Reuben's Home Inspection Blog


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 mostsmaller 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 CertainteedJohns 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


Comment balloon 24 commentsReuben Saltzman • March 29 2011 06:02AM


Good Morning Reuben, thanks for posting the excellent instructions, they are clear and complete.
Posted by Dan Edward Phillips, Humboldt and Del Norte Counties, CA (Dan Edward Phillips, Humboldt and Del Norte Counties, CA) over 7 years ago

Hi Dan, thanks for reading.  This stuff isn't quite as critical for you folks in the warmer climates :)


Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 7 years ago

Reuben, extremely detailed...i am curious about the cost of it over batt and finding reputable people who know how to do it in one's area...just not finding it done here.

Posted by Ginny Gorman, Homes for Sale in North Kingstown RI and beyond (RI Real Estate Services ~ 401-529-7849~ RI Waterfront Real Estate) over 7 years ago

So, Reubs, you thinking insulation is important?  Kidding...  I like you staying on topic!

Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) over 7 years ago

Ginny - there's a company in my area, International Insulation Contractors (they're not international), that offers what I call the gold standard for $4 / sf, and the silver standard for $2 / sf.  I'm sure this is more than fiberglass batts, and it should be.  I'm not sure what it would cost to have fiberglass batts installed.

Jay - ya think? :)  As long as it's still cold here, I'm staying on the topic of insulation.  It's currently 19 degrees here in Minnesota.  I'm ready for some warmer weather.  We got a little taste of spring back in February, but that sure didn't last long.

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 7 years ago
Great detailed explanation, thank you. Our local HD doesn't have cellulose in stock, or loose fiberglas. It contributes to the proliferation of the use of batts.
Posted by Ann Bellamy (Hard money lending for investors in NH and MA) over 7 years ago

Great information, Reuben! Very thorough.  I had to go to YouTube to see that it was actually you who was blowtorching the insulation!  It didn't burst into flames--just curious if that was what you were trying to show?

Posted by Peggy Chirico, REALTOR® 860-748-8900, Hartford & Tolland County Real Estate (Prudential CT Realty) over 7 years ago

Ann - and they dare call themselves a Home Depot?  Huh.  Shameful.

Peggy - Thanks!  Yes, that was me with the blowtorch.  Part of what I was showing was that this stuff doesn't just burst in to flames.  Sure, it burns quickly, but it's not like the stuff will explode if a cigarette gets too close.  


Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 7 years ago

Hi Reubin, like your Standard structure. I believe professionally installed fibreglass batts belong in the 'silver standard'. I get to inspect it frequently here in new construction and it is routinely well done here.

It does have to be installed as one of the components in an insulation system that has the highest perm rating achievable for the materials. This means that both the attic bypasses and the insulated box work has to be done meticulously.

Professionally installed vapour barrier systems mean widely overlapping taped joints on 6 mil and heavier polyethylene that covers all interior faces. All cuts, and pass throughs have specific sealing requirements and wall, ceiling and floor transitions are glue/caulked before the polly is applied and again prior to the gyproc being installed. This has been developed from the old R2000 standard, and is now the minimum professionals are allowed to install. Better is more common.

Experienced crews have been doing it well (and fast, therefor cost competitively) since the 90s.

Any installation without this level of vapour barrier standard is incorrect, even loose fill fibreglass or cellulose.

Of course you may not have anyone building to these standards in your area, and home owner installations are a free for all when it comes to DIY work.

Some people actually think they can just lay down fiberglass batts in the attic!

Posted by Robert Butler, Montreal Home Inspector | Aspect Inspection (Aspect Inspection) over 7 years ago

Hi Robert, it sounds like the people in your area really know how to install fiberglass batts.  

Have you ever inspected an attic with fiberglass batts using an infrared camera?  

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 7 years ago

Reuben, I enjoy reading your blog for two reasons:

  1. You're anal about doing it right
  2. You're in MN and I get to re-live my memories of being in that are vicariously through you.

Thanks for the great post!  I suggested it for feature.

Posted by Jeremy Wrenn, C.O.O., Winslow Homes (Winslow Homes) over 7 years ago

The pros do, the DIY guys, not so much.

As for IR work, not as common as you might think. The delta T requirement means reliable results can't be predicted. People here aren't paying for return visits when conditions are right.

Posted by Robert Butler, Montreal Home Inspector | Aspect Inspection (Aspect Inspection) over 7 years ago

Reuben---love the video---bet you are great with marshmallows :)

Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) over 7 years ago

Jeremy - better me crawling through these attics than you, right? ;)  Thanks.

Robert - I ask if you've used an IR camera on attics with fiberglass batts because I'm sure you're opinion of them would drop dramatically after you did.

Charles - LOL, I cook my marshmallows the same way.  What's the problem?

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 7 years ago

Yes, better you than me, Reuben!

Posted by Jeremy Wrenn, C.O.O., Winslow Homes (Winslow Homes) over 7 years ago

I've seem IR images that are dramatic and impressive, but...they are showing me what I expect to see. I'm not surprised by them.

The vapour barrier job is still the most critical component of the system. Loose fill fiberglass or cellulose blown in is more consistent and uniform but the high level 'low perm' air control system is key.

Posted by Robert Butler, Montreal Home Inspector | Aspect Inspection (Aspect Inspection) over 7 years ago

Jeremy - I was thinking today that I gotta train in a younger guy with better knees to start crawling through these attics.  It's starting to take it's toll this year :)

Robert - I agree, the prep work that gets done before insulating the attic is critical.  It reminds me of painting; if you don't prep everything properly, it ain't gonna be right.

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 7 years ago

Yeah, I've got three guys younger than me that I get to crawl around for me.  I"m only 38, but I'm really starting to feel it.  A good pair of kneepads at least makes it a little better.

In fact, that's one of the reasons I don't change over to inspections:  you're ALWAYS in the attics and the crawl spaces/basements.  How's that fun?!  :)

Posted by Jeremy Wrenn, C.O.O., Winslow Homes (Winslow Homes) over 7 years ago

Some how I feel responsible for that video :) I saw a home here in CT with the spray foam, loose fill fiberglass sandwich in the attic not long ago. first time I've seen. probably see it more.

Posted by James Quarello, Connecticut Home Inspector (JRV Home Inspection Services, LLC) over 7 years ago

Check out 'Fine Homebuilding' magazine a month or two ago, you'll see it there. it's becoming a best practice recommendation.

Posted by Robert Butler, Montreal Home Inspector | Aspect Inspection (Aspect Inspection) over 7 years ago

Jeremy - I don't blame you bit.  I'm a few years younger than you, and I've started having knee problems lately.

James - you should feel responsible!  You brought up the flammability thing.  Yeah, I'm sure you'll be seeing more and more foam houses.  It's definitely not typical around here, but most people have accepted this is the 'gold standard', or as Robert says, a best practice.

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 7 years ago

Reuben - just curious since construction varies so much from region to region - what is the impact of the foam on the attic wiring (in ceilings below)?  Also, I tend to see bathroom exhaust fan units and recessed lighting frequently in our attics  - how do they handle those with the foam installation?

Posted by Joseph Michalski, PA Home Inspector (Precision Home Inspection) over 7 years ago

Jim - the foam will have little if no impact on the wires in the attic. 

I've actually never dug through the loose fill insulation in these instances to see how the foam was installed around bath fans and recessed lights.  That's a good question.  Whenever I get the answer (and I will), I'll come back here and post an answer

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 7 years ago
I've heard that spray foam will be off the market in. About 3 years is this accurate?
Posted by Mary about 7 years ago