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Musings of an Energy Nerd

Installing Fiberglass Right

It’s hard to do a perfect job

Proper installation takes time. Few fiberglass contractors take the time to install batts carefully.
Image Credit: Fine Homebuilding

Of all of the commonly used types of insulation — including cellulose, rigid foam, and spray polyurethane foam — fiberglass batts perform the worst. As typically installed, fiberglass batts do little to reduce airflow through a wall or ceiling assembly; rarely fill the entire cavity in which they are installed; and sometimes permit the development of convective loops that degrade insulation performance.

Knowing this, why would any builder choose to install fiberglass batts? The answer is simple: because fiberglass batts cost less than any other type of insulation.

Before we totally dismiss all fiberglass batt installations, however, it’s important to note that there is a big difference between the typical fiberglass batt installation and a best-practice installation. If a conscientious builder installs fiberglass batts carefully, it’s possible — although not easy — to get the best of both worlds: adequate thermal performance at a relatively low price.

Lots of sloppy installations

Study after study has shown that most fiberglass batt jobs are sloppy. In 2002, the California Energy Commission contracted with researchers Marc Hoeschele, Rick Chitwood, and Bill Pennington to conduct a study of new California homes. In its March 2003 issue, Energy Design Update reported, “The performance problems uncovered by the study were particularly disappointing in light of the fact that all 30 houses [studied by the researchers] were enrolled in programs promoting building-envelope improvements and duct tightness.”

The article went on to note that “the California Energy Commission’s ‘envelope protocols,’ which include standards for air sealing and insulation installation, were widely ignored. … Not a single builder managed to implement any of the following standards:

  • Insulation batts cut to fit around wiring, plumbing, and electrical boxes;
  • Skylight shaft batts installed in contact with the drywall;
  • Installation of a facing to limit air intrusion on the attic side of kneewall and skylight well batts.”

Even builders enrolled in the EPA’s Energy Star Homes program have struggled to achieve good thermal performance in fiberglass-insulated homes. An article in the April 2005 issue of Energy Design Update, “Fiberglass-Insulated Homes Are the Leakiest,” discussed the findings of Bruce Harley, the Conservation Services Group’s technical director for residential energy services. “Harley assembled airtightness data on Energy Star homes (including single-family and multifamily homes) completed in 2004 in Massachusetts and Rhode Island. All of the homes were blower-door tested after completion,” EDU reported. “Harley found that houses with walls insulated with spray polyurethane foam were significantly tighter than those houses with walls insulated with cellulose, and that houses with walls insulated with cellulose were significantly tighter than those insulated with fiberglass.”

What’s required by code?

Many builders don’t realize that sloppy fiberglass installation is a code violation. For example, section 102.2 of the 2006 International Energy Conservation Code requires that “All materials, systems and equipment shall be installed in accordance with the manufacturer’s installation instructions and the International Building Code.”

This provision is relevant because the installation instructions provided by fiberglass batt manufacturers are widely ignored. Harley has written, “Most installation instructions require fluffing insulation to the proper thickness, covering continuously, filling cavities completely, and fitting products around all obstructions, such as wiring, plumbing, and framing.”

CertainTeed instructs installers of its fiberglass batts to follow recommendations of the North American Insulation Manufacturers Association (NAIMA). NAIMA recommends:

  • “When insulating side walls, place the insulation in the cavity and check to be sure it completely ï¬lls the cavity, top to bottom.”
  • “It is important that insulation be correctly sized for the cavity and ï¬t snugly at the sides and ends.”
  • “Even the smallest openings between framing members should be insulated.”
  • “Junction boxes for wall switches and convenience outlets at outside walls should be insulated between the rear of the box and the sheathing. Place insulation behind the junction box and if necessary, cut insulation to fit snugly around it.”

Doing it right

We’ve all seen sloppy fiberglass jobs — installations with a wavy surface that include sections of insulation recessed from the studs. Typical fiberglass jobs often have gaps at the edges of batts. The insulation is often pinched by wiring, and batts are rarely trimmed neatly around electrical boxes. (For a great selection of photos showing typical fiberglass batt installation problems, see Allison Bailes’s blog, A Visual Guide to Why Fiberglass Batt Insulation Underperforms.)

Moreover, many installation problems — for example, gaps behind electrical boxes — can’t be seen unless batts are lifted for inspection.

