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Cutting Cellulose Insulation Install Costs?

by Chad Ludeman on October 12, 2010 · 34 comments

in envelope,materials,process

As you may well know, we are big fans of cellulose insulation. The Skinny project was the first time we used cellulose and we learned some valuable lessons that we plan to apply to future projects like Avant Garage that Nic has been telling you about lately. As always, we are attempting to continually improve the quality, efficiency and cost of construction. That’s a big part of what this blog is about after all.

We learned two major things while trying to stuff 9.5″ of cellulose insulation into the walls of the Skinny Project.

  1. 10′ tall x 10″ thick of cellulose is very heavy and wants to make your drywall pop off the studs.
  2. The majority of the labor installing the cellulose insulation was actually in netting the stud walls prior to blowing it in the cavities.

Let’s dig a bit deeper into these two observations.

The first point is illustrated in the picture below. You can see that the cellulose is bulging the netting out and that the netting is stapled to the face of the studs. This invokes nightmares for the drywall crew. For starters, the crew has to throw their bodies against the drywall in order to press the cellulose back in allowing them to fasten the sheets to the studs. Secondly, due to the netting running over the studs, they can not glue the drywall to the studs to help prevent nail popping. The end result is that it takes longer for the drywall to be installed and we get more nail pops than normal that need to be fixed for the clients once they move in. Neither issue is a huge deal, but still an opportunity for improvement.
Skinny Project

The second issue relates directly to the installed cost of cellulose insulation. We did some research and found that the cellulose product itself accounted for about 1/5 of the total install cost we paid to our insulation contractor. The rest is labor and staples for the most part. As stated above, over half of the labor was spent simply installing the netting before a single scrap of cellulose was blown into our walls. You can see how many staples it takes to hold in the netting in the picture above to get an idea of why this takes so long.

Enough talk. On to our hopeful solution to both of these issues. Install the drywall prior to blowing in cellulose into the walls. Simple right?

This came to us when discussing new ideas with the Hybrid Construction crew for improving the air tightness of our envelopes. Installing the drywall first allows the hanging crews to easily glue all sheets to the studs with no hindrances. It also eliminates the need to press in the cellulose to get the sheets to lay flat because there is no cellulose. Once this is done, the insulation crew can come through and simple drill holes for their cellulose pumps just like they do for retrofit walls. They will do this in between hanging and mudding the drywall so there is no extra charge from the drywall crew to spackle over these holes.

We have since verified the effectiveness of this technique with a few other super insulating freak contractors across the country. If our calculations are correct, we should not only save time and improve quality, but we should be able to chop our cellulose insulation costs in half! Not bad.

Anyone out there trying something similar? Share it with us in those comments.

If you enjoyed reading this post I can promise you'll love our new writing over at Postgreen Homes. Yeah, we know that's the same thing your favorite band said and their new album is nowhere near as good as their early stuff, but seriously, we are actually still getting better.

There also isn't much conversation to be had here . . . at least not with us. So come on over to the Postgreen Homes Blog and tell us what you think of our new(ish) digs and crazy ideas. We will be sure to tell you what we think of your opinion.

{ 2 trackbacks }

can non ground contact rated treated wood be buried in sand.? | Repairing Water Damage
October 12, 2010 at 1:20 pm
Wireless Light Switches + Floor Outlets = Tighter & More Insulated Home — 100K House Blog
October 15, 2010 at 8:34 am

{ 32 comments… read them below or add one }

1 lavardera October 12, 2010 at 10:36 am

That sounds profoundly smart. How would you ever deal with the netting around an electrical outlet or switch anyway?

My only concern with this is its fine for vapor barrier to the cold side applications, but less so if you plan on following the convention of vapor barrier at the warm side of the wall. If so a vapor barrier layer is needed in place of the netting – not sure it would be strong enough to retain the loose insulation. Or you need to use drywall with a vapor barrier backing – typically a foil layer. That would gain back the process efficiency you are proposing. But outlets, switches, and other drwyall penetrations will make it very hard to achieve the air-tightness you’ve been shooting for. It would have all the same holes that typical construction has with batt insulation with an attached vapor barrier.

