Attic Insulation 101

Attic insulation is an important part of the home building envelope. Most attics do not have enough. Home design affects the type of attic insulation. Climate can dictate how much insulation is needed. Cost is an important part of insulation choice. The information in this article helps you make an informed choice.

Attic Insulation 101

Why Insulate the Attic?

Ninety percent of homes in the US are underinsulated. About 25% of home heat is lost through the attic. That is a big chunk of heating bills. A well-insulated attic will save most of that heat and money. The amount of money saved will only increase as inflation and energy costs increase.

The home will be more comfortable. Warm air rises and draws in cool air along the floors. The heating system warms it up and it rises. As a result, houses can feel drafty.

Attic insulation is not just for cold climates. Heat always moves to cooler areas. On hot days attic temperatures can be 20 – 30 degrees higher than the temperatures outside. The heat will be conducted through the ceiling into living areas by the process of conduction. Heat will also seep through any gaps or cracks. Air conditioning systems work harder to cool the extra heat.

How Much 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 will it cost is also a consideration. Ten inches of spray foam is significantly more expensive than ten inches of cellulose. 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

Insulation products are used to resist heat flow. A well-insulated attic keeps the heat in the house when cold and keeps the heat out of the house on hot days.

Batt or Blanket Insulation

Batt insulation is popular because it is a relatively easy DIY installation. Fit the first layer of insulation between the trusses or joists onto the drywall. Install a second layer perpendicular to the first. The insulation must be kept 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 if care is not taken to place them properly. Some insulation contractors recommend against installing batts in attics.

Blanket insulation can be installed between the rafters of an unvented attic. The friction fit properties hold them in place initially but strapping should be installed to keep them there.

  • Fiberglass. Available unfaced or with kraft paper on one side to act as 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. Better soundproofing.
  • Denim. Not always readily available. R-3.5 per inch. Batt sizes may not be consistent. Cost is approximately twice as much as fiberglass. Absorbs moisture easily–requires a vapor barrier.

Loose-Fill Insulation

Loose-fill insulation provides a thick blanket on attic floors. It covers all gaps and cracks. It is usually professionally installed but DIY options are available. Many home improvement outlets will supply the machines rent-free if you buy the material from them. Loose-fill can also be poured into the attic out of the bags and raked smooth. 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 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.

  • 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 becoming more popular despite its cost. It is used on attic floors and on the undersides of vaulted ceilings where it will adhere 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. It is now easier to DIY spray foam insulation by buying the product and renting equipment. Spray foam insulation can be toxic. Take all precautions if this is a DIY project. If the chemicals are not mixed correctly, spray foam insulation can be ineffective.

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

Reflective insulation does not have an R-value. Installed properly it will reflect heat out of the attic in hot climates. It is not effective in colder climates and may be detrimental because 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. They are a very good option for unvented vaulted ceilings. Foam board insulation between studs and rafters is a way to insulate the attic without losing ceiling height. Foam can also be installed perpendicular to the rafters. Rigid Insulation R-values range from R-3.5 – R-6.5.

Rigid foam can be installed on the floor of the attic between joists or trusses. Some types of products act as a vapor barrier if it is at least 2” thick and the gaps and it is sealed to the adjoining wood. A perfect fit between framing members is almost impossible to achieve. Seal these gaps with acoustic caulking or low-expansion spray foam from cans.

If you are considering a new roof, rigid foam can be installed on the exterior before reroofing. Extruded polystyrene is the most popular product–often called Styrofoam. It 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.

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Written by Murat

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