Q&A / 

Attic Insulation – Blown vs. Batt

DEAR TIM: I would like to add additional fiberglass insulation in my attic. I can install batt insulation myself. Blown-in fiberglass appears to require professional equipment. Which type offers better performance? Are there advantages in using one type versus another? Are there any installation tips you can suggest?

DEAR T. H.: Both systems will deliver excellent results. However, if you intend to do the job with batts, you had better be a detail oriented, patient individual. The thermal performance of fiberglass insulation is directly related to its ability to trap and maintain very small pockets of air. Blown-in fiberglass creates a seamless blanket in your attic. You might spend many hours kneeling in your attic to achieve this same result with fiberglass batts.

This zone map shows you how much R-value you should have depending upon where you live. GRAPHIC CREDIT: US Department of Energy

This zone map shows you how much R-value you should have depending upon where you live. GRAPHIC CREDIT: US Department of Energy

The thermal performance or resistance to heat flow (R-factor) is not the same when comparing fiberglass batts to blown-in fiberglass. Batts offer a slight advantage. You can usually obtain an R-factor of 3.1 to 4.2 or slightly higher per inch of material with batts. The R-factor of blown-in fiberglass is usually 2.3 to 2.8 per inch of material.

Once installed and expanded to the uniform, manufactured thickness, batts offer a known R-factor. Blown-in insulation thickness can vary in an attic space. This will lead to non-uniform R-values.

Before you make your final decision, do a cost analysis. You may be pleasantly surprised. I recently priced the cost to upgrade a 1,500 square foot attic from R-19 to the new thermal standard of R-49 in my climate zone. The total cost to have the fiberglass blown-in by professionals was $0.40 per square foot or $600. I then called the local retail outlet to price batt insulation. The cost of just the material was $0.45 per square foot! Clearly it made better sense for me to have the job done by professionals.

If you can find fiberglass batts deeply discounted, you possibly can tackle the job and save some money. Wear a respirator to minimize throat irritation from airborne glass particles. Gloves, long sleeved shirts and goggles are also highly recommended. Be sure to take your time while installing the batts around roof framing members. Carefully cut the batts so they fit tightly around each piece of wood. Oh yes, don't fall through the ceiling to the rooms below.

If you have existing batt insulation between the ceiling joists, install your new batts at a 90 degree angle to the existing material. This will help cover any gaps in the original installation. Check your existing insulation for dark spots. These dirty spots may be locations of large air leaks from your finished living space. The existing fiberglass traps dirt from this air as it passes up into the attic. Locate and close these holes before proceeding.

Be sure not to block the air passageways between your exterior soffits (eaves) and/or vents and the attic with your new insulation. Flow-through attic ventilation is very important. You can purchase foam or cardboard baffles which fit in between the roof rafters as they pass over exterior walls. These baffles create an unobstructed pathway for air to enter your attic.

Do not cover any recessed light fixtures with fiberglass unless you are absolutely positive it is permitted by your local electrical inspectors. If you decide to use blown-in insulation, install scrap lumber depth gauges in the attic. These pieces of wood can be pre-marked with a crayon or spray paint to the desired finished thickness of insulation. Tack them to the ceiling joists in various locations. After the job is completed, you can pop your head up into the attic to see if the pros hit the marks!

R-values needed in ceilings, walls and floors for your zone. GRAPHIC CREDIT: US Department of Energy

R-values needed in ceilings, walls and floors for your zone. GRAPHIC CREDIT: US Department of Energy




2 Responses to Attic Insulation – Blown vs. Batt

  1. I am surprised to find nothing being mentionied about the difference between faced and unfaced insulation batts or rolls . Blow in insulation is never faced as it doesnt come with kraft paper used as a moisture barrier.. Blow in insulation is primarily used as a add on layer not as an original layer or new replacement for old insulation on the bottom.. all insulation must be faced with kraft paper that is included in faced rolls or batts. If you already have original insulation with face paper , you dont need faced insulation to add on top as unfaced is sufficient. It is important that all surfaced to be insulated be faced with insulation first. If you decide to replace your old dirty insulation with new insulation, you must first lay kraft paper that you can buy in rolls at hardware . Either is brown or red rosin which is probably more durable than the former. I use red rosin then place either batts or blow insulatioin on top. This is an important thing to have in all insulation that is kraft paper .

  2. I have an attic that has blown in insulation. I am considering putting down a lot of plywood to convert the space into useable storage. Should I put the plywood over the blown in or would it be better to replace it with roll or batt insulation?

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