Batts, Blown, or Sprayed – What’s the Best Way to Insulate Your Attic?

July 21, 2011 — 53 Comments

Today, I have guest author Allison Bailes from Energy Vanguard sitting in for me. I specifically asked him to cover this topic since the question comes up so often.

Since I’ve acknowledged myself as the best, baddest, building science blogger of the internets, I get asked a lot of questions about windows, air conditioning, and other energy efficiency topics: What size HVAC system should I install? Is it true you can seal a house up too tight? Can I use a CFL bulb in my Easy-Bake Oven?

I also get asked about insulation, and that’s today’s topic. I’m going to stick to the attic here, but much of what I say could apply to walls or floors, too.

The first thing to know is that you really have only three choices here. Well, OK, you have more than three, but I’m mostly going to talk about those three because in terms of what you’ll be able to find someone to install, these are the ones you’re mostly limited to.

Before you ever get insulation anywhere near the attic, though, make sure that you get the air leakage sites sealed up. If you put an air permeable insulation material over a hole in your ceiling, you may have comfort, indoor air quality, durability, and efficiency problems.



These are large pieces of insulation that hold together because they’re made of long, interweaving fibers with adhesive binders. The two kinds of batts you’re most likely to encounter are fiberglass and cotton. In terms of their insulating quality, they’re pretty much equivalent. Cotton batts, though, are ‘cool’ because they’re made of recycled blue jeans.

The problem with batts, however, is that they don’t work well because they don’t fill the space well. For the best performance, an insulation material needs to fill the whole space, with no gaps, voids, compression, or incompletely filled areas. Batts are about the worst you can do here.


insulation batt fiberglass unfilled cavities energy vanguard

See that photo above? Notice that you don’t see insulation filling all the spaces between the ceiling joists. In this case, it’s because they weren’t cut to fill the cavity completely. Another reason that batts don’t do so well is that the house is full of other stuff where we want the insulation to go: wires, electrical junction boxes, framing, bathroom exhaust fans, can lights… Batts don’t do well when they have to compete against all that.



A better choice is insulation that comes in smaller chunks. The installer, taking his best firefighter pose, holds a large hose and blows the chunks into the attic. A large machine outside churns the chunks and uses air to blow them up through the hose.

The two main choices here are fiberglass and cellulose, and each has its advantages and disadvantages. They both insulate about the same, though, with R-vales in the 3 to 4 per inch range. Cellulose comes from recycled newspapers. Fiberglass comes from what I’ve heard one major fiberglass insulation manufacturer call a ‘rapidly renewable’ resource – sand. Hmmmm. I don’t know about that, but it’s a common insulation material that works much better in the blown form than in batts.


insulation blown cellulose complete coverage

The photo above shows an attic insulated with blown cellulose. Notice how you don’t see any of the ceiling framing down at the ceiling level. You also don’t see any gaps that allow you to see all the way down to the ceiling drywall. That’s because blown insulation is great at filling the gaps and giving you a good, complete layer of insulation.



The third major type of insulation is spray foam. Just as there are two types of blown insulation (fiberglass and cellulose) and two types of people (those who divide everything into two groups and those who don’t), there are two types of spray foam – open cell and closed cell. Each has its pros and cons, as well as its own set of adherents who will tell you never to use the other type. That’s an article for another day, however.

The main advantage of spray foam is that it allows you to move the building envelope – the boundary between conditioned and unconditioned space – from the attic floor to the roof line. If you’ve got your HVAC system and ducts in the stupidest place they could possibly be  (the attic), then moving the envelope to the roofline can be a good thing. In a new home, spraying foam in the roofline can bring the ducts inside the envelope without having to redesign the system and house.


insulation spray foam roof line

If you don’t have HVAC and ducts in the attic, spray foam on the roofline isn’t really necessary. I’d blow insulation on the attic floor (after air-sealing, of course). The big disadvantage with spray foam is cost. It’s generally 3 to 4 times what you’ll pay for blown cellulose or fiberglass.