In a good installation job:

  • Insulation should be installed without gaps, including at cavity corners.
  • Batts should be trimmed about 1 in. oversized so they fit the cavity snugly.
  • The entire framing cavity should be filled, without any air gaps between the insulation and the drywall.
  • Batts should not be folded.
  • Batts should be delaminated where necessary to fit around wiring. There should be no gaps behind wiring, electrical boxes, or pipes.
  • Attic insulation should be installed over wall top plates.
  • Batts installed in kneewalls and skylight shafts should be protected on all sides by a rigid air-barrier material like drywall, OSB, or Thermoply sheathing.
  • If the roof assembly includes ventilation channels, insulation must not block air flow.

Defining sloppy installation

Gaps in fiberglass insulation have a disproportionate effect on thermal envelope performance; in other words, a 5% insulation gap in a wall lowers the wall’s R-value by much more than 5%.

The Residential Energy Services Network (RESNET), a national association of home energy raters, has long struggled with the question of how to estimate the R-value of walls that vary widely in performance depending upon the skill of the insulation installer. Eventually, RESNET developed a useful rating system for insulation installation quality. The system is described in an article published in the January/February 2005 issue of Home Energy magazine, “Insulation Inspections for Home Energy Ratings,” by Bruce Harley.

The RESNET rating system recognizes three levels of insulation installation quality: Grade I, Grade II, and Grade III. “In order to qualify for a Grade I rating, insulation must … ï¬ll each cavity side to side and top to bottom, with no substantial gaps or voids around obstructions (that is, blocking or bridging), and it must be split, or ï¬tted tightly, around wiring and other services in the cavity. In general, no exterior sheathing should be visible through gaps in the material,” Harley wrote. “Compression or incomplete fill amounting to 2% or less of the surface area of insulation is acceptable for Grade 1, if the compression or missing fill spaces are less than 30% of the intended fill thickness (that is, 70% or more of the intended insulation thickness is present).”

The standard for a Grade II installation is lower. “A Grade II rating represents moderate to frequent defects: gaps around wiring, electrical outlets, plumbing, other intrusions; rounded edges or ‘shoulders,’ larger gaps, or more signiï¬cant compression. No more than 2% of the surface area of insulation missing is acceptable for Grade II.”

Grade III installations are the worst: “A Grade III rating applies to any installation that is worse than Grade II.” For further information on the RESNET grading system — including illustrations of good jobs and sloppy jobs — see “Assessing the Quality of Insulation Installed in New York Energy Star Labeled Homes.”

It’s hard to do it right

Although the steps required to install fiberglass batts well are easy to describe, they are fairly difficult to achieve. It is the nature of a fiberglass batt to want to be installed sloppily. Unlike cellulose or spray polyurethane foam, a fiberglass batt doesn’t volunteer to fill a cavity completely; on the contrary, it tends to fight an installer’s attempt to make it fit snugly.

It’s interesting to compare the structural performance of a commonly used construction technique — stick-frame construction — with the thermal performance of a typical fiberglass batt job. Stick-frame construction is redundant and forgiving. Even done sloppily — with a few missed nails or split plates — wood framing rarely experiences structural failure.

By contrast, almost every fiberglass insulation job fails to live up to the R-value promised on its label. Unlike stick framing, the system is unforgiving and totally lacking in redundancy. To achieve the R-value shown on its label, a fiberglass batt must be installed perfectly in a wall or ceiling cavity enclosed by a six-sided air barrier.

Knowing this, builders who want their installed insulation to achieve decent thermal performance have two choices:

  • Learn how to install fiberglass batts perfectly.
  • Choose a more forgiving insulation system.

Last week’s blog: “Getting More Efficient, But Using More Energy.”


  1. John Brooks | | #1

    The Air Barrier is the Key Component
    A perfectly tailored fiberglass installation ... installed by the A-team....will not make up for a less than perfect air barrier.
    A fiberglass job will require much more attention to the air barrier than cellulose or foam.
    So .. you will need the A-team for the air barrier and the Batt installation.

  2. Brennan Less | | #2

    Blame Energy Star and lackluster contractors
    Good post as always. One must wonder why the prescriptive performance programs out there have not simply banned the use of fiberglass batts. The constant refrain that I always hear is that all insulation products work well when installed right. That is true, but if one product has statistically higher likelihood of incorrect installation, then it should simply be relegated to those builders who don't care to participate in performance-based programs. Energy Star and others simply need to remove fiberglass from their acceptable roster of insulation materials. If they don't, they are not providing the performance benefit they are selling, as it is clear that no standard can make an installer do fiberglass correctly. Nor is it easy for the verifier to assess quality of install, which is often hidden behind kraft paper.

