The Best Way To Insulate An Attic

Unfortunately, the easiest way to add insulation to just about any place in your home is to install fiberglass batts.  Fiberglass batts are typically the worst insulation for any job, as I complained about in a recent blog.  I mentioned at that time that I would follow up with a blog about the other choices of insulation.  Today I’m going to discuss several different types of attic insulation.

The best way to insulate an attic or pretty much anything else in a home is to use spray foam insulation. There are two types of spray foam insulation; open cell and closed cell. I’ve also heard people call them ‘half pound’ and ‘two pound’ foams, respectively.  Open cell foam has an insulation value of up to R-3.9 / inch, while closed cell has an insulation value of up to R-6.9 / inch.  Closed cell foam will act as a vapor barrier when installed to a depth of at least 2″, while open cell foam won’t act like a vapor barrier at any depth.  For an in-depth discussion of the differences between closed cell and open cell foam, click here open cell vs closed cell foam.

When properly installed, either type of spray foam insulation will act as a perfect air barrier, sealing off all attic bypasses.  Spray foam insulation will also completely eliminate convection; air cannot move through foam insulation.   The downside to using foam insulation is the expense; foam costs way more money than anything else, and it’s definitely not a do-it-yourself product.

I’ve heard some people complain about the flammability of foam insulation; yes, it’s flammable, but it will typically be completely covered in the attic.  The fact that it’s flammable wouldn’t stop me from installing it.  If you’re curious, here’s a quick “don’t try this at home” video.

I’m such a firm believer in spray foam insulation that I had this done at my previous home, which was a one-and-one-half-story house.  For this style of house, foam is definitely the way to go; the insulation gets applied directly to the roof decking, and it’s called a hot roof.  The foam gets completely covered with drywall after that.  For traditional roofs, I’ve made up my own set of standards.

The Gold Standard

For a traditional attic, there is no need to use foam throughout the entire space, as you’ll get the most value out of the first couple inches.  A cost effective way to use foam insulation is to foam the lid of the house, then use loose fill insulation on top.  This means installing spray foam to a depth of at least 2″ on the entire attic floor to completely seal everything up.  After the foam is cured, loose fill fiberglass or cellulose insulation gets installed on top of the foam.  Because fiberglass costs more, has a lower insulation value per inch and makes my skin itch, my preference would be cellulose.

If it’s an older home with only a few inches of space between the tops of the outer walls and the roof, you won’t have much room for insulation here; extra spray foam needs to be installed here to help compensate for this.

Here in Minnesota, the minimum allowable insulation value for a new construction home is R-38 for an attic, but federal standards suggest R-50 for our climate.

Oh, and one other cool thing about spraying closed cell foam on the attic floor is that once the foam cures, you’ll be able to walk on the entire attic floor; not just the truss or floor joists.  I’ve done it.  It’s crazy.  That closed cell foam is strong stuff.

The Silver Standard

Prep the attic before insulation.  Have every attic bypass completely sealed.  Foam in a can is great stuff for most smaller attic bypasses (didja see what I did there?), but watch out for gas vents; they require a 1″ clearance.  Have insulated boxes constructed for any recessed lights – they contribute a ton of heat to the attic.  If it’s an older home where the rafters or trusses only leave a few inches for insulation at the outer walls, you won’t have enough room for proper insulation at the edges; hire someone to spray foam these areas with closed cell foam to get the highest insulation value possible.  Of course, don’t forget to install baffles at the eaves to prevent your soffit vents from getting blocked.

After everything has been prepped, it’s time to insulate.  My preference is cellulose insulation.  If you do it yourself, you can buy the insulation in bags from your local home improvement store, and they’ll probably let you rent an insulation blower for free.  The DIY cellulose insulation method is very dusty, but it’ll get the job done.  If you hire a pro, they’ll use wet-spray cellulose, which adds a small amount of water to the cellulose to help control the dust and to slightly increase the insulation value per inch.

If you choose to use loose fill fiberglass instead, don’t worry; it’s not bad stuff.  There was a widely publicized study conducted by Oak Ridge Laboratories in 1991 that said that loose fill fiberglass insulation lost a lot of its insulation value once temperatures dropped below 20 degrees, making loose fill fiberglass an inferior product when compared to cellulose.  I contacted Andre Omer Desjarlais at Oak Ridge Laboratories about this issue, and he said “This was true 20 years ago but all fiberglass manufacturers have changed their products appreciably since then and this is simply no longer an issue.”  I also contacted several insulation manufacturers about this, and they said the same thing and sent me some great information, which I posted on my web site; click any of these links to read the documents from Certainteed, Johns Manville, or Owens Corning.  Loose fill insulation will still experience convection, but not nearly as much as old fiberglass used to.

The Bronze Standard

Just use cellulose insulation in the attic.  Cellulose does a good job of controlling condensation in the attic and it’s a fairly dense product, so it will cut down on air movement from attic bypasses, but won’t completely eliminate them.

The Brown Standard

Roll out a bunch of fiberglass batts, proudly proclaim “done and done”, and have yourself a cold one.

Oh, and as for the attic access panels or pulldown attic steps?  I’ll cover those in another blog.

Reuben Saltzman, Structure Tech Home Inspections - Email - Edina Home Inspections