Learning Center


Frequently Asked Questions

  • What are the three types of cellulose installations?
  • Spray Applied: Also referred to as "damp spray" or "wet" cellulose. In this method of installation water is added at the tip of the hose in a very fine mist at high pressures. The cellulose is sprayed onto the exterior sheathing and builds up in the stud bay. We are filling the entire bay from back to front and from the bottom to the top of the stud bay with damp cellulose installed past it's settled density. This method can only be used in new construction on exterior or interior flat walls.

    The cellulose is sprayed slightly past the studs and then shaved off with a scrubber to be in line with the studs. All the cellulose that has been wetted is vaccuumed back to the truck and mixed with the dry cellulose. This method is generally quicker in new construction and therefore cheaper. It requires a clean work site as the spray hose is smaller than the vacuume hose so we can suck up something that will clog the spray hose on the way back out. It is a bit more "idiot proof" method of installation because it is clear that the insulation is completey filling the stud bay and has no chance to settle.

    Dense pack: Also referred to as "dry" or "blown in" cellulose. We use a specifically engineered fabric called Insulweb that allows the air to escape as the cellulose gets denser and denser within the stud bay. The cellulose is installed well past it's settled density and the entire bay is filled. The insulweb fabric is stapled up on all walls with pneumatic staplers and after installation we roll each bay with roller to ensure that there is no bulging that could interfere with the sheetrock.

    This method is generally a bit dustier than damp spray but there is overall less cleanup time as  less of the material ends up on the floor. There is however more prep time involved in stapling up the fabric so this method generally takes longer and is therefore a bit more expensive. It can be immediately sheetrocked. To ensure that the cellulose is installed properly we check each bay individually and occasionally we have to go back into a spot with a smaller hose and "tighten it up". Since you can see through the Insulweb fabric you can ensure that the insulation will not settle.

    In a cathedral slope the cellulose is packed densly into the bay either after sheetrock or with Insulweb fabric and strapping to prevent bulging. Dense pack does not require either ventilation or a vapor barrier.

    Loose fill: Dry celluose added over a flat attic. The cellulose settles a few inches in the first few days. This is the most cost effective method of installation. We recommend a settled depth of 16" or R60. This is where the payoff curve flattens out for insulation. It requires venting and a vapor barrier.

  • How long does it take for the spray applied cellulose to dry?
  • Depending on atmospheric conditions, you should wait 3-5 days after installation before hanging sheetrock. Cellulose dries from the face back towards the sheathing and loses 1/2 it's moisture in the first 24 hrs. The manufacturer, National Fiber, will stand behind 24 hour drying time. If there are significant sources of moisture in the house such as an improperly vented heating system, or a new concrete slab, the drying time may increase. If the sheathing is below freezing, or if the walls are thicker than 6" then it will take considerably longer to dry out. Cellulose can actually help wet framing to dry and it dries in a controlled manner. 

  • Is cellulose toxic?
  • No. It is considered a nuisance dust by the E.P.A. and the borates that the cellulose is treated with are non-toxic to humans. Fiberglass insulation is a known carcinogen and the smoke from smoldering foam is toxic. Cellulose has a great burn rate and although it's dusty when being installed, it is completely safe.

  • What is Boric Acid?
  • Boric acid is a naturally occuring mineral that is added to the cellulose as a fire retardant. It has the additional benefits of being a mold inhibitor and an effective barrier to vermin such as mice and insects.

Ice Dams

  • Ice Dams
  • Ice Dams are a sure sign of a building not functioning properly. They are sort of like a big leg cast on someone hobbling around on crutches: You know happy, healthy people aren't supposed to walk like that you're just not sure how they got that way. Ice Dams are caused by warm air coming in contact with the underside (sheathing) of the roof and melting the snow that sits on top of the roof.


    The warm air can get to the underside of the roof deck through inadequate insulation (conduction) or through heated air leaking up from below (convection). Actually it's not the hot air rising so much as the denser/colder air sinking that drives this but that's just splitting building science hairs.

    In my experience looking at numerous ice dams in lots of different types of buildings it's almost always caused by air leakage. Occasionally there's just not enough room between living surface and roof sheathing and the heat just moves directly through the ceiling material.

    Living in New England I often see people putting electrically heated wires onto the roof to melt the snow before it can freeze which is not fixing the problem it's just an expensive band aid.

    Often people will recommend adding venting to cool the surface of the roof sheathing to prevent ice dams (I'm looking at YOU roofers). While this can help if there is not much heat being added from the living area and can be very important in moving moisture out of an attic, I believe it is better to keep the heat and moisture out of the attic to begin with! This is your heat and your comfort and your money flowing through those holes. Sometimes the problem is easy to spot as when there are un-insulated heating ducts running through the attic:


    Sometimes it's a bath fan not being vented directly to the outside or leaky recessed lights. Sometimes in older homes, the interior partitions will not have any type of top plate on them as they were built differently back in the day so every interior wall below the attic is dumping warm air into the attic in the winter. This is why air sealing is so important. This is also something that you can do yourself if you are inclined before adding more insulation.

    Air Sealing an attic space

    Article on drafty attics

    Fine Homebuilding article on Ice Dams