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Musings of an Energy Nerd

How to Install Cellulose Insulation

Insulation expert Bill Hulstrunk provides detailed instructions for blowing loose-fill cellulose on attic floors and dense-packing walls

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A 3-inch hose works fine for loose-fill cellulose. The fastest way to move cellulose to an attic floor is through a big 3-inch hose.
Image Credit: National Fiber
A 3-inch hose works fine for loose-fill cellulose. The fastest way to move cellulose to an attic floor is through a big 3-inch hose.
Image Credit: National Fiber
With the right technique, cellulose can be dense-packed behind air-permeable netting to a density of 3½ pounds per cubic foot.
Image Credit: National Fiber

In some parts of the U.S. — notably northern New England — cellulose insulation has been widely used for more than 30 years. In other parts of the U.S., however, cellulose insulation is just beginning to gain traction.

Of course, cellulose insulation is installed with different techniques than those used to install fiberglass batts or spray foam. To help explain these techniques to builders who are unfamiliar with cellulose, we decided to interview Bill Hulstrunk, the technical manager at National Fiber, a manufacturer of cellulose insulation in Belchertown, Massachusetts.

Hulstrunk has worked as an insulation installer, an energy auditor, a weatherization program director, and a trainer. He has presented workshops at national conferences on a variety of topics, including the design of superinsulated buildings, air-sealing techniques, insulation performance, pressure diagnostics, and thermal imaging.

Q. What type of equipment is used to blow cellulose?

Hulstrunk: If you are going to be an installer, you need to own your own blowing equipment, which typically costs from $5,000 to $10,000. We don’t recommend that our installers use rental machines.

These machines will be reasonably sized. Typically an installer will show up in a box truck or pulling a trailer. The equipment draws from 15 to 30 amps, depending on the machine. The 15-amp machines can be plugged in, but the 30-amp machines need their own generators.

Q. What is the most important thing to remember when installing loose-fill cellulose on an attic floor?

Hulstrunk: Since you are installing the insulation at a lower density, be sure you do all of the necessary air sealing work beforehand. Air sealing is critical. When homeowners say, “I don’t have enough money to do both air sealing and insulation,” I tell them, “Then it’s better to wait until you have enough money to do the air sealing — otherwise the insulation doesn’t make any sense.”

Many of our cellulose installers, especially the ones who are BPI-certified, will do both air sealing and insulation work. With some of the utility efficiency programs, though, there will be a separate air-sealing crew that comes in first.

Q. What diameter of hose is used for installing loose-fill?

Hulstrunk: Usually a 3-inch-diameter hose, the same size that comes off the machine. We find that when you get over 200 feet of hose you have a harder time pushing the material, but it depends on the equipment. If you’re installing loose-fill on an attic floor, that’s one application where you could run a machine rented from a local hardware store — it’s a less critical application than dense pack.

Q. How much settling should an installer expect?

Hulstrunk: Approximately 13% settling. That occurs weeks or months after the installation. I always recommend that owners specify the settled thickness or R-value they want, and leave it up to the installer to do the calculations to determine how much material to put up there.

The installed thickness is somewhat dependent on how you hold the hose. The settling chart on the bag assumes that you hold the hose at a 45-degree angle and shoot it upward. But if you don’t have the room to do that, and you shoot it straight out or downward, you increase the installed density — and in some cases it won’t settle at all. The coverage per bag goes down, but you don’t have any settling.

Most installers, if they are looking for 12 inches, will put in 14 inches just to be sure they don’t have any issues with settling. I always tell installers to be careful with these calculations, because you don’t want a customer to call you back to install more insulation. Once you have to go back a second time, you’ve lost money on the job.

Q. Is it possible to cover recessed can lights with cellulose?

Hulstrunk: It depends on the rating on the can. If it is an IC-rated can, you can cover it — depending on a couple of factors. The National Electric Code seems to indicate that you can’t put any insulation above can lights, but in fact we do it all the time. The code addresses worries that you can trip the thermal switch with some high-wattage bulbs. In all areas except Massachusetts, we just cover the can lights, and we don’t have problems. But in Massachusetts, many of the utility programs don’t allow coverage of the can lights.

There are really two kinds of IC-rated cans: ordinary IC and IC airtight. The regular IC lights are really leaky, so if that’s what we’re dealing with, we need an airtight enclosure around the light anyway. Instead of building a box, a lot of installers are using a large Sonotube. You cut a length of the Sonotube and foam it to the ceiling below. Then you make a top out of sheetrock and foam the top in place. Once you’ve done that, you can cover the whole thing with cellulose. That stops the air leakage and ensures that the thermal switch won’t blink on and off.

Q. Is it possible to cover bath fans with cellulose?

Hulstrunk: Yes. That’s routine. However, if it is an older combination fan/light that takes a standard screw-in incandescent bulb, it’s better to treat as if it were a non-IC recessed light fixture, and enclose it in a box before insulating.

Q. Can you blow cellulose against a chimney?

Hulstrunk: No, you need to keep cellulose away from chimneys and metal flues. A masonry chimney requires 2 inches of clearance. We advise installers to air seal the crack where the chimney penetrates into the attic, using metal flashing and high-temperature caulk. Then the installer should take a 3½-inch-thick Roxul batt — a mineral-wool batt — and wrap it around the chimney, securing it with a wire so it doesn’t fall off. Then you can blow cellulose right up against the mineral wool batt.

We do the same thing with metal chimneys, although with B-vents the clearances get a little higher. National Fiber has published a technical bulletin listing the clearances.

Q. Have you ever heard of condensation problems when air-conditioning ducts are buried in cellulose?

Hulstrunk: No, I’m not familiar with that problem.

Q. Is there a maximum depth of cellulose insulation that can be installed above a drywall ceiling before you have to start worrying about the weight of the insulation causing the drywall to sag?

Hulstrunk: We have never seen a sagging issue due to the weight of the cellulose installed above a ceiling. That may be because some of the weight of the cellulose is being redistributed onto the ceiling joists. We have blown very high R-values, up to R-100, and never had any issues with the ceiling sagging.

Q. How should air-permeable netting — for example, InsulWeb — be attached to the studs?

Hulstrunk: The best technique is to staple it with a pneumatic stapler right to the face of the stud. By ‘face,’ I mean the 1½-inch-thick edge of the stud facing the room. The staples have to be less than 1 inch apart, so you’ll go through a good number of staples.

Some installers prefer inset stapling or “lip stitching” — they put the staples at the corner of the studs, wrapping the InsulWeb a little bit around the corner as they staple. But that really isn’t necessary. I don’t like the lip-stitch method, because I want the insulation to be close to the sheetrock. Installers who use lip-stitching are usually worried about bulges, but we’ve found that an aluminum roller works well to eliminate any bulges.