Alternative Materials

There are other materials and systems that you can use to insulate your attic. One that I like a lot (since I built a house out of them  is the structural insulated panel. It’s a sandwich of rigid foam insulation and plywood or OSB (oriented strand board, the flaky plywood).


insulation alternative materials beer can

One that I don’t like so much, and which you can see below, is the beer can.




Print Friendly

even better stuff from Life of an Architect

  • gojogo15

    Hi! My husband and I would like to better insulate our attic, but are a bit confused what the best combination would be. Currently there are some foam batts along the rafters of the attic, but we are pretty convinced they do almost nothing. Additionally we have our heater in the attic, along with all of the duct work. The air conditioner is outside. We live in Colorado where it is very hot in the summer and very cold in the winter. Also, we have a lot of vents in the attic. Currently, our plan is to insulate the floor of the attic with blown in cellulose, but after reading this I’m thinking that perhaps it would be a good idea too to either insulate the rafters of the attic with foam or close the vents during the winter. What do you think?

  • larrybloom

    Please check the code in your jurisdiction. Better yet, call your local code official. There are requirements for amount of ventilation when enclosing the spaces between rafters, which the sheet attached to the bottoms of the joists would do.

  • Curmudgeon44

    I wish we homeowners could understand these issues better, but I am also convinced many of the so-called experts are blowing smoke. A true expert should be able to explain in detail what happens when we follow a rule, and equally well what happens when we violate a rule. Only if you can discuss both sides with expertise, do you really understand a rule.

    That said, the rule from experts I respect: Ventilation is necessary to make radiant barrier (RB) work its best. So when I installed radiant barrier I tried to improve ventilation. When the roof was replaced I had lots of ventilation, in excess of the 150:1 recommendation… to my surprise air conditioning bills went down measurably. I am in Texas, near Houston.

    So ventilation is good, but is it necessary? The published research I have seen tried to measure the benefit of going from 300:1 to 150:1, and their experimental result was zero. The published research I have seen on RB itself, indicate about a 10% reduction in cooling bills.

    You worry about creating a hot spot between your RB and the roof structure, but would that hurt anything? Joe Lstiburek (look him up!!) argues eloquently that temperature matters for asphalt shingle life, and NOT for roof decking… or certainly tile shingles. I hypothesize the result of a hotter roof on your garage, would be a slightly cooler temperature below (reasoning: more hot tile means more radiation to outdoors, no factor introduces more heat into your structure).

    FWIW my opinion is your contractor’s proposal would help. It might be yet better if you found a way to ventilate at the roof peak. Joe Lstiburek is a big advocate of a more expensive solution: spray foam insulation at the roofline. In my hot-humid climate I believe there are legitimate worries about the roof structure rotting, but your hot-dry climate should minimize those.

  • Brenda

    I’m trying to cool down an attic over a finished garage that we’ve just added a/c to. It’s in the southern california desert where being cold is never a problem. The contractor has suggested radiant barrier stapled directly to the underside of the roof between the trusses and blown cellulose on top of the garage ceiling. As far as I can tell, there is no venting at all in this attic and the roof is tile. Will the radiant barrier be better stapled to the underside of the trusses? If so, will that create pockets of hot air between the radiant barrier and the roof decking since the attic is not vented?

    • read the response from Curmudgeon44 directly above your question –

  • RudyGr

    “If you’ve got your HVAC system and ducts in the stupidest place they could possibly be (the attic), then moving the envelope to the roofline can be a good thing. ”

    I’m not sure that Allison is recommending this at all. Having read several of his blog posts, he seems to have an opposite opinion.

    • That quote was written by Alison – in fact, he wrote this entire article so I think he would be the foremost expert in what he recommends (or not)

  • Chris M.