    One other thought, Harley indicated that all spray foam homes were better than cellulose homes, which in turn were better than fiber glass homes. Do you think this actually has to do with the material itself, or is the insulation choice simply indicative of a contractor's overall dedication to quality controls? Probably it's a mix...

  3. User avater GBA Editor
    Martin Holladay | | #3

    Good question, Brennan
    You asked "Do you think this actually has to do with the material itself, or is the insulation choice simply indicative of a contractor's overall dedication to quality controls?"

    I pondered the same question when I wrote the April 2005 article for EDU. Here's what I wrote:

    “Although the data clearly show a consistent correlation between insulation type and airtightness, the reasons for the correlation are unknown. It is possible that builders who choose cellulose or spray foam insulation — both of which have a reputation for resisting air flow — may be more meticulous in performing air sealing tasks than builders who choose fiberglass batts. Perhaps builders’ attention to detail in certain areas unrelated to insulation performance — for example, careful sealing between wall bottom plates and subfloors — partially explains the measured differences in airtightness. If this theory is true, the extra dedication to air sealing may be motivated by the builders’ desire to justify the added cost of spray foam or cellulose over fiberglass batts. However, the principle of Occam’s Razor favors a simpler explanation: that the measured differences in airtightness are due to differences in the material characteristics of the different insulations.”

  4. John Brooks | | #4

    Minimum Standard for Air Tightness
    If code required a blower door test and had a minimum standard for air tightness...
    then the half-arse builders would have no choice.
    They would either upgrade their insulation material or they would be forced to employ some other air barrier strategy that meets the standard.
    The result would likely be a lot less use of fiberglass.

  5. User avater
    Michael Maines | | #5

    Compressed batts
    One of the rules for good installation you mention, avoiding compression of batts, gets undo attention. For years I've heard repeated that you can't compress the fiberglass or you ruin the R-value. The truth is that you reduce the potential R-value, in other words waste material, but you still end up with higher R-value than you would by using less material.

    I think this one myth has caused more gaps to be left in the insulation system, resulting in reduced performance, than any other one of the many things that can be done wrong with fiberglass installation.

  6. User avater GBA Editor
    Martin Holladay | | #6

    You're right
    Michael, you make an excellent point. Compressing a fiberglass batt reduces the R-value of the batt but raises the R-value per inch. In other words, batt manufacturers have not optimized the product for R-value per inch; they have optimized the product for a low manufacturing cost.

    Owens Corning has published a chart showing the advantages of fiberglass compression. According to the chart, a 2x6 cavity can be filled with a so-called R-19 batt, resulting in a cavity R-value of R-18. The same cavity could also be insulated with a 9 1/2-inch R-30 batt compressed into the 5 1/2 inch cavity. That would result in a cavity R-value of R-21. Better R-value through squishing.

  7. User avater
    Michael Maines | | #7

    Thanks for clarifying my description. Just after I posted that, I flipped open the new JLC to see your Q+A on this same topic, which made me second-guess what I said. So I pulled the same Owens-Corning chart to double check. Glad we're in agreement! And curious that they don't seem to publish that chart anymore....

  8. User avater GBA Editor
    Martin Holladay | | #8

    More on fiberglass compression
    Let's consider a wall cavity with a horizontal electrical wire that bisects the studs. A lazy subcontractor decides to insulate the cavity by tucking the insulation behind the wire (instead of delaminating the batt to go on both sides of the wire).

    The lazy installer has violated the "no compression" rule. But as Michael points out, there really isn't any problem resulting from the compression of fiberglass batts, so the "no compression" rule is a misnomer. The problem with the lazy installer's action isn't the compression — it's the uninsulated air gap on the interior side of the wire.

  9. User avater
    John Semmelhack | | #9

    Energy Star 2011
    Great article as always, Martin. For Energy Star homes beginning in 2011, it looks like fiberglass batts might be (for all practical purposes) removed from the equation. Part of the proposed revamping of Energy Star for New Homes (which starts January 1, 2011, see for more info) is to REQUIRE Grade I insulation installations. This would, in my opinion, effectively eliminate fiberglass batts entirely. In my 18 months of working as a RESNET Home Energy Rater, I've never seen a fiberglass batt installation that came anywhere close to Grade II, let alone achieve Grade I!