There’s another method of installing the InsulWeb: using glue. First, you tack the InsulWeb up quickly. Then you apply watered-down Elmer’s glue to the studs through the InsulWeb, using a trim roller — you roll through the InsulWeb to push in the glue. It gets the InsulWeb really tight and nice. By the time you get to the last wall in the house, the first wall you glued is usually dry enough to start blowing cellulose. It’s best to wait at least 6 hours for the glue to dry.

Q. What do you do about bulges?

Hulstrunk: After you’re done blowing the walls, the material will be bulged out a little bit between the studs — maybe out an inch or so in the middle. So you take an aluminum roller — it’s about 1 foot long, maybe 3 inches in diameter, and sold specifically for this purpose — and you roll the bulge really quickly. If you do it right, it will leave the cellulose flat, and a long metal straightedge will touch the studs. We recommend that owners or builders include this language in their wall insulation specs: “The material will be rolled flat, ready for drywall.”

Q. Is it really possible to achieve a density of 3½ pounds per cubic foot behind netting?

Hulstrunk: Yes, if you use the right technique. We recommend using a rigid tube — a 4-foot aluminum tube, 2 inches in diameter. The tube should have a 45 degree bevel at the end — the bevel lets you pop the tube through the InsulWeb without cutting a hole first. It also allows you to aim the cellulose with good directionality.

With experience, the installer can determine the density by the way it feels. Between 3 and 4 pounds per cubic foot, the feel of the installed material goes from soft (at 3 pounds) to actually hard at 4 pounds. At 3½ pounds per cubic foot, it will feel like a firm mattress. It really changes dramatically between 3 and 4 pounds.

When I first heard people claim that they could determine density by the feel, I thought there would be no way to tell. But it does change in a dramatic way, and you can feel the difference.

The density is important — partly for increased resistance to air leakage, but also because you want the material to be self-supporting. If it is dense enough, there is no downward force, and it can’t settle.

In each stud bay, you need a single hole about halfway up from the floor. The trick is to move the pipe around and back and forth. You start at the bottom, and you move the tube back and forth. The corners are the areas where you might end up with a soft spot, so you should start at the corners and then work your way up through the hole. Then when you reach the hole, you flip the tube upwards and work from the top down.

You can always feel the material through the InsulWeb, and if you find a soft spot, you can move the tube to that area and inject a little more. Then you move on to the next stud bay.

Q. Do you have to patch the holes in the InsulWeb once you’re done insulating a wall?

Hulstrunk: No. The cellulose won’t fall out, so there is no need to patch the holes.

Q. What about holes in the InsulWeb near penetrations and electrical boxes — will a lot of cellulose blow out of these holes and cracks when the stud bay is being filled?

Hulstrunk: No. You don’t have to worry about holes or cracks — even cracks up to 1 inch wide. If you have a really big hole, you can staple up a new strip of InsulWeb, from stud to stud, before you start insulating. The stud bay is filled from the bottom up to the middle, and from the top down to the middle. When the hose nozzle gets close to a big hole or crack in the InsulWeb, the installer might temporarily put his hand over the hole to minimize the amount of cellulose that blows out. But not much will blow out. Once the bay is full and dense-packed, the cellulose won’t fall out. In any case, it all gets covered with drywall soon enough.

Q. Is damp-spray cellulose installed at a lower density than an InsulWeb installation?

Hulstrunk: No. With a damp-spray installation, we are looking for the same density —3½ pounds per cubic foot.

Q. Why do some installers prefer the damp-spray method?

Hulstrunk: Some installers choose to spray because it takes less time for the installer to get in and out. For some jobs, a spray installation could cost a little less. One limitation: when you spray, the floor has to be clean, because the installer is recycling the material. If there is a lot of lumber or other construction material in the way, it can interfere with the installer’s ability to recycle the material that falls to the floor.

Q. How is water added to damp-spray cellulose?

Hulstrunk: The spray truck contains a water tank. When the installer is spraying, there are usually three nozzles close together — a flattened nozzle with cellulose blowing through, and two water-misting nozzles. The water is added to the dry cellulose as the material exits the nozzle.

Q. Can the installer adjust the water application if there are signs that the insulation is too wet?

Hulstrunk: Yes. They have a couple of adjustments. Usually the easiest way to adjust the amount of water is to change the tip size at the end of the water nozzles. There are very small orifices that atomize the water — the water-pump pressure is usually at 500 to 1,000 psi. If you see that the material is going on too damp or too dry, you can swap to a different orifice size.

Q. Do damp-spray cellulose applications ever include glue?

Hulstrunk: It’s possible, but we feel that if you have a well thought-out system you don’t need to add glue. I haven’t had much experience with the glue method. We can spray very dry without adding any glue. Glue can make it easier to spray cellulose at a lower density, but I am not an advocate of dropping the density down very much. I’d rather have a density of 3½ pounds per cubic foot.

Q. Have you ever heard of a damp-spray job were the insulation just wouldn’t dry and had to be removed?

Hulstrunk: The only times we have only had problems are when we waited a long time before installing the drywall. Now that buildings are significantly tighter than they used to be, the construction moisture loads in these buildings tend to be greater.

We recommend waiting 24 hours before hanging the drywall. After that, the surface of the cellulose is dry. It isn’t all dried out, but if we leave it for a longer time, the construction moisture in the building can migrate into the cellulose. If it remains uncovered for a month, the construction moisture can make the material very wet. It can end up with a greater moisture content than when it is sprayed.

If I get a call that a spray job won’t dry, the next thing the caller usually says is, ‘I’ve been waiting a month.’ But we like to see the sheetrock on there after 24 hours, because the sheetrock actually slows down the moisture migration from the indoor air into the cellulose.

The cellulose wants to dry out, and it always dries out eventually. The idea that the cellulose in a wall could stay wet and not dry out is not correct.

Q. Can you describe the steps of a damp-spray installation?

Hulstrunk: You start at the bottom of the wall and build the materially up vertically. After the stud bays are filled, there will be some high spots and low spots. To make the high spots even with the studs, we use a ‘scrubber’ to scrape the wall and even it out.

The scrubber is like an elongated paint roller with a rubber face. When you scrub the wall, the scrubber removes anything beyond the face of the studs. The material falls to the ground, where it is collected by a vacuum recovery system hooked up to the truck. The truck has two hoppers — one for the dry material, and one for the recycled material. By keeping them separate, you make sure you end up with a consistent moisture level when the material is sprayed.

Q. Can you determine whether a wall is dry enough to hang drywall just by looking at it?

Hulstrunk: Visually, we can get a really good idea of the moisture content of the material once it is scrubbed. After scrubbing, if the cellulose looks mottled, with some darker areas and some lighter areas, that means that the moisture content is how we like it.