2 Jesse Thompson October 12, 2010 at 10:49 am


Vapor barriers are really antiquated terminology, and the IRC now reflects this. There are varying levels of vapor RETARDERS mandated by the code, but vapor barriers on both sides of walls are to be avoided at all costs (OSB outside & sheet poly inside, for example) due to the risk of trapped moisture rotting out assemblies.

Much discussion of this issue here:

And here:

3 Chad Ludeman October 12, 2010 at 11:48 am

Thanks Jesse. To answer some of Greg’s other concerns, we do not have outlets or switches in our walls, so this is not a problem. Also, we are not relying on our drywall to provide air tightness. We use an exterior air tight assembly that utilizes the ZIP System of sheathing.

4 Jesse Thompson October 12, 2010 at 12:16 pm

Chad, did that other post about cellulose make it through?

5 Chad Ludeman October 12, 2010 at 1:02 pm

Yes. You mean the one linked in the first sentence of this post?

6 Jesse Thompson October 12, 2010 at 1:32 pm

Hmm, must have been eaten, maybe it will show up later. If not:

Our numbers back up what you’re seeing about netting labor being the majority of the cost for cellulose insulation. We only get upcharged a small amount to go from 2×6 to 12″ walls in our area (10 – 15% extra), the actual cellulose is not where the cost is.

One major disadvantage to dense-packing behind drywall is that you won’t get the chance to verify the effectiveness of the insulation, and a typical Energy Star / HERS / LEED insulation inspection would be impossible without xray vision. Hopefully thermal imaging and a quality contractor could avoid any technical problems from hidden voids or walls that didn’t get dense packed up to 3.5 lbs / cu ft.

This is speculation, but my impression is that the stories of cellulose settling aren’t from “settling” at all, but from renovation and weatherization contractors not filling cavities all the way up since no one can see inside the walls and check their work easily. Once walls are truly dense-packed, they can’t settle, it doesn’t even fall out of the cavity when you open walls up.

I posted a link to a series of videos by JLC Magazine taken by a contractor we’ve worked with showing how some of the more experienced cellulose contractors install 12″ walls in our area:

Key difference from the picture above is that they staple the netting to the sides of the stud framing, which pulls the face of netting back from the drywall, giving room so any bulging won’t make life miserable for the drywallers.

But, if you can make it work with drywall alone, it will save a great deal of cost.

7 Dan Beideck October 12, 2010 at 1:38 pm

Cellulose is being blown into my walls as I type this. Very timely! We are using a double frame for the outer walls and have attached netting to the outer part of the inner frame. That cavity gets filled with dense pack and it can bulge all it wants. The remainder of the inner frame is filled with damp spray cellulose and won’t create bulging issues for the drywall installers.

Big fan of using cellulose. This article about the global warming potential of insulating materials should be must reading for anyone making insulation decisions, The bottom line is that XPS and closed cell spray foam using HFC’s should be strongly avoided since the payback in terms of global warming is two orders of magnitude worse than cellulose (and others materials to a lesser degree)!

8 Jesse Thompson October 12, 2010 at 1:38 pm

I think my comment is being held back because of links I placed in the posting that might look like spam. If so it will show up twice eventually.

9 Chad October 12, 2010 at 6:34 pm

Jesse – I found your post and brought it back. Sorry for the confusion. You are correct with practice of stapling the netting back a few inches. We were speaking to our contractor about this also, but it would’ve added even more cost. In the picture above he used a much stronger netting than standard to help with the bulging.

The possibility of gaps was also a concern. Luckily, our contractor has purchased a thermal camera and can scan the homes after a week or so of installing the insulation to determine if there are any gaps.

10 Steve Greenough October 12, 2010 at 8:53 pm

As a long time insulation contractor I’ve done lots of dense pack dry cellulose as well as damp spray. A problem that can happen with closed cavity applications is nail pops from the machine pressure during application. You have to have enough pressure to get to a non settling density and half inch drywall with a normal screw pattern and 24″ framing can make for a bad day. I just finished my own house with a double 2 by 4 wall 12″ thick stapled and glued the fabric to stud face and my drywall crew had no problem with it. Yes it is more labor but it worked really well.

