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.”