  10. User avater GBA Editor
    Martin Holladay | | #10

    That's interesting
    John, I didn't realize that Energy Star builders were quite that bad. Very interesting information! We all imagine that Energy Star builders are at least a notch above the run-of-the-mill builder -- and even Energy Star builders can't seem to manage even a Grade II fiberglass insulation job. More evidence, if any is needed, of a gross contradiction: the code requires installing fiberglass batts according to the manufacturers' instructions, and these instructions require installation techniques which, for all practical purposes, are never achieved.

  11. Thorsten Chlupp | | #11

    Martin - great article!
    Good point also about the compression vs created airspace and your clarification that fiberglass needs to be installed by a six sided air barrier and fit perfect to work as advertised. I agree with you about the drastic effects of gaps and voids in performance-we measured over 40% reduction in R-Value of a R-13 fiberglass bat installed in a 2x6 wall cavity-which leaves a sizable air gap to the sheetrock. My experience with fiberglass batts is that they only work perfect under one condition: laboratory testing conditions at 70 degrees in the hotbox with ideal moisture contents and no airflow. As soon as you introduce some airflow and moisture to fiberglass it's performance deteriorates rapidly. Around 99% of all moisture that works its way into the building envelope is due to air flow and we currently have no regulations or set goals about air tightness, which about guarantees the two things to happen that fiberglass insulation can't handle well.

    And I agree with John that I have hardly seen any Grade 1 installs...however I doubt that the new Energy Star reg's will get fiberglass batts removed from the equation in 2011 - the fiberglass industry is just to entrenched and powerful in its current marketplace. They have by far the biggest market share and are most widely used. I believe that to be the only reason why we also don't see more better batting alternatives like Mineral or Rockwool on our market. As we are creatures of habit most people also like to use what they always used - and are simply unaware of the problems. That's way we need folks like Martin and his good writings to keep the rest of us straight. Thanks, and keep up the good work.TC

  12. Darin Zurliene | | #12

    Insulation ?
    Fiberglass isn't an insulation it is simple filter and a poor one at that. It only can resist heat and cold from 30F to 90F or to a Delta T no greater than 50F.
    It is interesting that there are continuing questions raised on what is or isn't insulation and what material performs with actual insulating values.
    It started with the Federel Trade Commission in its faulty testing reuirements of materials used for insulating homes, 75F is what the material is tested at, which doesn't doesn't meet real world applications, that is when the HVAC is idle. The FTC also ruled just recently (Tuesday, May 31, 2005: 16 CFR Part 460) that even though there is sufficient proof that fiberglass is ineffective as insulation, the FTC ruled it would be confusing to the consumer, if manufacturer labeled the product to which would inform them to the effective parameters of resisting heat and cold. The FTC opinion also stated that it would not have significant effects on the national economy, or on the cost of home insulation products, or on covered parties or consumers.
    The confusion is greater now than it has ever been, with no help from the FTC who is clearly afraid of loosing the tax dollars from the fiberglass industry, because they consume a large amount of gas to manufacture and fiberglass is still used in most homes.
    When working with the home owner, a HVAC contractor, an energy provider, or a builder. It is simply put. If fiberglass works so well, why are you calling on other products to give you a quote on insulation. They have little or no faith in fiberglass to provide them with efficiency or comfort.
    When insulation is being discussed, fiberglass shouldn't even be allowed at the table.

  13. User avater GBA Editor
    Martin Holladay | | #13

    Thanks for your comments
    Thanks, Darin. For those who don't know him, Darin Zurliene is the owner of Spray Insulation, Inc., an installer of cellulose insulation with headquarters in Bartelso, Illinois.

    While many technical objections to the approved methods for testing insulation have been raised, the current testing methods at least require competing manufacturers to adhere to similar standards and test methods. For more information on how insulation products are tested, see "Understanding R-value."

    Contrary to Mr. Zurliene's assertions, fiberglass is an insulation product.