11 Rob October 12, 2010 at 10:49 pm


How do you not have outlets or switches in your walls? Perhaps I am mistaken but I thought it was code that electrical outlets had to occur every so many feet along a wall?

12 October 13, 2010 at 3:08 am

Not sure if the new 100k homes do this, but switches and outlets on the outside walls can be run in conduit for an industrial look. Some SIPS guys build a combo baseboard/raceway for outlets. Wiremold boxes also look pretty good for a reasonable price.

Years ago some codes didn’t allow wiring in the common wall of attached housing, so you’d see those outlets in the floor.

On another forum somewhere, someone proposed installing the drywall in two steps, and the process was well-critiqued. I’ll try to find it.

13 Chad Ludeman October 13, 2010 at 7:18 am

Hmm. It doesn’t look like I’ve done a dedicated post to our outlet and switch strategy. I’ll draft one up for later this week or next.

14 max October 13, 2010 at 11:57 am

Why not try using the wet applied cellulose; we’ve used Green Fiber in the past. It’s available with a non-toxic binding agent (borate?), it stabilizes in the wall cavity, and is screeded off once dry. The stabilization / drying process will alleviate the outward pressure on your gyp board you are seeing with the dry stuff. Also the wet applied will also eliminate the need for the gazillion staples per sheet netting, saving major dough on labor. On a recent job, we did what you suggested where we installed the gyp board first, drilled holes, the pumped the dry cellulose into the cavity. If you do this you MUST perform a thermal scan of the wall after installation – this will show you where you have gaps in your insulation (be prepared for big ones) be it from settling, or the insulation getting hung up on crossing members, wires, outlets (I know you guys magically build outlet-less wall cavities, but trust me, when you’re flying blind it’ll get hung up somewhere and not flow down as expected). After the thermal scan your insulation contractor will have to come back and fill in the gaps (read: more labor on the back end). Even though the thermal scan and report will cost you around 1k-2k, it is worth it if you go this route, and want to end up with a well insulated envelope. If it were me, I would go with the wet applied cellulose when the walls are open for all to see.

15 Chad Ludeman October 13, 2010 at 12:09 pm

Some good input here. We did look into the wet applied cellulose. The problem is that with 12″ of it, we’d be waiting weeks for it to dry out potentially and we don’t have that kind of time. Good idea though.

16 lavardera October 13, 2010 at 1:04 pm

Chad, I’m aware that you have the vapor barrier, or retarder if you wish towards the outside. Although you have confused me now with your mention of ZIP. I know from prior blog posts you’ve been usind this material. My understanding is that ZIP was vapor permeable, although perhaps not to the degree of traditional house wrap or building paper. So are you using an additional plastic layer to created your tight envelope, or are you relying on the ZIP?

Pardon me for finding this strange, but you would be using the ZIP as a cold side vapor barrier when the typical installation of this material is as a house wrap replacement as a vapor permeable air barrier. IF you don’t have another vapor barrier element you would essentially have a wall system with no vapor barrier?

17 lavardera October 13, 2010 at 1:10 pm

I am curious about the properties of the cellulose insulation. Would the insulation alone create a sufficient barrier to water vapor – and if so I can assume it would be an air barrier as well.

OR would the potential for a void the loose fill process make this impossible despite its properties?

18 Jesse Thompson October 13, 2010 at 3:18 pm

This is all in the links I posted above, but here goes again.

The ZIP is retarding the flow of moisture, but its primary function in terms of this discussion is that it performs as an effective air barrier. If you stop air moving through your walls, your risk of moisture damage goes way, way down.

Once you’ve stopped bulk air movement, you’re left with diffusion, which is a very weak process, and much less of a risk of having bad things happen, but still can cause damage.

You still don’t want moisture from the building moving from warm to cold in winter going through your walls, so you need a Vapor Retarder in the warm side as appropriate for your climate. Phili is Zone 4. Zone 4 does not require a vapor retarder at all on the inside face, Zone 4 Marine requires a Class II Vapor retarder on the inside face (NOT a vapor barrier).