    I got open cell spray foam insulation done on the main envelope cavity of my roof (vaults inaccessible to spray foam). The spray foam installer recommended that I remove the 40 year old blown insulation from the attic floor to circulate the air into the attic cavity. I followed that advice. My attic is spotless clean, easy to breathe, I can wire stuff, walk around freely up there. In the higher rate summer season, my 3750 sqft house with 2 electric HVAC systems in Texas averages about $220/month. In the winter however, my bill averages $450-500. This past January $652!! Just for electric. That’s with the lower winter electricity rates too. It doesn’t even get below 30 degrees here. THE PROBLEM is because I have no attic floor insulation, the heat doesn’t stay down. Cool air drops, hot air rises. My attic is very warm in winter because my heater runs constantly. I don’t want to lose the empty attic floor, but my bills in the winter are absurd. So I thought about doing batt, but I know that won’t seal all gaps. My question is, will putting blown new blown insulation on my attic floor AND having open cell spray foam on my roof hurt my electric bill or the efficiency of the spray foam in the circulation? I only ask because the difference the spray foam made in summer is astronomical. It’s already paid for itself after 2 years.

  • Mike Huber

    Has anyone spray foamed 1″ and then batt the rest on the walls? pros cons

  • Sawgrass

    Does anyone see there being an issue with leaving the batt insulation in the ceiling joist cavities when the roofline has been spray foamed? I’ve frequently received conflicting opinions on this.

  • bird

    Our house is 35+ years old with NO insulation where the roof is. You can see the holes where the nails come thru- it makes the bedroom up stairs hot and cold all year round. What should we use to fix this- our electric bill in the summer is close to $600 every month and that’s keeping the AC at 76.

  • augie

    Hi, I am in the process of redoing my roof, which now exist of architectural singles. I am going to install Drexel metal roofing. I do see now with my old roof on, in the attic a lot of condensation dripping from the nails that are exposed. also some kind of mildew/moist. I do have insulation, i believe the spray fiberglass(grayish color) on the attic floor. What can you advise for insulation after the metal roof is completed?
    Bye the way, I am in the Boston area, near the ocean. Thanks for your help.

    • thefoamguru

      Have the blow in removed. Have the roof line sprayed with 4.5-5″ of closed cell foam with 2″ or 3″ on the gables or perimeter mansard walls (Boston?). It will give you R-30ish and improve the racking strength of the roof at least 200%. Some say 300%. You will never have to worry about the integrity of the metal, safety of the air quality, not to mention your utility bills will drop significantly.. If you leave the blow in you’re inviting moisture, mildew, mold and rust to become future problems unless you mechanically ventilate, but as your heat escapes through the ceiling in the winter, which it will, it will fill the attic space and eventually meet with freezing cold air on the surface of the metal. Always Coca-cola. Moisture. This also happens in the summer as the attic space is baked all day while the temperature can drop wildly low at night. Again. Moisture.

  • G.Yu

    My insulation guy suggested spray foam on the base of attic as vapor barrier and blown cellulose on top since he believes much heat is coming from my living space to the attic, which contribute to my moisture issue up in the attic….any comment on this recommendation?? I have multi vent around the perimeter of the roof line as intake and ridge vent as exhuast currently, there is not enough overhang space to do soffit vent…thanks

    • thefoamguru

      First you have to address the real problem. Your attic isn’t ventilated well enough, it’s passive. In fact, without some serious mechanical ventilation, it is nearly impossible for traditionally insulated buildings to be anything other than miserably hot in the summer and absolutely freezing in the winter. As more and more duct work and HVAC mechanical units make their ways into attics, this becomes more of a problem, with serious consequences.

      If you have no mechanicals, and you NEVER plan on using the space again (although I would never recommend it), the “flash and blow” approach is acceptable. All lights and fans must be boxed with rigid foam. Outlets must be taped. The lights must be IC rated or a 3″ air space must be created around them. Any tiny hole WILL lead to over spray.

      The ceiling joists need to be coated. In order to actually benefit from this application, you need a MONOLITHIC membrane across the flat ceiling. If you would like some pics of thermal bridging on a frosty roof, I can show you, it happens. If your going to use foam, closed cell I suppose, you have to seal it or you lose a lot of the benefit. 1.5″-3″ on in the cavities, 1″ wrapped around the remainder of the joist.