  14. Ben Ainslie | | #14

    First off, Martin, it made my day to see you taking a stance against fiberglass insulation. Your stance against the ASTM testing c1363 which rated a wall system with P2000 a 10.2 per inch led me to believe you were some a fiberglass-guy. For that I apologize. I thought that because the standard astm c571 was created by the fiberglass industry 40 years ago. What you and Mr. Zurliene may not know is that when the ICC did the testing for P2000 they also tested an identical wall system with R-19 fiberglass and it tested at R-11. They then added moisture to both wall assemblies and the fiberglass tested at a R-9. This is the fiberglass plus the wall system. Oak Ridge labs, which both of you are familier with I'm sure, stated that a wall with perfectly installed fiberglass in a perfect climate will never perform higher than R-13.
    The FTC is slowly coming around though. Every year new insulation product are being switched over to the C1363. As of now, by law P2000 has to display both the old ASTM C571 which rates it a 4.1 and the new "wall system" test which rates it 10.2, on the product. Fiberglass still holds 90% of the insulating industry. Money is power in this country so don't keep your hopes up that they will have to switch to the new testing anytime soon. Like you said Martin, people by it because it is cheap. The biggest hurdle in green buiding is getting the consumer to believe the price hike on high performance products like hard pack cellulose, XPS, EPS, and Icynene is a worthy investment.

  15. Mark Williams | | #15

    Compression is a Misnomer
    Martin, I disagree about your statement saying that compression, e.g. around a wire is a misnomer problem. You point out that the air gap is a problem, but due to compression, you've changed (reduced) the R-value in the vicinity of the compression. Both the compression and the air gap contribute to the problem.

    Overstuffing, but filling the cavity IS OK. You 'waste' R-value in a sense, but you got more R per inch and as long as the cavity is filled, we have a higher insulation level.

  16. User avater GBA Editor
    Martin Holladay | | #16

    Hmm ... not sure I get you
    You wrote, "due to compression, you've changed (reduced) the R-value in the vicinity of the compression. Both the compression and the air gap contribute to the problem." Either I misunderstand you, or I disagree.

    If you take a batt designed to fill a 3 1/2 inch deep cavity and you compress that batt by sliding it behind a wire bisecting the space, then the insulated volume behind the wire will be insulated to a higher (not lower) R-value than the same volume would have had if the batt had been split.

    Unfortunately, the area in front of the wire will probably remain uninsulated. The air gap is the problem.
    However, if, after the drywall is installed, the insulation contractor comes back and carefully drills a hole at the center of the air gap, and fills the air gap with blown cellulose or blown fiberglass, the wall will perform better than it would have with a split batt. The compression was never a problem.

  17. John | | #17

    and the derating would be....
    I am curious how much a class 2 installation should have its effective R value derated.

    If for example a R-19 batt installed to class 1 standards achieves R-19, what does a class 2 installation achieve? Would you derate it to say R-16?



  18. User avater GBA Editor
    Martin Holladay | | #18

    More information
    I'm not sure of the answer to your question, but the answer can be inferred from an example given in Bruce Harley's article: "Consider a typical example: a 2 x 6 wall with R-19 insulation rated at 6 1/2inches,that gets a rating of Grade II at the rough inspection. The HERS rater would assign the assembly an R-17 wall, with appropriate on-center stud spacing and interior and exterior sheathings, and would choose Grade II from a set of radio buttons. The software would do the rest."

    Presumably the de-rating from R-19 to R-17 refers just to the R-value of the installed insulation, and doesn't include the de-rating attributable to the wall framing.

  19. Rick Jenkins | | #19

    How to deal w batt faced wings and drywall glue?
    Please discuss the wings on batt facing that I want the contractor to staple to the studs for full compression. Then glue (which the builder feels is the best, most permanent way to attach drywall correctly to the studs) won't work. I am looking for full compression of the cavity and can't get it when the insulation guy tucks it in instead of using the face wing as intended. Maybe we get rid of the wings and go with new friction fit so the drywaller can glue, I get full fill on all 6 sides. Is gluing the superior way to attach the sheetrock? I am a rater frustrated w the insulation guy for not knowing the standard and figuring out how to do it correctly and still leave a stud for the drywaller to attach to.

  20. User avater GBA Editor
    Martin Holladay | | #20

    Response to Rick Jenkins
    Manufacturers of kraft-faced fiberglass batts recommend that the stapling flange should cover the edge of the stud facing the room, and should be stapled to the edge of the stud (the side measuring 1 1/2 inch). See the illustration below, from "How to Install Fiberglass Insulation," a guide from Guardian Fiberglass, a batt manufacturer.

    If you want the edges of the studs to be left exposed so that you can glue drywall to the studs, then you should choose unfaced fiberglass batts. Because these batts don't include a vapor retarder, your local building inspector may insist that you paint the drywall with vapor-retarder paint.