OSB (and ZIP is essentially OSB + a water resistant layer) is a low perm material, 2 perms / inch, or about 1 perm as typically installed as sheathing. This makes it a Class III Vapor Retarder (right on the edge of Class II). It’s not very vapor permeable. This is a good thing in a mixed humid climate, because once you air condition your building and it’s 95 degrees outside, moisture is trying to move INTO your building from the outside, especially when the hot sun is baking your siding right after a rain storm gets it wet.

A vapor barrier is a Class I Vapor Retarder (polyethylene or foil, typically).

Cellulose is 75 perms / inch, it’s not a vapor retarder at all.

Permeability of materials:

19 October 13, 2010 at 3:39 pm

Robert Riversong has a lot of field experience with cellulose-filled fat walls, and shares his methods at:

In regard to the stapling and drywall issues, his design allows him to fill behind the first floor walls from the second floor before the second floor drywall goes in, since he doesn’t have a conventional top plate.

20 October 14, 2010 at 12:04 pm

Here’s a quote from Building Products Magazine (, just about the only negative information I’ve seen about cellulose recently.

“Because cellulose can absorb moisture, its R-value decreases over time. And if the insulation is exposed to moisture for long periods, it can rot and grow mold. The Cellulose Insulation Manufacturers Association notes, however, that few cases of significant mold growth on cellulose insulation, which is treated with boric acid, have been reported.”

The ability to store water vapor can be a good thing actually, as long as the cellulose can dry to the inside or outside during later times. Some folks, like Riversong, recommend drying to the inside AND outside.

21 lavardera October 15, 2010 at 10:33 am

Jesse, I’m not sure if you are commenting on, or representing what Chad has done with their builds. In any case relying on the Zip as vapor retarder rather than barrier may be a convenient solution in our locale, but I don’t feel this points the way to a wider solution for other places. Of course this blog is not about that – its about the value PostGreen is putting into their houses. I’m always digging a little deeper because I’m concerned about finding models that can have broad application, and energy efficient model that is easy and familiar for builders to adopt.

That said I still have reservations about relying on ZIP in this manner. Yes, I understand as you say a good air barrier is the first step to keeping moisture out of your wall system. But it ignores the potential for water gaining entry through failures, construction errors, or design errors. A small leak in a well vented wall may dry easily and never be noticed. In a wall design with an exterior vapor retarder/barrier and a condensation zone close to the sheathing a small leak can become a big problem. Given ZIPs reliance on the tape for making a primary barrier to water that gets behind siding, I’m not comfortable with it for the long term. If it eventually allows water behind the tape and into the wall it can easily become the big problem.

Philadelphia enjoys a climate that offers the full range of seasons, yet none too extreme. And so the construction code allows for a system as you have described. But our climate can also have exceptional weather that could be outside these design parameters for significant periods of time. I don’t think this arrangement deals with those situations. There has to be a better way which can still achieve the efficiency in construction they are looking for without these risks. My experience is they always come back to bite you.

22 lavardera October 15, 2010 at 10:43 am

Kevin – in the Larsen truss article he states he does not use a vapor barrier, but rather “air tight drywall” (not sure what this refers too) and a vapor barrier enabled latex primer. So essentially he is using a warm side vapor barrier, although unconventional.

I’m curious how the air-tightness of such a house would compare to a system such as Chad is using, where his air tight envelope is at the exterior sheathing and decking?

BTW he also says that he bypasses the need for fire blocking because his building inspectors have accepted the dense pack cellulose as a fire stop. That sounds great – and a big help in the thermal break double stud system that Chad is using. Fill from above, and for houses with an attic, you can do the second floor from there even if you make a hole in the top plate and fire-stop it after. But unless that becomes part of the IRC can we expect that every building inspector would be on board with that interpretation? Seems unlikely.

23 Jens October 17, 2010 at 4:27 am

Would it not be easier to use use blocks of cellulose insulation, instead of blowing it into the wall. Then you wouldn’t have the problem of a bulging wall. Here in Finland a company sells this kind of insulation => (no english translation but you get the idea from the pictures).