But if it is consistently dark, we know the material is wetter than we like. That’s not good, because when it is that wet, it can sag. If that happens, you can see a gap at the top of the stud bay. In the industry, these gaps are referred to as ‘smiles.’

If you have a stud bay that looks dark in the entire bay, you probably want to pull the material out so there is no chance of it settling, and then re-spray with less water.

Q. Are moisture meters useful?

Hulstrunk: Most moisture meters are set up for the density of lumber or drywall. If you talk to the manufacturers, they’ll tell you that the meters are not calibrated for the density of insulation, which is a low-density material. That’s why moisture meters tend to be misleading when used on cellulose. And you tend to get into questions about how deep you should poke it in. You’ll find that the numbers vary quite a bit depending on how deep you insert the probes.

So we just say, if you cover the walls after 24 hours, we guarantee there will be no damage to the wall or finishes as the moisture migrates out. People don’t realize that the longer you leave the insulation exposed, the chances are good that it can actually get wetter. It’s because we have tight buildings with lots of construction moisture. It is counterintuitive. It often helps to crack a window or two.

Q. Is it possible to install damp-spray cellulose overhead, in an open ceiling?

Hulstrunk: No, cellulose insulation cannot be sprayed overhead — except for specific cellulose-based fire-retardant products with a high glue content that are used as thermal barriers over steel or foam insulation.

Q. Some builders install drywall with a 4-inch horizontal gap in the middle of the wall, 4 feet off the floor, and then blow cellulose behind the drywall. Does this system work well?

Hulstrunk: I’m not a fan of that method. I prefer to dense-pack behind InsulWeb, because you can feel the cellulose through the InsulWeb, and you can see what is going on.

Another problem: you are injecting the cellulose pneumatically, so the wall needs air relief. If the wall is too tight, the installer can mistakenly think he has reached the proper insulation density when there is really a problem with air relief.

Q. Does the same problem with air relief occur when you install cellulose behind polyiso foam on a ceiling?

Hulstrunk: Usually we don’t see a problem with air relief when the installer is blowing into a ceiling. I think that installing polyiso foam on the ceiling and injecting cellulose behind it is a good application method.

Q. When you are blowing behind rigid foam into rafter bays, how far apart do you space the holes?

Hulstrunk: Usually one hole per rafter bay is enough — one hole in the center of the span. But it depends on the diameter of the hose. The bigger the diameter, the lower the velocity of the cellulose leaving the end of the hose, and the shorter the distance that we can dense pack. Smaller diameter hoses give you better density and better consistency.

If you’re dense-packing a ceiling, we recommend that you pre-fill the assembly using a big hose — the 3-inch diameter hose. You insert the big hose and pre-fill the cavity. That will result in a lower density than what we’re aiming for, so then you want to go back with a smaller tube — a 1½-inch or 2-inch tube. You want an aluminum or TigerFlex tube that you can insert through the pre-filled material.

If you have a 12-foot ceiling span, you’ll be accessing the ceiling from one 3-inch-diameter hole in the center of the room, so you can blow 6 feet in either direction. Once you reach the desired density, 3½ pounds per cubic foot, you can’t push a tube back through the material any more, because it’s too dense. That’s one way to tell if you’ve reached the density you’re aiming for. But if there is a weaker area, you will be able to move the tube toward that low-density area.

If the rafters are 16 inches on center, you can usually blow down the center of the cavity and get good consistent density. But if the rafters are 24 inches on center, you want to move the tube first to the right, and then to the left, and then back to the center as you pull the tube through.

The installer should also know how many bags they need to fill the bay to the right density, and they should double-check the bag count. If it’s not dense enough, you can always put the tube back in and blow some more.

Q. Does the same technique work with 12-inch-thick double-stud walls?

Hulstrunk: Yes. With very thick walls, we also use the pre-fill method. First you pre-fill the wall loosely, using the 3-inch hose right off the machine, with the feed gate opened right up and the air all the way up. You poke a hole through the InsulWeb near the top of the wall, every 24 inches, and using a 3-inch-diameter hose, you just dump the material into the wall and fill it up.

Once you’ve done that, you go back with the 2-inch-diameter dense-pack hose — the hose attached to the 4-foot aluminum tube. You come back and pop a hole near the center of the cavity, about 4 feet off the floor, and you aim the aluminum tube downwards. Beginning at the bottom of the wall and working your way all around the cavity, feeling for soft spots, you dense-pack the wall and then draw the tube back up. Then you aim the hose up, and you do the same thing, working from the top of the cavity downward toward the hole. You do this all around the wall, every 24 inches, until you’re sure you’ve got the density you need.

When it comes to thick walls, it’s all about density. One thing I have learned over the years is that when we get to thicker walls, we have to increase the density a little more — to just over 4 pounds per cubic foot for a 12-inch thick wall, and to as much as 5.1 pounds per cubic foot for a 27-inch-thick R-100 wall.

These recommended densities are based on a study performed by the Danish government. The Danish researchers built cavities of different thicknesses and installed cellulose with different densities. They put the wall assemblies on a vibrator. Based on that research, I modified the information on our expanded bag coverage chart to reflect the need for greater densities for wider walls.

Q. When dense packing cellulose in the walls of an existing old house from the outside, how many holes are drilled per stud bay?

Hulstrunk: One hole per stud bay. The best technique is to remove a section of the siding — my preference is to pull the siding nails and drop the siding, and then to put the siding back on at the end of the job. I’m against drilling through siding, and I don’t like siding plugs.

If you pull off some of the siding and it starts disintegrating, it’s best to stop the job and talk to the homeowner. You don’t want to wait until the end of the job to tell the homeowner that the siding can’t be reinstalled. There are some siding types that are very difficult — old clapboard that is ready to fall off the building, or blind-nailed asbestos siding. In some cases we can blow the walls from the inside, but that’s not a great way to proceed because of all the plaster dust or drywall dust.

If you can get some of the siding off, you drill a 2 9/16-inch hole in each stud bay. If you can, it’s a good idea to bevel the hole a little on the inside by tilting the drill as you finish the hole.

For ergonomic reasons, I want the holes to be between waist and chest high. That means the holes will be toward the bottom of the bay. You’ll be using a 1¼-inch or 1½-inch diameter hose, a 10-foot tube made out of TigerFlex — a helix-reinforced rubber hose with plastic braid that runs around it.

You start at the bottom of the stud bay. You push the hose to the bottom plate, then pull it up 6 inches. You turn the machine on, and you wait until the material stops flowing, then you quickly pull up the hose until the material starts flowing again. You do that until you reach the hole. Then you do the same thing starting from the top, working down. If the hose doesn’t go up to the top of the stud bay — if you can’t twist the hose past the obstruction because of fire blocking or something else, you have to drill another hole.

