I would like to add more insulation to an attic. What measurements and points do I need to look at to decide what materials to buy?

How do I determine if I need airflow baffles?

How do I determine how much insulation to purchase? Measure the area of the attic and then figure out the necessary thickness to get the desired R value?

What ancillary items should I plan for? Insulated attic access, critter-proofing, etc?

What tools or extras will make the job easier?

There is lots of information about why to insulate and how to install insulation, but not much on how to buy the right materials and only make one trip to the store and do the job right the first time.


Actually it is quite simple.

  1. Take your dimensions of the area to be covered. Example: 25 feet X 20 feet = 500 square feet.

  2. Determine what kind of insulation you are going to use. Fiberglass blanket, blow in cellulose, etc. Blanket is easy, select the R-value type you need, look on the package for how much area it covers. Example; a package covers 88 sq ft. (typical). Divide the area to be covered by the amount in a package 500 / 88 = 5.68 or 6 packs.

    You may have to double the amount if you want to double the r-value of the product. Example: two layers of R-22 give you R-44. (R-40 and above recommended for attics)

  3. If you are considering blown-in cellulose, consult the chart on the bag to determine the amount needed for the R-value you want to achieve. The chart will have amount per square foot factors you need to determine how many bales it will take for your area.

  4. Use "Propervent" panels, the air flow baffles you asked about, only if you have vented soffits and either a ridge vent or gable end vents. These must be installed before you add the insulation. They simply rest down on the soffit vents and staple between the rafters.

There are no special tools required to install blanket insulation. A good utility knife, blades, straight edge, dust mask, eye protection and maybe a cheap tyvec bunny suit. If you are considering blow in, you can rent the machine at many places including Lowes or HD for around $25 or less if you buy enough insulation from them.

If you give me the exact specs and dimensions you want, I can calculate the material list for you.

  • Help! can't seem to get my formating to work properly. spacing between paragraphs etc. Need a format edit. – shirlock homes Feb 6 '11 at 10:26
  • Attic measures 22'x38'. I see a common size roll of R-30 fiberglass is 9.5"x15"x25'. Assuming thats thicknessxwidthxlength, it'd be 28 rolls of insulation. Since there is already some insulation in the attic I would want unfaced insulation so I don't trap moisture in the existing insulation. Attic has soffit and gable vents. So I'd need baffles too. – Freiheit Feb 6 '11 at 16:32
  • my calcs 22X38=836 836/31.25=26.7 So if you use all the scrap end cuts 27 rolls (unfaced) should do it. Yes you need to install propervent first. – shirlock homes Feb 6 '11 at 19:07
  • Also, gloves. I've skipped them and it's usually not a problem but my hands did itch a little bit afterwards. – Reed Hedges Feb 8 '11 at 20:13

I found some of the articles and videos from This Old House helpful:


Blown in is good for getting good coverage and doing it quickly and easily. I went with faced batts (which are rectangular pieces about 3X5 feet) plus unfaced rolls (for a layer on top of the faced batts) because I may want to redo or move them eventually to do some work and finishing in my attic.

It was a pain getting them all transported and up into the attic (needed help from a few people) but I was able to easily put them all in place myself.

Do pay attention to allowing ventilation in the soffits and on the roof surface. I just left a gap of a few inches at the edges, didn't use any special baffles.

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