24 October 18, 2010 at 12:42 am

Jens, thanks for the information. Maybe someone will start importing the stuff.

Greg, and Chad,

The GBA forum is sizzling with informative posts and differences of opinions on this topic:

Some highlights:
1. Chandler notes that JM Spider blown-in-batt has better R value and infiltration resistance than cellulose at a similar price.
2. Riversong compares the effect that cellulose’s hygroscopic behavior has on indoor humidity to thermal mass’s effect on indoor temperature. (both effects are good, BTW)
3. Chandler and others have seen mold because cellulose dries too slowly after say, a plumbing leak.

25 Jesse Thompson October 18, 2010 at 12:25 pm

Greg, you’re still missing the terminology.

You’re not “relying on the ZIP as vapor retarder rather than barrier”, it’s that if you use ZIP to build a house, its physical properties are that of a Class II or III Vapor Retarder, not a Class I Vapor Retarder, which is the only material category that qualifies as a Vapor Barrier.

EVERY wood framed house with Plywood or OSB as exterior sheathing has a cold side Vapor Retarder, and this issue has to be managed so your wall can dry out if indeed you do get a leak down the road. You are correct, ZIP tape might not be tough enough long term, that’s why some builders add house wrap over the top for extra protection.

The only way to avoid having a cold side Vapor Retarder is to not use any plywood or OSB on the outside of your walls, and instead sheath them with a vapor permeable sheathing like Structural Fiberboard (15-20 perms), DensGlass Gold (20 perms), diagonal 1x pine boards (10 – 30 perms) or fancy european fiberboard sheathing like Agepan (15 perms). Then your walls can dry to the outside if water gets inside them.

Also, in Riversong’s system, he uses Air-Tight Drywall, which is not a Vapor Barrier at all, it’s painted drywall, a Vapor Retarder with a specific permeability. (

Riversong gets his houses down to ~ 2.0 ACH50, and believes that getting tighter is unnecessary and inappropriate. Passivhaus demands a much more airtight building shell, 0.6 ACH50. See Greenbuildingadvisor for lots of conversation about this.

26 lavardera October 21, 2010 at 12:23 pm

I understand the retarder vs barrier distinction Jesse. You are just misinterpreting my “rather” in that sentence. I also understand that conventional sheathing also retards passage of vapor. I understand where you are coming from, even though I don’t have the discipline to type the retarder correctly in place of barrier. ;^)

I might feel more comfortable with this arrangement if we were further south of here, or if I was not conditioned to be more conservative in this regard by growing up further north of here.

Chad has done what he has needed to do which is make the wall construction more lean in terms of material and labor. Its well done. My personal interests lie in similar lean designs that will work in climates where this will fail, and ultimately set an effective model for widespread adoption, similar to the building paper, plywood, stud, batt, drywall of the past. Something so simple that it does not even pay to reconfigure for a mild climate like Phila. That solution will likely not achieve passive house, but I’m of the opinion that we need a way forward that allows 100% of builders to gain 75% of passive house performance over allowing 5% of builders to gain 100% of passive house performance.

27 Steve Leighty November 1, 2010 at 11:09 am

1) A more efficient way of installing net type fabric for retaining cellulose in a sidewall is after you staple the fabric in place then roll on a single coat of glue over the stapled studs. A water based glue such as Elmer’s glue will work. This provides a monolithic seal which helps greatly to reduce the “stretching” effect of the fabric due to the weight of the cellulose. In freezing temps a solvent based glue will be needed. Also the contractor can still use his/her normal gluing method for installing drywall if….
2) Your mentioning of blowing cellulose behind newly installed drywall is misleading. If this method cuts cellulose cost in half I assure you the installed density will be less than required for elimination of settling. Irregardless of what kind of install method used the installed density must be 3.0 lbs. pcf minimum in each sidewall cavity. Anytime you blow a sidewall where the delivery hose’s outlet into the cavity is above the insulation as it is filling the cavity you WILL NOT achieve the necessary density per cu.ft. to eliminate settling. Energy efficiency can only be obtained from efficient means of installation, not the cheapest means.