Q. What about the rim joist area?

Hulstrunk: You climb up the ladder and remove the siding at the rim joist. You drill a 3-inch hole and look in the hole. If you are looking down a joist bay, you take a feed bag — we used to use one of the 100-pound grain bags from Blue Seal Feeds, the plastic ones, but now that most grain is sold in 50-pound bags, they’re harder to get. But we’ve found a source for the big bags — we can still get them.

You pull the bag over the end of your tube, so the tube is ready to fill the inside of the bag, and you insert the tube with the bag over the end into the 3-inch hole. You want to keep holding on to the bag so the bag doesn’t go flying when you start to fill it. The bag is mostly in the joist bay when you do this. You fill the bag with cellulose — you literally fill the bag.

Now the key to this technique is getting around the edges of the filled bag. The bag doesn’t always completely fill the joist bay that it is in. So after you have filled the bag, you pull the hose out. You stuff the end of the bag into the hole. You push it back 6 inches or a foot, and then you dense-pack the gap between the bag and the hole. You aim the hose to seal off any gaps.

Q. Is there a good way to patch holes in stucco when retrofitting cellulose in an existing stucco home?

Hulstrunk: Typically stucco homes are done from the inside, because stucco is hard to patch. You can drill through stucco with a carbide-tipped hole saw, but patching the stucco is difficult.

Q. What type of ventilation chutes can resist the pressures of dense-packed cellulose?

Hulstrunk: The baffles need to be rigid enough, so the styrofoam ones are completely out. We have had good luck with AccuVent baffles. They will withstand the pressure of the dense-pack without collapsing.

Q. Do you have any comments about the controversy surrounding dense-packing unvented cathedral ceilings?

Hulstrunk. We feel comfortable enough to warrant the material in that assembly for the life of the building. We will stand behind these installations. The installations that have had issues have always been related to density problems. We tell our installers, ‘If you aren’t sure of the density of your installations, you shouldn’t be using this technique.’ The material has to be at the right density, and in full contact with the exterior sheathing.

Q. Do you think that the latest building codes allow for unvented insulated sloped ceilings with cellulose?

Hulstrunk: The argument we make is that when you consider the combination of the dense-packed cellulose and the drywall, the assembly is air-impermeable. The intent of the code is to prevent moist air from having contact with the sheathing, and all we need to do is convince the code official that that won’t happen.

[Editor’s note: Although Bill Hulstrunk advocates the installation of dense-packed insulation in unvented cathedral ceilings, most building code prohibit this approach. Green Building Advisor recommends that builders include a ventilated air channel between the top of the cellulose insulation layer and the underside of the roof sheathing. For more information on this issue, see How to Build an Insulated Cathedral Ceiling.]

Q. What about vapor retarders?

Hulstrunk: We have been injecting cellulose into buildings without vapor retarders since the mid 1970s, and there have been no moisture issues with these buildings. We know from experience that cellulose can be installed without an interior vapor retarder.

We recently had a professional engineer run some WUFI models for us. According to the models, a cellulose wall without a vapor barrier in Massachusetts will remain dryer than a wall with a vapor barrier because of summer vapor transmission into the wall assembly from the exterior — walls with a vapor barrier have accumulation behind the barrier in the summer. Vapor barriers shouldn’t be used in buildings that have both heating and cooling because vapor flow reverses during the summer.

However, even if you decided to install a vapor barrier, I have never seen a problem when a vapor barrier is used with cellulose. If somebody really wants a vapor retarder, we recommend installing a vapor-retarder paint.

The only type of building where a vapor barrier might make sense would be in something like a swimming pool facility.


Last week’s blog: “The Energy Star Homes Program Raises the Bar with Version 3.”


  1. 5C8rvfuWev | | #1

    thanks again
    This blog entry has answered more questions than I knew I had. Very much appreciated, especially as I go forward and anticipate or set expectations for contractors.

    Joe W

  2. Aj Builder, Upstate NY Zone 6a | | #2

    Another very very good blog
    Another very very good blog Martin. You consistently deliver what we need to know to do our best work.

  3. Kevin Dickson, MSME | | #3

    "The argument we make is that
    "The argument we make is that when you consider the combination of the dense-packed cellulose and the drywall, the assembly is air-impermeable."

    "I tell them, “Then it’s better to wait until you have enough money to do the air sealing — otherwise the insulation doesn’t make any sense.” "

    Martin, do you think this is a contradiction? Could you have Bill explain it ?

  4. Jack Woolfe | | #4

    Thanks, Martin and Bill, for
    Thanks, Martin and Bill, for a very helpful blog.

    Kevin, the air-sealing comment pertained to a loose-fill attic, while the air-impermeable comment had to do with dense-pack walls or ceilings.

  5. User avater GBA Editor
    Martin Holladay | | #5

    Response to Kevin Dickson
    I agree with Jack's comment. Obviously, attention to airtightness improves the performance of any assembly.

    The ceiling below an unconditioned ventilated attic tends to be leaky, unless attention is paid to sealing air leaks. This air-sealing work is particularly important with older buildings, where holes can be large enough to swallow up a small child. So of course Hulstrunk is right that it makes no sense to blow insulation on the attic floor of an existing house unless air sealing work has been completed.

    The second case -- negotiating with a building official about whether it is permissible to insulate an unvented cathedral ceiling with cellulose -- is obviously a new-construction situation, not a retrofit situation. In such a situation, the builder has control over the airtightness of the drywall. I agree with Hullstrunk that, with care, it's possible to make a drywall ceiling airtight. It certainly behooves the builder to do so, because if the ceiling assembly leaks air, the level of risk rises -- and the builder will be at risk of facing an expensive callback.

  6. Raymond Evangelista | | #6

    Best product I have used to date.
    I can’t speak highly enough about the national fiber product. We started installing their product over this past summer and fell in love with the product right away. The project in question was in Hoboken and we had to dense pack a 16” cavity with cellulose. The thing that caught my attention right away was the lack of dust and how cleanly we were able to fill the cavities. At the end of the project we found that we have 30 bags left that we didn’t expect to have. Using less product is always a nice plus!

    This past fall we visited their facility for a training class and to see how the product was made. I was surprised to see that they exclusively use over print materials like news papers and phone books. It’s good to know that the New Your times, Boston Globe, and some of the less reputable New York papers we being put to good use. I was hoping for an overdraft playboy or two, but no luck. FYI that was a joke.

    If you have a distributor in the area I highly recommend trying their product. We are building a passive house and we just placed an order for three truck loads. If you are interested in reading about our passive house project visit

    Feel free to call me if you have any questions about their product or how to install cellulose insulation.

