Blown-in or spray foam for a Houston attic?
Find your air handler first. If your ductwork and air handler are already inside the conditioned space, insulation belongs on the attic floor and blown-in does that job for substantially less than a sealed roof deck. If they are in the attic, which is true of most Houston homes, blown-in on the floor leaves them above the insulation in air that reaches 140 degrees in August, and the Department of Energy puts leaky attic ducts at 20 percent or more of a home's heating and cooling losses. That is the case a sealed roof deck is built for. R-value, depth and price all matter less than that one question.
On this page
Almost every comparison of these two products argues about R-value per inch. That is the wrong argument, and it is the one you will be handed by whoever wants to sell you the bigger job.
The two products do not go in the same place. Blown-in goes on the attic floor. Spray foam goes on the underside of the roof deck. Everything that matters follows from that, because it decides which side of the insulation your air conditioning equipment ends up on.
Go and find your air handler
This is a thirty second job and it settles the question.
If it is in a closet, the garage or a mechanical room, your ducts are already inside the conditioned space. The attic is just a hot box above an insulated ceiling, which is exactly what attic floor insulation is designed for. Blown-in is likely the right answer and it will cost you a fraction of foam.
If it is in the attic, which it is in most Houston homes, you have a different problem. Your system makes air at around 55 degrees and pushes it through metal and flex duct sitting in 140 degree air. Insulating the floor underneath all of that does not help it. The ducts stay on the hot side.
Adding R-value to the attic floor while your ducts cook above it is the most common way Houston homeowners spend money without fixing the thing that is bothering them.
What each one actually does
| Blown-in | Spray foam at the deck | |
|---|---|---|
| Where it goes | Attic floor | Underside of the roof deck |
| Attic stays | Vented, hot | Inside the envelope, near house temperature |
| Your ducts end up | Above the insulation | Inside the conditioned space |
| Air sealing | Little on its own | Seals as it insulates |
| Settles over time | Yes | No, it bonds to the deck |
| Cost | Lower, quoted once we have seen the attic | Published by the square foot |
| Area covered | Attic floor, the smaller surface | Roof deck, larger because of pitch |
The last two rows compound. Blown-in costs less per square foot and covers less area, so the gap in total job cost is wider than a per foot comparison suggests. We publish foam pricing in what spray foam insulation costs in Houston; blown-in we quote after seeing the attic, because depth and prep move it too much to put a number on a web page honestly.
Depth, and why old attics are shallower than they were sold as
Blown-in fiberglass is about R-2.2 to R-2.7 per inch. Houston is IECC Climate Zone 2A, where the prescriptive ceiling minimum is R-38 or R-49 depending on which code edition your city enforces. We cover which applies in what R-value Houston code requires.
That means 14 to 17 inches of loose fill for R-38, and 18 to 22 inches for R-49. Go and look at yours. In most Houston homes built before 2010 it is nowhere near that, partly because it was installed shallow and partly because loose fill settles.
Open-cell foam is about R-3.5 to R-4 per inch, so 5.5 to 7 inches at the deck meets the same requirement. It does not settle, because it is stuck to the roof rather than lying on the ceiling.
The honest case for blown-in
We sell both, so here is the case we actually make for the cheaper one.
- Your equipment is not in the attic. Then the floor is the right plane and foam is overbuying.
- Your existing insulation is just thin. Topping up serviceable material to full depth is the cheapest useful thing anyone can do to a house.
- You are selling in two years. Payback on a roof deck conversion does not happen in that window. Comfort is real but it is not a return.
- Budget is the binding constraint right now. Half a job done well beats a whole job you cannot afford. We would rather blow your attic to full depth this year than talk you into financing foam.
The honest case for foam
- Your ducts and air handler are in the attic. This is the big one and it is most Houston houses.
- The upstairs never matches the downstairs. That is usually an envelope problem at the roofline, not a thickness problem at the ceiling.
- You are tired of topping it up. Foam is installed once.
- You have knee walls, a bonus room over the garage, or a complicated roof. Those are geometry problems that attic floor insulation handles badly.
What about doing both?
People ask. Generally no, and not because of cost.
A sealed roof deck brings the attic inside the envelope. Once it is inside, insulation on the attic floor is insulating one conditioned space from another, which does nothing useful. Pick the plane. Do not pay for both.
The exception is a partial job, where only part of the attic is converted and a wall divides the two. Those exist but they are unusual, and we would tell you if yours is one.
Frequently asked questions
Which is better, blown-in or spray foam?
Neither, in the abstract. They go in different places and solve different problems. The question that decides it is where your air handler and ductwork are, not which product has better numbers on a brochure.
Can I put blown-in over my existing insulation?
Usually yes, and it is the cheapest thing we do. Not if what is up there is wet, rodent damaged or contaminated, because covering a problem seals it in where nobody can see it. We will tell you which case you are in before quoting.
Does blown-in insulation settle?
Yes, which is why the installed depth is specified above the finished target. An attic blown fifteen years ago is not at the R-value it was sold at. Spray foam does not settle because it bonds to the deck.
Is blown-in a waste of money if my ducts are in the attic?
Not a waste, but it will underdeliver against what you are hoping for. It will make the ceiling plane better and do nothing for the ductwork. If the complaint is "the upstairs is unbearable and the system runs constantly," the floor is not where the problem is.
What about a radiant barrier instead?
Different thing again. It has no R-value, it reflects radiant heat rather than slowing conduction, and it does nothing without an air gap. We go through it properly on the radiant barrier page.
What to do next
Find your air handler. Then book the free assessment or text a photo of the attic to (281) 703-2700. We quote both, so we have no reason to point you at the expensive one if the cheap one is right.