28 Javier November 10, 2010 at 10:00 pm

Chad (and other experts in the group)

Have you heard of an insulation product called AirKrete? If so, what are your thoughts on it? Do you think it could be used in a similar fashion as the cellulose?…. no more questions :)

29 Robert Riversong December 20, 2010 at 2:43 pm

I just Googled to this site and it appears that my name and methods are mentioned several times. So let me correct several misconceptions.

I use dense-pack dry-blown cellulose for all my Riversong Truss or double stud wall houses. I always blow it after hanging drywall for least effort, best density and lowest cost. I also do this so that I can apply acoustic caulk to the bottom and top plates, door and window rough openings and Lessco polypans around electrical boxes as I hang the drywall, to complete the Air-Tight-Drywall system which was developed decades ago in Canada by Joe Lstiburek (now of Building Science Corp.), and which also requires sealant or gaskets between framing assemblies.

It is not difficult to achieve 3.5 pcf density with a double-framed wall, but it requires some special techniques such as the “leap-frog” method of pre-inserting several 3″ hoses in adjacent stud bays so that each blow starts from the bottom as the open cavities gradually fill up. Secondary densifying can also be done with a stiff 2″ or 1½” hose with a beveled end for inserting into partly-dense cavities.

If there are no solid top plates, such as in my Riversong Truss wall, then filling can be done from above, one floor at a time. If there are top plates (I use CDX double top plates on my 12″ double stud walls), then a better method than drilling holes through the drywall is to staple a 12″ wide strip of Insulweb or filter fabric horizontally across the middle of each wall, leave a 4″-6″ gap between the upper and lower drywall sheets, blow through that gap from the bottom plate upwards, and then fill the gap with 3/8″ strips of drywall for a seamless finish.

All my exterior framing is 24″ on center, and I’ve never had problems with screw pops (the double wall system actually reduces localized air pressure). The brand of cellulose I use, Cel-Pak by National Fiber, has been third-party certified as a fire stop and I’ve never had an inspector even question the effectiveness of borate-treated cellulose as a fire stop in balloon-framed walls – it is better than solid wood.

I’ve also not had a problem with Energy Star doing the thermal bridge inspection before drywall is hung and the cellulose is installed.

I am a firm believer in walls that can breathe in both directions in a cold climate. I use either board sheathing or no sheathing (let-in steel T-bracing for shear), though CDX is far preferable to any kind of OSB, including ZIP, because it’s vapor permeance is not only slightly higher but also increases with increased saturation (sort of like the high-tech MemBrain). On the inside I use nothing more than Benjamin Moore Super Spec vapor retarder primer (less expensive than most good quality primers), and 1 perm to meet IRC standards.

30 Bryan Shephard December 30, 2010 at 7:00 pm

I’ve found that strips of half inch plywood or OSB work well for holding the cellulose netting in place. They allow the cellulose to bulge out a bit without causing a problem. Obviously any electrical boxes need to be spaced out an extra half inch.

31 Robert Riversong December 30, 2010 at 8:06 pm

Bryan, you’d rather install netting with staples 2″ on center and then rip strips of sheetstock and nail them up and have to move all electrical boxes out than just install the drywall and get a higher density cellulose installation without all the fuss? I don’t get it.

32 Devin Miller March 6, 2011 at 12:18 pm

Blown in fiberglass would be the way to go I believe. Listening to the problems that you are having with cellulose, fiberglass insulation would take care of those. It is blown into walls similar to cellulose, fills all voids and is screeded off during installation so no drywall pops. It has a powder activated glue that requires much less water to activate and hold it into place than the cellulose. The manufacturer of my product recommends a 24 hr drying time which is way less than the 72 hr drying time recommended by most installers of wet sprayed cellulose. Blowing loose fiberglass into attics also makes a better job. It only settles around 1 percent, significatly less than the 20 to 25 percent cellulose settles. I have used cellulose on a few of the homes ive built and had some issues with it settling and falling out of walls even being installed wet sprayed. Do some research on the fiberglass and see what you come up with it may suprise you.

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