      A bunch of blow in cellulose. Ugh. I’ve removed thousands of cubic feet of this in preparation FOR spray foam.

      Then you need an attic exhaust fan, maybe more than one depending on the square footage. The fact that the attic won’t have soffit vents means that loose fill blowing from one side of the attic to the other is less of a problem, but settling will start and continue. As the loose fill compresses, you actually lose R value.

      Foaming the roofline solves all of these problem. The science of THAT can be for another day.

      First, remove ALL fiberglass from the flat ceiling. Seal any ventilation with rigid board, 1/2″-1″ and have a PROFESSIONAL spray foam company foam the roof line with the approved R-value for your zone in open cell foam. By professional I mean talented.

      If there is walk up access or there are mechanicals, the finished foam will have to be sprayed with an intumescent coating. Depending on how tight the rest of the home is, a Heat Recovery Ventilator may need to be used to mechanically exchange the air.

      My attic when it’s 10 degrees out, 70. When it’s 100 degrees out and the traditional attic is 140-160, 80.

      Many birds with one foam.

  • Joe R

    I’m considering foam insulation on the attic ceiling. 2 questions. How do you apply foam over a gable vent? The second is what do you if you have a small attic exhaust fan? How do you work with the gable vent and fan for this type of job?

    • foamed attics basically mean that you are changing the plane of the building envelop from around the living areas to around the building … this means you can’t have a gable vent and the attic exhaust fan is no longer a requirement (since there isn’t a need to remove the super-heated attic air)

    • thefoamguru

      Rigid board painted black over the gable vents. Disconnect attic fan. Rigid board over the hole. Call roofer at your convenience. Have professional spray foam company do the applications.

  • doug

    I am a fireplace installer, on spray foam roofs we are getting a lot of condensation/frost on vertically vented chimney Any suggestions

    • thefoamguru

      Are the vents double walled or single? Your industry may need to come up with an airtight, one way vent that is acceptable for fireplaces to prevent the cold air from dropping down the vent and “coke canning” on the stack. Interesting problem.

  • Liz M.

    I’m buying a house in Kingston, NY — cold and wet in winter, hot and humid in summer. The attic, which I’d like to use for living if possible, has no insulation — neither between the rafters or on the floor. Because the exposed rafters seem fairly evenly spaced, would batting be an okay for me if it’s cut and installed carefully between the rafters; or, does it have to be installed over the rafters with a ceiling installed over the batting? On the other hand, if I put up a ceiling first would blown cellulose or cotton be a good bet? I want to avoid condensation up there between the roof and the wood structure of the house.

    • There are some moving parts here but this is not too complicated of a question. Normally if you have an attic that has ventilation as part of the intended design (do you have soffit or ridge vents, turbines on the roof? – something that allows outside air to get pulled in and through your attic?) you would typically insulate the ceiling between the joists leaving the attic at the roof line un-insulated. Based on your comments/ questions, I am very confident that this is the system you have in place. Blown cellulose would be fine as long as you get an adequate depth and do not cover the vents where the outside air is coming into the attic space.

  • moderate

  • Larry

    We are thinking of buying a new house in San Antonio (HOT/HUMID). In the attic, they use radiant barrier on top and and blown cellulose and the attic floor. I believe the attic is vented by ridge vents. Would there be any benefits or consequenes blowing spray foam over the radiant barrier? I went to one still being worked on and put my hand on the barrier…It was very hot!

    • you would not mix those systems. Radiant barriers provide a moderate amount of protection but they would still feel hot to the touch. Foam insulating systems are used with the idea that you are moving the insulated cavity of the house out to the roof line and not introducing air flow into and through your attic space. You are trying to create an insulated box (so to speak) around the habitable areas. If you insulate at the roof, you wouldn’t need to insulate at the ceiling between the rafters.

  • Dan Cowell

    Hey Bob,
    I’m wanting to install a bathroom fan, and upon inspecting my attic I have blown in Cellulose. What is the best way for me to find the framing underneath so I can access electrical without disturbing it too much? Any help would be great!


