Attic insulation increases energy efficiency and prevents mold-causing humidity from entering the space. There are five common types of attic insulation, and the best choice depends on the local climate, your home’s architectural type, and your budget.
Why Insulate the Attic?
Ninety percent of homes in the US are underinsulated, and about 25% of home heat is lost through the attic. A well-insulated attic will save most of that heat, lowering your energy bills.
Since warm air rises, it will make its way to the attic and out of the house through leaks and cracks when you don’t have proper insulation. The more heat that escapes through the attic, the harder your HVAC system must work, and the higher the chance your attic will develop mold or moisture issues.
Attic insulation isn’t just for cold climates, though. Heat always moves to cooler areas, and on hot days, attic temperatures can be 20 – 30 degrees higher than the temperatures outside. The high attic heat can make its way through the ceiling into living areas through the process of conduction. Heat will also seep through any gaps or cracks, causing the air conditioning systems to work harder.
How Much Attic Insulation Is Enough?
All insulation has an R-value rating. R-value is a measure of the thermal resistance (how well it stops heat transfer) of a product. Different types of insulation have different R-values per inch. Once you decide on the type of insulation to use, the thickness becomes important.
The map below shows different US climate zones. The chart lists recommended attic R-values for each zone. The suggestions for hot and cold zones are very similar. The map, chart, and R-value of insulation products combine to answer the “How Much” question.
How much attic insulation costs is also a consideration. Ten inches of spray foam is significantly more expensive than ten inches of cellulose, but the R-values are also quite different.
Recommended insulation levels for retrofitting existing wood-framed buildings
|Zone||Add Insulation to the Attic||Floor|
|Uninsulated Attic||Existing 3–4 Inches of Insulation|
|1||R30 to R49||R25 to R30||R13|
|2||R30 to R60||R25 to R38||R13 to R19|
|3||R30 to R60||R25 to R38||R19 to R25|
|4||R38 to R60||R38||R25 to R30|
|5 to 8||R49 to R60||R38 to R49||R25 to R30|
Types of Attic Insulation
A well-insulated attic keeps the heat in the house when cold and keeps the heat out of the house on hot days. Below are the most common types of attic insulation.
Batt or Blanket Insulation
Batt insulation is popular because it’s easy to DIY. The installation consists of fitting the first layer of insulation between the trusses or joists onto the drywall and a second layer perpendicular to the first. It’s important to keep this insulation away from heat-emitting electrical fixtures such as pot lights, regardless of the fire-retardant properties of the batts.
Many attic spaces are home to plumbing pipes, HVAC ducts, and electrical wiring, making batts difficult to install and inefficient when not properly placed. Some insulation contractors recommend against installing batts in attics.
In an unvented attic, blanket insulation can go between the rafters. The friction fit properties hold the insulation in place initially, but you’ll need strapping to keep them there.
Batt or blanket insulation types:
- Fiberglass. Available unfaced or with kraft paper on one side to act as a vapor barrier. Costs between $0.40 and $1.20 per square foot, depending on thickness. Between R-3.1 and R-3.4 per inch of thickness.
- Mineral Wool. R-15 mineral wool costs approximately $0.80 per square foot. Between R-3.0 and R-3.85 per inch. More rigid and difficult to cut and place than fiberglass but provides better soundproofing.
- Denim. Not always available. R-3.5 per inch. Batt sizes may not be consistent. The cost is approximately twice as much as fiberglass. Absorbs moisture easily–requires a vapor barrier.
Loose-fill insulation provides a thick blanket on attic floors, covering all gaps and cracks. It’s often installed by a professional, but DIY options are available. Many home improvement outlets will supply the loose-fill insulation machines if you buy the material from them. You can also pour loose-fill insulation into the attic out of the bags and rake it smooth, but this often results in a less consistent application.
Loose-fill covers the entire attic floor with a blanket of insulation that fills around any obstructions, wires, pipes, and ducting. It is inexpensive, easy to install, fire retardant, and has a good R-value.
Blown-in insulation can easily block soffit vents if it is not installed carefully–especially in older homes without air chutes or stops at the exterior wall. Attic ventilation is needed to equalize air temperature.
Loose-fill insulation types:
- Cellulose. R-3.5 per inch. Costs between $0.80 and $1.30 per square foot. Very dusty during the installation process. Does not lose R-value if it settles. Will absorb moisture in humid conditions.
- Fiberglass. R-2.2 – 2.7 per inch. Costs between $1.56 – $2.72 per square foot. Has some air convection problems because it lacks the density of cellulose or spray foam.
Spray Foam Insulation
Spray foam insulation is popular for attics, despite its cost. It goes on attic floors and the undersides of vaulted ceilings, where it adheres permanently. Foam provides a complete insulation job by expanding into all cracks, gaps, corners, and crevices.
Until recently, spray foam was only installed by professional applicators because of the equipment requirements. DIYers can now rent the equipment, but because spray foam is toxic, extreme caution is required. In addition to toxicity, this product is ineffective if not mixed correctly.
Spray foam is available in two formulations: open and closed-cell foam with different characteristics. Both provide excellent R-value and have an 80-year lifespan.
- Open Cell Foam. R-3.8 per inch. Costs $0.25 – $0.50 per inch of thickness. Provides an air barrier but does not act as a vapor barrier. Not recommended for the northern half of the US, where outside air temperature and inside building temperature can have a 70-degree F difference. Excellent soundproofing product. Expands approximately 3” after application allowing it to get to most gaps and cracks.
- Closed Cell Foam. R-7.0 per inch. Costs $0.90 – $1.50 per inch of thickness. Provides an air barrier. Acts as a vapor barrier when over 1.5” thick. Expands approximately one inch after application. May require multiple layers to reach desired R-value.
Reflective insulation does not have an R-value. Instead, it reflects heat out of the attic in hot climates. It’s not effective in colder climates and may be detrimental since it prevents solar heat gain.
Reflective insulation is most effective when installed on the undersides of rafters. It requires a minimum of one-inch air space on the reflective side to allow for the heat to move away. Installing it on top of the joists is not as effective but will keep most of the heat away from the home’s living spaces.
Rigid Foam Board Insulation
Rigid foam boards are difficult to install between trusses or rafters but are a good option for unvented vaulted ceilings. Using foam board insulation between studs and rafters is a way to insulate the attic without losing ceiling height. Rigid Insulation R-values range from R-3.5 – R-6.5.
Rigid foam can go on the floor of the attic, between joists or trusses. Some products act as a vapor barrier if at least 2” thick. A perfect fit between framing members is almost impossible to achieve, so you’ll need to seal these gaps with acoustic caulking or low-expansion spray foam from cans.
If you are considering a new roof, you can install rigid foam on the exterior before reroofing. Extruded polystyrene is the most popular product–often called Styrofoam and is available in multiple sizes and thicknesses
Rigid foam is difficult or impossible to install properly on attic floors if the space is filled with wires, plumbing, and HVAC ducting. Loose fill or spray foam are better options.