    411 Energy Services LLC

  7. Ken Field | | #7

    Reasons to use that brand
    I tried a cellulose product that is manufactured about 20 miles from us and thinking that they were all the same was a mistake. My installers complained about plastic bags and inconsistent texture while blowing. In addition to having Mr. Holstrunk available for tech support, the product is great. Dust is almost nil and we havent clogged a hose in weeks. I will happily pay more for the shipping from CT. to PA because it allows me to give my customer a better job and charge for it accordingly. Garbage in=garbage out.

  8. Walter Gayeski | | #8

    Great Article
    Really enjoy the tricks of the trade section. Still trying to work out setting on machine when dense packing walls. Gate closed 50% with air reduced to 70% of max. Kendle 425. Any suggestions?

  9. Chris White | | #9

    Walter- contact me, Chris White at 413-531-4308, to set up a machine optimization visit. As long as you are using National Fiber, there should be no problem figuring out what to do to get the results you want (I work for the same company Bill Hulstrunk does).

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

    Bill Hulstrunk responds to Walter Gayeski
    [Bill Hulstrunk sent the following response to Walter Gayeski by e-mail:]

    With the Krendl 425, the air should be set at 100% and the feed gate closed to approximately 25% depending upon the size of the installation hose being used. The bag coverage chart can be used to confirm proper installation density of 3.5 lbs/cu. ft. If not enough bags were used, than close the feed gate more, reinsert the installation hose and reinsulate the cavity to increase the installed density to achieve a dense pack.

    Bill Hulstrunk

  11. John Klingel | | #11

    very good
    I, too, learned from this. Thanks very much for that. I will have to check on whether or not that particular cellulose is available here. john

  12. Dan Kolbert | | #12

    Another vote for Bill
    I've known Bill for a while and he's my go-to guy for cellulose questions. Thanks for the piece, Martin.

  13. Alexis Greene | | #13

    Comprehensive Cellulose Article
    This articles matches up well with Community Environmental Center’s experience installing cellulose (mostly damp-spray) in New York City residential buildings. It definitely took trial and error before we came to the right mix of installation strategies, but we’ve been enjoying great success with both home performance and weatherization jobs. We recently wrote an article on cellulose insulation from an advocacy perspective (much less comprehensive on the installation details!) that you can check out here:

  14. Chris Brown | | #14

    To Raymond Evangelista: 30 "left over bags"?
    Really, a few may be explainable, but 30? If your customer reads this blog he might be asking for an explanation!

  15. Tim Miller | | #15

    Dense pack R-value is higher?
    Great article. Thank you for the information. I am confused -- how does dense-packed cellulose have a higher R-value then loose fill? Is it not the air spaces that influence the R-value?

    Oh, this sounds way dumb, but reading the labeling on cellulose, how can I tell if it has borates in it? Lowes sells "Greenfiber" but nothing on the bag about additives for insects or fire resistance. Their web page has a different package stating it's got borate on it. (I thought all of it had borate.)

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

    Response to Tim Miller
    The R-value per inch of installed cellulose generally increases as installed density increases -- up to a point. Exactly where that point is depends on the characteristics of the cellulose manufacturing process. Modern "fiberized" cellulose is manufactured using a process that results in smaller pieces of material than older hammer-mill manufacturing processes; this "fiberizing" results in better thermal performance.

    You'll get slightly different R-value information depending on which manufacturer you talk to. According to one source, loose-fill cellulose has an R-value of R-3.8 per inch, while dense-packed cellulose (at 3.5 pounds per cubic foot) has a slightly lower R-value of R-3.65 per inch.

    However, according to Bohdan Boyko, a technical rep from U.S. GreenFiber, the fiberized cellulose introduced in 1990 has about the same R-value per inch whether it is installed as loose-fill or dense-pack. The main advantage of dense packing is NOT an improvement in R-value; rather, it is a reduction in air infiltration rates.

    Dan Lea, the executive director of the Cellulose Insulation Manufacturers Association, has a slightly different take from Boyko. He told me, "The guys at ORNL and R&D Services ... [performed] research that we paid big bucks to do. Their conclusions: at nominal settled density, cellulose R-value is R-3.6 to R-3.7 per inch; at wall density it is R-3.8 to R-3.9 per inch. They base this on studies of multiple materials."

    Dan Lea also sent me an e-mail in which he noted, "It is clear that the R-value of current fiberized cellulose products increases with density up to a point somewhere between 3.3 and 4.0 pounds per cubic foot." So if you are installing the product at a very high density, you will probably see the R-value per inch start to drop off.

    Concerning your final question: I'm fairly sure that the Greenfiber cellulose sold at Lowe's includes borates.

  17. Raymond Evangelista | | #17

    To Raymond Evangelista: 30 "left over bags"?
    I based my number of bags on how much green finer I would have needed. When I use green finer I find that I normal y need 14% - 20% more then they recommend. With the national fiber product I needed a little less than they said I would need.

    Trust me when I tell you the job was done right. I don’t take a job if I don’t think I can do it correctly. I also scanned the project with the infrared camera when we were done. The infrared camera doesn’t lie!


  18. Steven McCarthy | | #18

    coverage charts
    Most of my work is retrofit of existing buildings but in the last year, I have blown some new attics. On all of these jobs, I have used 18 to 20% more insulation than the coverage chart called for. My supplier has recently had their material recertified but the coverage chart changed very little. These blows ranged from R-30 in a commercial building to R-50 and R-60 in residential. I never point the hose up at 45 degrees and I constantly measure carefully. What do you think is going on here? Am I to blame?

  19. User avater GBA Editor
    Martin Holladay | | #19

    Response to Steven McCarthy
    Re-read Bill Hulstrunk's answer to the question, "How much settling should an installer expect?"

    In his answer, Bill said, "The settling chart on the bag assumes that you hold the hose at a 45-degree angle and shoot it upward." But you just told us that you don't do that. So the coverage chart is likely to be inaccurate for installers like you.

    You seem to be installing the insulation at a greater density than the chart assumes. There's nothing wrong with that -- in fact, the cellulose won't settle as much in your attics as in attics insulated by your competitors -- but your method uses up more bags.

    If you are willing to risk the possibility of unexpected settling, you could try to install less cellulose next time, but I think it's safer to give the customer a little more than expected rather than risk installing too little.

  20. J Chesnut | | #20

    dens pac install from exterior
    Much appreciated article.

    My office has designed a retrofit scenario where we would nail I-Joists vertically over air tight re-sheathing of a 1930s stud wall assembly. Currently we are considering the 9 1/4" I-Joists bays be filled with dens pac cellulose.