    • I’ve always used a narrow rod, 1x or something of that sort that I could stab down through the insulation to find my structure

  • ron

    I used foam in attic and now the stuff is leaking yellow oil drops down wall..the roofer says I need vents 8 , to let the attic breath ,what to do.

    • Really!?! I’ve never heard of that happening before. Typically a foamed roof is a closed (unvented) system. Something sounds dodgy about this one. Let me see what I can find out.

    • EnergyVanguard

      The contractor who sprayed the foam insulation in your attic did something wrong. The problem was probably either that the proportions were off in the 2-part foam or they had the wrong temperature in the hose. It’s a bad foam job and they need to fix it or give you your money back.

      The roofer doesn’t know what he’s talking about. If you’ve encapsulated your attic with spray foam insulation, the last thing you want to do is vent the attic. That’s just stupid, and a lot of roofers propose it because they understand nothing about how homes work.

      • See what happens when you have quality friends who write guest posts for you – they deliver the goods even a year later!

        Thanks Allison, your the best.

  • KiraC

    We’re renovating a home built in 1925. We want to use the attic space as a library or meditation area. What would be the best insulation if we plan to use that space regularly?

    • I tend to gravitate towards open-celled spray foam these days, I like the performance compared to the expense, and I like that you can install it on the underside of your sloping roof and it will stay put.

  • The type of insulation you should choose depends on how you will use it and on your budget. While closed-cell foam has a greater R-value and provides stronger resistance against moisture and air leakage, the material is also much denser and is more expensive to install. Open-cell foam is lighter and less expensive but should not be used below ground level where it could absorb water. Consult a professional insulation installer to decide what type of insulation is best for you.

  • Donald

    We are remodeling a home that is around 30 years old.  It is a single story home and we are putting a vaulted ceiling in the kitchen / living area.  We are thinking about using a closed cell polyurethane foam on the roof deck and keeping the batt insulation in the walls.  The rest of the house has blown insulation in the attic and batts in the walls.  We were told that we should seal off the new vaulted area that will be foamed from the other attic space and add a small duct to allow air flow into vaulted area attic space.  What are your thoughts on this?? And will we have moisture issues on our windows??

  • Netua

    I am building a new small home and because of space constraints, must put the HVAC in the attic.  I was planning on doing a white metal roof (with ventilation) instead of sprayed foam.  I’ve struggled to find a comparison of performance between those two options.  Any suggestions?  BTW, I’m the south where summers can be brutal.

  • I have recessed light cans in my kitchen, and am planning to blow in greenfiber insulation in my attic after i am finished installing the radiant barrier on the underside of the rafters. For each light can i made a protective cylinder made out of 14″ sheet metal so it extends about 8″ above the fixture, the top is open, they are 10″ diameter with the light being 6″ dia..  How do I insulate around these?  Can I lay batt insulation over the top of the sheet metal cylinders and then blow around them? 

  • kyle

    Mr. Bailes — I’m interested in closed-cell foam as roofline insulation. You mention that a successful installation requires it to be detailed correctly, and that few installers do so. Where can I find information on the properly detailed installation of closed-cell foam?

  • BobM

    What are your thoughts on radiant barriers in the attic? I live along the Pennsylvaniaaryland border. Today it’s one of the hottest days so far this year with some thermometers topping off at 102degrees. Certainly we don’t get the extra hostile sun like the south and southwest but it is clear that my 2nd floor still gets more heat gain from the attic than I would prefer. I have the recommended spray cellulose insul in the attic for this region (and I k ow I could add more for greater R-value) I was curious however in the likelihood of using a reflective/radiant barrier to reject some of the heatgain so that it doesn’t eventually filter to my 2nd floor. I’ve seen some sheathing that has a foil barrier for this exact reason, as well as having seen plastic/foil type rolls that can be stapled in the attic space. (admittedly complicated to make a full barrier) it seems like a radiant barrier of some sort rejecting the radiant energy before it super heats the attic space could help combat the heat gain.

