.

Moisture – The exterior moisture controls remain the same as standard framing…siding for protection, water resistant barrier and rain screen (usually the siding) for drainage and drying and flashing details to direct water flow.

Normally fiberglass or cellulose would be paired with a vapor retarder. The closed cell spray foam is a Class I vapor retarder and doesn’t require the addition of another vapor barrier. Open cell spray foam is much more permeable and should be paired with a vapor retarder. The one moisture related downside is that because there’s no air flow, there’s limited drying if the foam gets wet.

Ease – How easy is this approach? It’s standard framing plus ‘just add spray foam contractor’. If anything it will save time. One of appeals of fiberglass is that little training or experience is required. As long as you’re detailed around outlets, light fixtures, wires and pipes anyone can do it. What could be easier?

Not doing it is easier.

The contractor frames up and sheathes the exterior and rather than committing a day or two to fiberglass installation, the crew moves on to other tasks. The spray foam contractor rolls in after electrical and plumbing is roughed in.

Thermal Bridging – Thermal bridging issues are not a plus, but I also don’t feel like starting a ‘+/-‘ category. Open and closed cell foam in a standard wall cavity are still compromised by thermal bridging. The R-values, especially with closed cell, are so superior to fiberglass that it’s still produces a high performing wall assembly.

Closed cell spray foam achieves around R-30 to R-32 in a 2×6 wall cavity (assuming standard practice of filling most of the cavity to avoiding trimming the foam). Open cell foam is usually overfilled then trimmed, achieving a full R-21 or 22.

Reduce these by about 20% for standard frame thermal bridging for your effective whole wall R-value. Compare this with the R-19 fiberglass batt; both frames perform better, air seal and the closed cell achieves significantly improved R-value levels despite the thermal bridging.


The disadvantages unfortunately hit all the major consumer and energy auditor buttons.

Cost – Spray foam is not cheap. At all. Compared with a baseline fiberglass, spray foam can be 10 times more expensive. Superior performance but a premium price.

Materials – Compared with a standard wall, there is the same amount of construction materials (makes sense since we only replaced the insulation). The spray foam has significantly more embodied energy (the total energy required to make a project or material) than the fiberglass or cellulose it would be replacing.

Green – This is a loaded question, as it has so many variables. Very generally speaking, between the gas propellents, aged off-gassing and petroleum base of polyurethane foam makes it less green than cellulose or fiberglass. There are foam variants that address each point (water propellant, low gassing soy based), but that’s why I said ‘generally.’

As code requirements for insulation rise and air tightness minimums are being implemented, contractors are finding ways to meet them. Spray foam addresses these needs in a straightforward manner that wouldn’t require a crew to learn a new skill set. Just keep the foam contractor on speed dial.

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