  21. Tim Johnson | | #21

    All this talk about fiberglass is about batts. Any batt, no matter what the material, is not a good choice. Fiberglass done right is called BIBS. BIBS is approved according to FTC 460. BIBS is a tested in the field insulation system. Cellulose, or any other insulation, is not. It is foolish to say fiberglass is no good, based on a poor installation. I have blower door tested homes with "dense packed" cellulose installations and found settling in as little as 3 months. Proper installation is key with any system. Dense packed fiberglass in a retrofit application, blown in at 2.25# cubic foot can no way settle and has the same infiltration as 3.5# cellulose and is R-4.23 per inch. Cellulose is great in an open blow, but I would no way put it in a wall.

  22. User avater GBA Editor
    Martin Holladay | | #22

    Response to Tim Johnson
    I agree than blown-in fiberglass is far preferable to fiberglass batts.

    I disagree, however, with your disparagement of cellulose insulation. Certainly cellulose is a "tested in-the-field solution" -- whatever that means. The R-values of insulated cellulose walls have been extensively tested, and cellulose installers have decades of field experience.

    Your statement that dense-packed cellulose installed to 3.5 pounds per cubic foot can settle is simply untrue. I agree with your statement that "Proper installation is key with any system" -- a statement which is just as true for blown-in fiberglass as it is for dense-packed cellulose.

    Properly installed, dense-packed cellulose will not settle. Of course, any insulation can be installed improperly, so it is always important to choose a skilled installer -- whether you specify fiberglass or cellulose.

  23. Matt Feldmann | | #23

    Recommended vapor barrier for wood TNG walls
    Great article, very helpful. I am currently building a home and am strongly leaning towards mineral wool insulation in the outside walls (2x6). Martin, I don't know if you like that any more than fiberglass, but I prefer not to use spray foam. There will be no drywall in the house (well, garage only) - all the interior wall and ceiling coverings will be 3/4 inch TNG basswood. While technically a hardwood, basswood is very light weight. I have no idea what the air or vapor permeability will be of the finished product. Would you still recommend a "smart" vapor retarder in this case? I am on the Zone 6/7 border. The sheathing will be 1/2 inch of plywood. No insulation exterior to the sheathing. Exterior finish is stucco, and I plan to follow GBA's advice of using a drainage mat (the 10cm version if I can find it). Any advice appreciated.

  24. User avater GBA Editor
    Martin Holladay | | #24

    Response to Matt Feldmann
    With any air-permeable ("fluffy") insulation, including fiberglass batts or mineral wool, it's essential to have an air barrier on both sides of the wall. So the main problem with your plan is that tongue-and-groove boards are leaky. (The vapor permeance isn't a huge deal, but technically in your climate zone, the building code requires an interior vapor retarder, so you have to think about that issue as well.)

    The easiest way to ensure that you have both an interior air barrier and an interior vapor retarder is to cover your walls with drywall and tape the seams, and then paint the drywall with vapor-retarder paint. Once this work is complete, you can install any type of finish material -- including tongue-and-groove basswood if that's what you want -- on the interior side of the drywall.

    For more information on air barriers, see Questions and Answers About Air Barriers.

  25. Matt Feldmann | | #25

    Response to Martin
    Thanks Martin. So you don't think a careful application of MemBrain would suffice in this case? I am a little concerned about tearing the plastic during install of the tongue-and-groove, but considered putting drywall shims or furring strips over the studs (after MemBrain installation) to keep the boards from rubbing against the plastic during their installation. This would be less expensive, but if you think drywall is the way to go, would 1/4 inch suffice? Or here is another thought - what about 1/4 inch of plywood, with taping or caulking the joints?

  26. User avater GBA Editor
    Martin Holladay | | #26

    Response to Matt Feldmann
    MemBrain is quite fragile. Moreover, I don't know if anyone has studied whether the many fastener penetrations that are required to install tongue-and-groove boards would reduce the effectiveness of the MemBrain as an air barrier. I suspect they would. (Builders have plenty of experience putting fasteners through drywall -- an approach that doesn't seem to hurt the ability of the drywall to work as an air barrier.)

    Drywall is an inexpensive and time-tested air barrier; it's the best material in this case, in my opinion. I would use 1/2-inch drywall if I were you. With thinner drywall (or thin plywood), the fastener penetrations might cause air leaks.

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