    Yes/no? Curious your thoughts… (apologies for spelling&grammar I’m tapping this out on a phone)

    • Anonymous

      Hi, BobM, I think a radiant barrier isn’t advised for your location. Yes, it’ll help in hot summer weather like you’re having now, but you’ll lose the benefit of keeping the attic a little bit warmer in winter. The best thing you can do is make sure you’ve air-sealed and insulated your sealing as much as you can. Also, don’t install power attic ventilators (!

  • Mark Johnson

    I am told that in the Cinco Ranch (Houston) TX area, there were a number of homes built as an experiment using foam insulation at the roofline. Some local energy-aware builders who I don’t think are BS-ing me here, say there are pending lawsuits relating to roof decking problems widespread with this method. Perhaps you would care to spill some professional gossip on what can go wrong in a hot-humid climate. A nicer way than “gossip” would be to say what precautions must be taken to prevent problems specific to a hot-humid climate.

    • Larry Bloom, AIA

      Yes, there are some lawsuits over some spray foam; either old foam that had  formaldihyde or new foam that’s open-celled, in that it can hold water. I’ve seen some people say they had problems with Icynene, but I can’t find any more proof of that. Icynene is closed-celled and therefore shouldn’t hold water. As for decking, The only thing I can think of is that there was a roof leak and the water soaked the decking over the insulation, and was trapped. Icynene is non-flammable, but it does smoke. That’s all I know. Oh, and it’s extremely expensive.

      • Anonymous

        Both closed-cell and open-cell spray foam can have problems if not installed correctly or if the roof leaks. Also, Icynene is a company that makes both closed-cell and open-cell spray foam.

    • Larry Bloom, AIA

      Yes, there are some lawsuits over some spray foam; either old foam that had  formaldihyde or new foam that’s open-celled, in that it can hold water. I’ve seen some people say they had problems with Icynene, but I can’t find any more proof of that. Icynene is closed-celled and therefore shouldn’t hold water. As for decking, The only thing I can think of is that there was a roof leak and the water soaked the decking over the insulation, and was trapped. Icynene is non-flammable, but it does smoke. That’s all I know. Oh, and it’s extremely expensive.

    • Anonymous

      I’m not aware of widespread lawsuits relating to this, but whenever you use a material that can trap water, as closed cell spray foam does, you have make sure you detail it correctly. Generally that means air gaps. Create a ventilation channel between the roof deck and the foam, and you’ll avoid most of the problems that happen with spraying foam in the roofline. It’s more expensive and time-consuming, though, and pretty much no spray foam contractors are doing this right now. Spraying foam directly onto the deck can still work, but you have to be careful with the details.

  • Larry Bloom, AIA

    Love the comparison article. One more advantage of sprayed foam is that it seals leaks and penetrations. Since it’s not permeable and fills every crevice, it seals the envelope. As long as you also have fresh air coming in for air exchange, this is a perfect solution for air infiltration, bug control, etc. I believe it’s also non-flammable.

    • Anonymous

      Yes, Larry, spray foam also serves as an air barrier when installed with enough thickness, and that’s one of the reasons it’s become so popular. Ventilation, as you say, is going to be required in most spray foam houses because they’re so tight. 

      The flammability issue is getting a bit heated lately, though, because a lot of building departments are starting to ask for thermal or ignition barriers. I’m not really up to speed on that issue, but I’ve heard it’s starting to rock some boats.

      • One problem I’ve heard of with the closed cell… if moisture gets in (through the roof deck to the truss), it has little way of getting out. Borate treated top cords will address potential issues of a “rot sandwich”.

        • Anonymous

          Yeah, the open cell vs. closed cell debate gets into the details of moisture because closed spray foam has a low permeability and can trap water. Open cell foam will show a wet spot as the water moves through it. Either can work well, in my opinion, if you detail them correctly.

          • Absolutely! AND if you insulate the inside of the roof deck, you open opportunites to utilize the new conditioned space. 😉