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Question DetailsAsked on 11/8/2013

Cost of foam insulation

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Well, that is a wide open question - do you want it delivered in a dump truck on your drive, installed loose in an attic, sprayed into open stud or joist openings, or foamed into enclosed wall space - makes a lot of difference in cost, as you can imagine.

VERY rough numbers - for board insulation in open exposed cavities (like open studs or ceiling joists) roughly $0.25-0.50/BF (that is board foot - 1 square foot one inch thick). Of course, you usually do not install one inch of foam insulation - usually more like 3-6. Sprayed in place foam can run in a wide range too, but for discussion purposes $0.40-0.60/BF is a typical number for open-cell (which has a lower insulating value), to $0.80-1.50 for typical closed cell, to $1.50-2.00 for extremely high R value closed cell.

Of course, there is a proviso - each has a different insulating value, depending on how manufactured or applied, and what the chemistry is. Rough typical R values (heat flow is inversely proportional to R value - so double your R vallue and you cut heat flow by half, for enclosed spaces without direct airflow) for comparison (check numbers for your specific products), from the Department of Energy (sample article in Source, below) - per inch of thickness:

Fiberglass batt - 3 to 4 per inch thickness

Open Cell foam in place - 3 - 3.7

Expanded Polystyrene board - 3.5-4

Extruded Polystyrene - 5-6

Closed cell foamed in place - 6-6.5

Polyisocyanurate board - 6.5-6.8

As you can see, there is more than a 2:1 difference in R value, and as much as an 8:1 range in price per inch of thickness. Commonly, the highest efficiency products are only used where there is not adequate space for the more economical products or where there is moisture exposure, because for the same R value they are more expensive. The lower R value products generally degrade more with time and have a greater propensity to pick up moisture, so they are commonly used where cost is a key factor, in new construction on speculation homes or where the buyer did not specify high performance materials, and where moisture contact (direct or from condensation) is not a significant concern.

Before insulating, you really need to look at where your dollars are best spent - I have seen clients demand certain insulation when the payoff is never going to happen, while refusing to insulate where it would do the most good. A lot of people have misconceptions too - like that a lot of attic insulation is always more effective than wall insulation where that is generally not the case beyond a limited R value, that insuylating floors is a waste because everyone knows heat rises and never sinks, and that insulation is a more effective use of their money than sealing air leaks or putting thermal barriers around doors and windows or putting thermal drapes over exposed windows.

My recommendation - do some reading to educate yourself, and if looking at a whole-house upgrade, look into rebate programs from local government agencies or utility companies, and use a free web calculator (lots available at insulation company and Cooperative Extension Service websites) to figure, with your actual energy rates, how much $ you will actually save. In my area, with what you would call real winters for about 6 months of the year, you would be surprised how many whole house insulation projects have a 20+ year return period, but about 70-80% of that energy savings can be recovered by spending only about 10-15% as much on the worst energy hogs - which might be by insulation, sealing air leaks, energy efficient windows, or more efficient heating system, depending on the specific house. As an example, I cut my early 1980's house heating bill by almost half in the second year with less than $300 worth (with no labor charges) of weatherstripping, insulation, and minor home and hot water heating system modifications.

One other thing - you need to pay close attention to water vapor flow as much as heat flow, because if you make a situation where you trap moisture inside walls or ceilings, or cause the freezing front (point where temperature is below zero) to move too far into your walls or ceilings in the winter, then you can cause moisture condensation in the insulation resulting in water or ice buildup and eventual rot and decay. This is the typical case where the homeowner who runs amuck with just installing many inches of cellulose or board or spray insulation in the attic without actually using a designed vapor barrier and insulation plan, and then wonders why his roof joists or trusses are rotting and failing 5-10 years down the road.

For a significant insulation job, you need to first get a professional energy audit and insulation recommendation, with the type and thickness keyed to your specific situation and house construction conditions, and taking into account the need for vapor control as well as thermal control.

Bear in mind also that foamed in place insulation makes subsequent work in that wall or ceiling space near impossible - so make sure any electrical or plumbing is handled first, that the electric wires are rated for insulation confinement (routine NM/Romex wiring is NOT), and that you are not going to need to do any additions in that area, and cross your fingers the area never floods from above.


Answered 6 years ago by LCD

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