    In this article Bill Hulstrunck mentions two things that informs how we should do this:
    (1) "When it comes to thick walls, it’s all about density. One thing I have learned over the years is that when we get to thicker walls, we have to increase the density a little more — to just over 4 pounds per cubic foot for a 12-inch thick wall, and to as much as 5.1 pounds per cubic foot for a 27-inch-thick R-100 wall."
    (2) [Concerning dens packing behind drywall] "Another problem: you are injecting the cellulose pneumatically, so the wall needs air relief. If the wall is too tight, the installer can mistakenly think he has reached the proper insulation density when there is really a problem with air relief."

    We were thinking of spacing the I-Joists 32" apart and sheathing with a fiberboard or exterior grade gypsum sheathing and then installing the cellulose through the sheathing. However in this scenario maybe the density of the dens pac needs to be increased above 3.5lbs/cf and it would be difficult for the installer to achieve the proper density installing through a hole in the rigid sheathing.

    I'm wondering would Bill's recommendation then be for the 9 1/4" I-Joists to be spaced at 16" O.C. and the dens pac installed through the InsulWeb prior to sheathing? (Note we are trying to cut costs by installing the I-Joist @ 32" O.C. and create a 16"O.C. nail base for lap siding with 2 layers of furring strips the first horizontal across the I-Joists and the next vertical for a drainspace).


  21. User avater GBA Editor
    Martin Holladay | | #21

    Response to J Chesnut
    First of all, your plan to install the I-joists at 32 inches on center, sheathe with fiberboard, and insulate with dense-packed cellulose won't work. Even at 24 inches on center, your fiberboard can belly.

    You should install your framing at 16 or 24 inches on center -- no more -- and use Insulweb so you can see what's going on. Then when the insulation work is done, and the cellulose bellies have been rolled, you can install your fiberboard.

    I just visited a job site in Washington state where they tried to dense pack cellulose through fiberboard sheathing. Some of the fiberboard bellies were 3/4 inch proud -- enough to touch the back of the siding (the job had a 3/4-inch rainscreen gap). Moreover, the installers had a lot of density problems, and they ended up drilling a zillion holes to get enough cellulose in there. Conclusion: "We're never going to do that again."

  22. Steven McCarthy | | #22

    coverage charts
    Thanks for your response. I did read that coverage charts are based on cellulose being blown up at 45 degrees. I would add, that no one installs cellulose that way. No one! Including the person in the image at the beginning of this article. I believe that manufacturer supplied coverage charts for open blown cellulose are grossly inaccurate and not helpful for any contractor that wants to be fair to customers, legal and competitive. I suggest to contractors that they develop their own coverage charts for common insulation blows in new construction by going back to check on settled depth. In my case, Blowing a quarter ton of cellulose EXTRA and the time to do it is a tad too much of a giveaway.

  23. Robert Dickinson | | #23

    To Insulweb or not to Insulweb
    I'm curious about the pros and cons of dense-packing using Insulweb versus just using the drywall as the cellulose container and leaving a 12 inch gap in the middle of the installed drywall in which to blow the cellulose -- i.e. the method described by Riversong in a previous article (and also in an old Fine Homebuilding).

    I can see the benefit of using Insulweb in terms of visibility into the completeness of the job and also a sense of the denseness, versus the opaque nature of blowing behind drywall.

    But it would seem like a much increased cost to install Insulweb on all of ones walls, both the added material cost (mostly the Insulweb) as well as the extra time to properly install it.

    And I feel like I will have less to worry about with the cellulose bulging if I install it behind thick (5/8") drywall than if I install behind Insulweb and then have to roll out the bulges before installing drywall. I could well be wrong about this, and would love to hear if that is the case. Which method best controls the "bulges"?

    Thanks for a great article! Really useful.

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

    Response to Robert Dickinson
    I think you have adequately summarized the advantages of using InsulWeb. If you want more perspective on your question, you are likely to get more answers if you post it where more readers can see it -- on our Q&A page:

  25. Jill Neubauer Architects | | #25

    2 Questions
    1. Is Insulweb needed with a damp spray application?

    2. Is there a way to confirm the appropriate density of a dampspray installation?
    I.e. should you be able to compress it with your hand?

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

    Response to Chris Harris
    Q. "Is Insulweb needed with a damp spray application?"

    A. No.

    Q: "Is there a way to confirm the appropriate density of a damp-spray installation -- I.e., should you be able to compress it with your hand?"

    A. Good question. I assume the answer is yes; you can always feel the density by holding a scrap of housewrap or poly against the exposed face of the cellulose and pressing with your hand to see if the cellulose feels like a firm mattress. But I will e-mail your question to Bill Hulstrunk.

  27. Bill Hulstrunk | | #27

    Damp Spray Application Question

    In vertical damp spray applications, the starch in the wood fiber allows the material to be self supporting if applied at the 3.5 lbs/cuft density. For overhead applications, slopes and flat ceilings, than the Insulweb will need to be stapled up in order to support the dense pack cellulose overhead.
    In both damp spray and dry dense pack applications, the cellulose should have the feel of a firm mattress when pressed with the palm of your hand. In damp spray applications, low density, too much moisture, or poor spray technique can cause small gaps "sags" to develop, usually overnight, between the top plate and the top of the cellulose. If this is observed, the installer will need to recompress the cellulose below at least six inches and than respray these areas at a higher density. Spray applied cellulose is more sophisticated from an equipment and applicator perspective and is best performed by those installers who spray frequently with large equipment specifically designed for damp spray cellulose, i.e. NuWool 1600, Krendl KS-260 or equivalent.

  28. Chris Steiner | | #28

    Hello, I'm getting closer to
    Hello, I'm getting closer to insulating the addition I put on my house this summer and I have a few more questions, I have an unvented cathedral ceiling made of 2x12's 24 o.c supporting a SIP made of two layers of 1.5" extruded foam between a sandwich of 5/8 advantech.

    1) Given that the ceiling cavities are free of wires and electrical fixtures, could I achieve a comparable level of insulation using fiberglass batts rather than cellulose, since that would be much easier for a DIYer like myself to install? Would fiberglass batts be better for the ceiling in some ways?

    2) Does the aluminum roller trick work on the ceiling or is that a good application for lip stitching?

    3) Are there any products like insulweb that are available in smaller quantities? When I inquired with my local supplier the smallest quantity they had was about 4x what I needed. Granted it was only $120 a roll but still I hate to buy more than i need if I can help it.

    4) I see now in the latest addition of FHB that I messed up when choosing my 3" foam thickness for my climate zone 6 house. All the info I had at the time (J. Lstiburek fhb 221, and C. Wing The Visual Handbook) showed an R10 layer of foam over the sheathing on a hybrid roof insulation plan. They seemed to be only concerned mostly with the final R value. Why does the roof require more insulation than the walls to prevent the dew point form being reached? Is my roof doomed to premature failure?

