Advantages to Dense Packing
There are several major advantages to dense packing (completely filling) a cathedral ceiling cavity with cellulose.
First, cellulose has a natural density about two times that of fiberglass batts, and four to five times that of loose blown fiberglass. This makes it much more difficult for air to move through the insulation.
Second, cellulose has a stated R-value of 3.8 per inch as compared to 3.2 for commonly used batts and 2.2 for blown fiberglass. Cellulose insulation’s higher R-value is very important in limited space areas such as a cathedral ceiling cavity. Plus, with fiberglass batts, you must leave ventilated space, further diminishing the insulating potential of the cavity. Cellulose offers much greater insulating power per inch, and, compared to fiberglass, provides more insulating room because ventilation space is not required. This means that in a limited space such as a cathedral ceiling, cellulose offers a much higher stated R-value. Keep in mind that cellulose not only holds a big advantage in stated R-value, but the advantage skyrockets when considering the "effective" or "installed" R-value of cellulose compared to fiberglass.
Third, you eliminate the need for an air or moisture (vapor) retarder, because as researcher J. K. Latta says in his report, Vapor Barriers: What are they? Are they effective?:
Air leakage is now considered to be the prime cause of most condensation problems in walls and roof spaces. If, therefore, a building can be made tight against air leakage, it may not need a vapor barrier, as defined.
In a correctly insulated cathedral ceiling (using the tight fill, moisture elimination method), the high density of cellulose reduces moisture penetration to the point that no vapor retarder barrier is needed. As mentioned earlier, cellulose has a natural density that is usually two to four times that of fiberglass. However, in a cathedral ceiling application cellulose is installed at a much higher density. The density is important because it answers two questions at once. The first is moisture penetration, which we have discussed at length, and the second is settling.
"Will cellulose settle?" is the question that inevitably comes up when discussing cathedral ceiling applications. According to Terry Applegate, President of Applegate Insulation Manufacturing, Inc., if it’s done properly, settling is no problem. "What you must make sure of is that the cavity is filled to the proper density and that means using a reputable, knowledgeable contractor."
To back up his position, Terry points to HUD standards which "establishes as acceptable a settling (installed) density of 3.5 pounds per cubic foot (pcf) for installation of dry loose-fill insulation…
It is now generally agreed that an installation density of 3.5 pcf. is adequate to eliminate the problem of voids…"
"At 3.5 pcf." says Terry, "the cavity is pressurized as the cellulose is actually compressed into the cavity at 150% to 200% of its natural density. Actually, instead of settling, cellulose compressed to 3.5 pcf. wants to expand back to its natural condition. This tension keeps the cathedral ceiling cavity filled for the life of the home."
Blowing the Roof Off the Shingle Myth
Lastly, myth has it that the ventilation serves to keep shingles cool in summer. This was debunked by a study discussed in the November 1990 issue of Energy Design Update (EDU). In a section entitled "The Non-Effect of Roof Ventilation on Shingle Temperature," they discuss the findings of an experiment conducted by the University of Illinois Small Homes Council which found that:
Contrary to common belief, attic or roof ventilation in cathedral ceilings appears to have little or no effect on shingle or sheathing temperature.
Furthermore, EDU reports,
tests at the Florida Solar Energy Center showed no more than three- to five- degree difference in sheathing temperature between vented and unvented roofs…" and "although ventilation appears to have little effect on roof surface temperature, it may have a significant effect on attic air temperature and heat gain into the house.
Dense pack non-ventilated cathedral ceilings are, as a building science study entitled Report On Roof And Wall Details: Upper Canada Post And Beam points out, "nothing more than well insulated exterior walls with insulating sheathing which is sloped."
Cellulose has been used for years to dense pack cathedral ceilings with excellent results, outperforming the ventilated method. It is now being accepted as the preferred method of installation by building experts across the country.
To avoid moisture problems in the future, consult a knowledgeable insulation contractor in your area or call Applegate Insulation Manufacturing, Inc. at (800) 627- 7536.
About the Author:
Aaron Applegate is co-founder and C.E.O. of Applegate Insulation Manufacturing and has over 50 years experience in the insulation industry. Aaron is well known for his expertise and frequently serves as an industry consultant.