Spray Foam Insulation Explained
Spray foam is the most oversold and most misunderstood insulation on the market. It is genuinely excellent in the right application and genuinely problematic in the wrong one.
Spray polyurethane foam is applied as a liquid that expands in place and hardens. Its defining advantage over batts or blown-in materials is that it fills irregular cavities and seals gaps as it expands — so it insulates and air-seals in one step. That combination is why it matters in a hot, humid climate, where air leakage carries both heat and moisture.
It also has real limitations, requires correct installation more than any other insulation, and is not automatically the right answer. Anyone presenting it as universally superior is selling, not advising.
Open-cell versus closed-cell
These are different materials with different jobs, and the distinction is the most important thing on this page.
Open-cell foam has cells that are not fully enclosed, leaving it soft, light, and spongy — roughly half a pound per cubic foot. It delivers somewhere around R-3.5 to R-4 per inch. It is vapour-permeable, meaning moisture can pass through it, and it is the cheaper of the two per unit of R-value. It expands dramatically, filling cavities well.
Closed-cell foam has fully enclosed cells packed tightly together — around two pounds per cubic foot, rigid and hard. It delivers roughly R-6 to R-7 per inch, the highest of any commonly used insulation. It is a vapour retarder at sufficient thickness, resists water, and adds structural rigidity to the assembly. It costs considerably more.
Where each belongs
The permeability difference drives the decision, and in a humid climate it is not a detail.
- Closed-cell makes sense where moisture control matters, where space is limited and you need maximum R per inch, and in any application likely to encounter water. It is the usual choice in flood-prone locations, because it does not absorb water and hold it against framing.
- Open-cell makes sense where you have depth to work with and where vapour permeability is acceptable or desirable — allowing an assembly to dry. It is often used on the underside of roof decks in unvented attic assemblies, partly because a leak in the roof above will show itself through the foam rather than being hidden and trapped.
That last point is worth sitting with. Closed-cell against a roof deck can conceal a leak until the damage is substantial. Open-cell lets it telegraph through. Neither behaviour is universally correct; it depends on the assembly.
The air-sealing benefit
This is the real argument for foam, and it is a strong one. In most Texas homes, air leakage is the largest single loss. Batts and blown-in materials slow conduction but do essentially nothing about air movement — air routes around them. Foam seals as it expands, addressing the leak and the conduction together.
Consequences worth understanding. First, a well-foamed house is a tight house, and a tight house needs its ventilation and its HVAC sizing considered deliberately rather than by default. Second, foam changes the moisture behaviour of an assembly, which is exactly why the open/closed choice is not a preference.
Installation, curing, and ventilation
Spray foam is a chemical reaction performed on site. Two components are mixed at the nozzle in a precise ratio at a controlled temperature, and the reaction is what forms the foam. Everything depends on getting that right.
When it goes wrong — off-ratio mixing, wrong substrate temperature, wrong ambient conditions, passes applied too thick — the results are not cosmetic. Improperly cured foam can fail to reach its rated R-value, shrink and pull away from framing, or produce a persistent odour. In serious cases, the remedy is removal, which is expensive and unpleasant. There is no touching it up.
During and immediately after installation, the work area must be unoccupied and properly ventilated. The chemicals require respiratory protection for installers, and re-occupancy times are specified by the manufacturer. Anyone who tells you that you can stay in the house during the work, or that you can move back in immediately, is not following the product's own instructions. This is not overcaution; it is the manufacturer's requirement.
The gap between well-installed and badly installed foam is larger than the gap between foam and any other insulation. Product choice matters less than installer competence.
Thickness is the whole game
Because foam is priced and rated by inch, the temptation to spray thin is structural to the business. An inch short of closed-cell is several R-values gone, invisible once the job is done and impossible to check later without cutting into it.
This is why choosing an installer emphasises written thickness specifications and verification during the job, while the foam is still visible. Depth gauges, photographs, and being present matter. A bid that is dramatically cheaper is usually cheaper because it is thinner.
Is it right for your house?
Honestly: sometimes. Foam earns its cost where air sealing is the problem, where cavities are irregular, where space is tight, or where moisture demands closed-cell. Where a vented attic simply needs more blown-in depth and a weekend of air sealing, foam is an expensive way to buy what cheaper measures would have delivered.
See attic insulation for how foam fits into vented versus unvented attic strategies, R-value for why per-inch numbers mislead, and the main guide for the wider picture.