    Thanks so much for all your help so far, -Chris

  29. User avater GBA Editor
    Martin Holladay | | #29

    Reply to Chris Steiner
    In your climate zone (zone 6), the code requires at least R-25 of foam above your roof sheathing for your approach to work. Have you talked to your local building code official about your plan?

    As far as I know, the reason that the code is more conservative about roof dew points than about wall dew points is that roof failures are more common than wall failures. Because of the buoyancy of warm air, there is more air movement through roofs than walls.

    I can't really recommend your approach, since it doesn't meet code. However, if your installer does a very good job dense-packing the rafter bays, your roof will probably be fine.

    Concerning whether you will get the same performance with fiberglass batts as dense-packed cellulose: no, you won't get the same performance. Although perfectly installed batts might have the same R-value, they will always be more air-permeable than dense-packed cellulose. The fact that fiberglass batts don't slow down air movement will make them more risky in your case.

    If I were you, I would spend the $120 and just buy the Insulweb. Are you doing the dense-packing yourself? If so, I hope you have a powerful blowing machine.

  30. Chris Steiner | | #30

    My local code officer is
    My local code officer is about 70 years old and really only cares about the footprint of the addition. If I tried to describe what I was doing he wouldn't understand/care. I called him today to set up an appointment to have him review my work and he said he had totally forgotten that I was over here building something. I had him come look at my framing in July and he seemed confused as why I was having him look at my construction project. I live on the mid coast of Maine. If I went inland a few towns I'm pretty sure I could build WHATEVER I wanted without ever having to worry about the building code. For me building codes are like this inaccessible text that really doesn't apply to me anyway, except of course when it comes to the laws of physics.

    I tried to do my homework on this one but it never occurred to me that the dew point would be different for the roof. All the references I had at the time explicitly showed R10 foam on the roof deck with no mention of the foam thicknesses varying for different climate zones. I thought I was playing safe by making it R15.

    So what should I do now, should I add 2'' of foam to the inside of the rafter bays or would that just create a new moisture problem? Whats going to happen if I do nothing? Is it just that on super cold days I will get some condensation on the underside of the sheathing? If I have a dense packed rafter cavity 11 1/4" thick is there really going to be any interior heat getting to sheathing?

    I had this house insulated last year (there was absolutely none, not even the attic!) and they dense packed the rafter bays in the sloped ceiling part of the interior knee walls with cellulose. The rafters are only rough cut 2x4's (It's an old house). I was a little freaked out when I came home and saw what they had done, but my energy auditor assured me that this was common practice and proven to work. It seems like if that is OK, then what I did should be 10x better.

    As far as doing it myself, yes I was going to, I had gotten an estimate to have it done and it was way more than I had expected. They had broken it down into its parts and seemed a little outrageous to me, for example they were going to charge me $840 just to blow the 24'x14' open attic space. That's almost $600 in labor for a couple of hours of work. I can't afford to waste that kind of money on something I can do myself. They confessed that they had never done a job with insulweb before, they only did retro fits. I figured why pay them to learn when this is something that i want to know how to do anyway. But I think now I'll shop around a little for some other estimates.

    Thanks again for your prompt replies to my questions, I only wish I had read your articles before putting my roof up, Oh well, live and learn. I did it once, I guess if I have to I can do it again.

  31. User avater GBA Editor
    Martin Holladay | | #31

    Another response to Chris Steiner
    My advice is the same: "If your installer does a very good job dense-packing the rafter bays, your roof will probably be fine." If you are the installer, and it's your very first dense-packing job, you'll need (a) a powerful blower -- not a rental machine -- and (b) a lot of self-confidence.

  32. Dennis Cornhill | | #32

    Cellulose weight against sheetrock in cathedral ceiling
    How do you keep the dense pack cellulose in a cathedral ceiling from excessively pillowing out against the ceiling sheetrock?

    We are building a house in western Wisconsin with an 8:12 cathedral ceiling formed from parallel chord trusses. There is room for 20" of insulation (distance perpendicular to the face of the ceiling - truss heal is over 2'), and it would seem that no matter how tight the fabric forming the cavity is stretched, the weight of the cellulose will cause it to bulge out and interfere with installation of the drywall.

    One suggestion is to staple the fabric on the inside of the truss, about a half inch from the edge, to create a space for the bulge above the sheetrock. Is this practical for the weight of the cellulose in our ceiling? Is 1/2" the right distance?

    I'm also having difficulty finding a contractor to do the work, so any references would be appreciated. The house is located in Wisconsin about 50 miles east of Minneapolis.

  33. User avater GBA Editor
    Martin Holladay | | #33

    Response to Dennis Cornhill
    There are two possible ways to do this:

    1. Install the fabric, and then strap the ceiling with 1x4s or 2x4s. The bulges between the strapping won't interfere with the drywall.

    2. Install a continuous layer of rigid polyisocyanurate insulation under the trusses, followed by 1x4 strapping installed with screws. Cut holes in the polyiso and blow the cellulose through those holes. Patch the holes with tape or canned spray foam. (See more information on this technique in the article on this page.)

  34. Dennis Cornhill | | #34

    Cellulose weight against sheetrock in cathedral ceiling
    I've had three cellulose insulation contractors tell me how they propose to insulate the cathedral ceiling:

    1. Install a layer of fiberglass at the bottom of the truss cavity, staple fabric to the bottom of the trusses and dense pack the cellulose above the fiberglass. The purpose of the fiberglass is to keep the cellulose in place. I've asked via email why this is necessary in a dense pack installation; awaiting his response. He is confident that the bulge below the trusses formed by the pressure of the dense pack cellulose will not impede the installation of the sheetrock.

    2. Install sheetrock up to within a few feet of the top of the ceiling. Loose fill the truss cavities up to the opening, fill the last few feet with fiberglass and then cover the fiberglass with sheetrock. He is confident that the loose fill cellulose, as it settles, will not slip down the truss cavities.

    3. Install sheetrock up to within a few feet of the top of the ceiling, using one extra screw on each joist (6 screws per joist per sheet) to help mitigate the pressure of the cellulose on the sheetrock. Install fabric over the last couple of feet by stapling inside the joists to create a small cavity for the cellulose to fill before coming into contact with the sheetrock. Pack the truss cavities to 110% (1.8#/cu. ft.) and finally complete the ceiling by installing sheetrock over the top couple of feet.

    Alternatively, the third contractor could put fabric over the entire ceiling (stapled to the inside surfaces of the trusses), but at an additional cost of a couple days of labor.

    Given that we have 21" of space for insulation, even a loose fill would settle to 18" which should still give us more than R-60. How important is dense pack for our application? Do we need the dense pack to keep the cellulose, as it settles, from slipping down the 8:12 truss cavities?

  35. Bill Hulstrunk | | #35

    Cellulose weight against sheetrock in cathedral ceiling
    Hello Dennis,

    The installation process will vary depending upon if this roof is vented or not. Before proceeding with a non-vented roof, I would first check in with your local code official to see if he/she is comfortable with using cellulose in this application. Also, all non-vented roof assemblies must be able to dry in at least one direction, and in both directions if possible. This means if an Ice and Water Shield type product is being used on the roof sheathing, than a vapor barrier or any material acting like a vapor barrier (i.e foil faced foam board) should not be used on the inside of the roof assembly to allow drying.

    In a non-vented cathedral ceiling of this rafter depth, you should first staple up Insulweb vertically along each rafter trusses, creating individual rafter bays to make it easier to fill and provide some extra lateral support for the cellulose to be self supporting. Next, staple up Insulweb to the underside of the rafters and than strap under this with 1 x 3 or 1 x 4 strapping ever 16" of 24" o/c. In this non-vented roof case, the entire rafter cavity should be dense packed with cellulose at 3.8 lbs/cuft density (the deeper the cavity, the denser the cellulose needs to be self-supporting). I prefer using the Insulweb in this application over foam board since it allows us full quality control of the density above. At the 3.8 lbs/cuft installed density, the cellulose should be very firm when pressed against from below. The installer may need to roll any bulging below the strapping, an aluminum roller works good for this. For a 21 inch rafter bay, each 25 lb bag of cellulose will cover 3.3 sqft.

    For a vented roof cavity, you should first install the wind blocks, vent baffles up to the ridge vent. Next, staple up Insulweb vertically along each rafter trusses, creating individual rafter bays to prevent the cellulose from one bay from spilling over to the next and preventing the hose from sliding downward. A three foot wide piece of Insulweb can than be installed on the underside of the rafters down from the peak on each side. Next, dry wall can be installed on the under side of the rafters and covering the first lower foot of the Insulweb. This will leave a two foot strip of Insulweb on each side of the peak so that the insulation hose can be inserted through and dropped down, loose filling the rafter below. Once filled, the cellulose will not slide down the rafter bay.

  36. Dennis Cornhill | | #36

    Cellulose weight against sheetrock in cathedral ceiling
    Thanks for your help. Bill. The roof is vented.

    The process proposed by my third contractor is similar to the one you describe. There are one or two differences that I need to check out.

    First, he did not mention covering the trusses with InsulWeb to prevent the cellulose from migrating between truss bays. Second, I don't know that the fabric he uses is air permeable, like InsulWeb. He described it as a reinforced poly.

    I'm expecting that the InsulWeb covering the trusses should be installed a bit loosely to allow the pressure of the cellulose to fill the space around the truss members.

  37. Dennis Cornhill | | #37

    Cellulose weight against sheetrock in cathedral ceiling
    We insulated the ceilings using the process for a vented roof described in Bill Hulstrunk's message above. The insulation contractor asked us to install the ceiling sheetrock with an extra screw in each row. The contract called for 110% fill so the bays should stay close to full even after settling. The same contractor also blew dense pack cellulose into the walls.

    We are having a mild winter but even so the heating system seems to be loafing, so at least the early signs are good. I guess I'll know better when we start paying the utility bills.

    Sound insulation is also good. The noise from a snowmobile passing by within 150 feet of the house was severely muted.inside the house.

  38. Richard Maxwell | | #38

    Ceiling Sag - Maximum depth of cellulose insulation
    I find it hard to believe that 20 in. of cellulose in a vented attic on a 1/2" drywall ceiling with trusses 16" o.c. wont cause some ceiling sag over time....

    I have plans on doing flat smooth ceilings and very concerned that here in the hot humid Lagrange, Ga area given enough time the sheet rock will begin to sag. The home will be located 400 feet from West point lake which is a 26,000 acre lake.

    Assuming this is a problem and I believe it to be so, I plan on attaching Insulweb to the bottom the ceiling joist across the entire ceiling through out the home. Next running 2x4 furring strips 16" o.c.

    Reason for 2x is it will give the Insulweb a maximum deflection of 1 1/2 before it could touching the top side of the ceiling drywall. I have not tested the deflection of Insulweb, but will perform some test to get a rough idea of what the maximum deflection is at 16" o.c.

    Not quite sure how an 1.5" gap between the drywall and cellulose will affect the r value etc...... but I would assume it will add r-value.

    I have thought about maybe using 1/2" ridged insulation board attached to the underside instead of the Insulweb that way I could use standard 1x4 for the furring strip..... not quite sure how that would work out as the ridged insulation board is rather soft. Might make it easier leveling the ceiling boards, or could make it more difficult. Not sure if building codes would allow it also.

  39. User avater GBA Editor
    Martin Holladay | | #39

    Response to Richard Maxwell
    Your approach -- strapping the ceiling -- is common in New England. While New Englanders normally use 1x3 or 1x4 strapping, there is no reason you can't use 2x4s.

    Make sure that the perimeter of your ceiling assembly is carefully air sealed, however, since you are creating an air gap between your air barrier and your insulation. If any exterior air circulates in that air gap, the thermal performance of your ceiling will be compromised.

  40. James Christy | | #40

    Storage attic with cellulose
    Hi, we use our attic for storage. We just had a contractor insulate our attic. They did dense pack cellulous under the floor boards in the main parts, and then loose fill around the edges, separated by a barrier. my wife doesn't like the barrier and losing the space around the edges, and she's concerned about any health/dust issues related to going up to the attic to put or retrieve storage stuff. Is it typical to do loose packed on the sides and dense pack under floorboards this way? Any comments about this approach would be helpful.

  41. User avater GBA Editor
    Martin Holladay | | #41

    Response to James Christy
    You wrote that the two sections of your attic are "separated by a barrier." What do you mean by a "barrier"?

    Q. "Is it typical to do loose packed on the sides and dense pack under floorboards this way?"

    A. Yes. There are no particular health concerns arising from visiting the attic occasionally. It's true that attics can be dusty. If your wife is allergic to dust, she probably shouldn't spend too much time in the attic.

  42. James Christy | | #42

    Thanks for the quick reply!
    Thanks for the quick reply! Trying to enclose a pic, the barrier is just made of cuts of wood. Does this look typical? (they aren't done with cleanup yet).

  43. James Christy | | #43

    Here's that pic.
    Here's that pic.

  44. User avater GBA Editor
    Martin Holladay | | #44

    Second response to James Christy
    That looks like a good job, as long as there are insulation baffles to maintain an open ventilation channel connecting your soffit to the attic.

    The main issue raised by this photo is whether the insulation installed between the floor joists provides the R-value you need. My guess is that the R-value of the central part of your attic is too low.

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