Insulated, Enclosed, Unventilated Attic Question

[Marcel R. Cyr]

Sorrry Gerry, you did not specify.

Maybe this will help,
http://www.sprayfoam.com/spps/ahpg.cfm?spgid=6
and in anycase the closed cell contributes to structural uplift as you were concerned about. Or should say curious about?

Marcel

[Dennis Kenman]

I'm planning to build a house in Tucson with a flat roof. I recently discovered the claimed benefits of unvented attics and I'm intrigued; unfortunately, my builder isn't familiar with this practice.

I've seen many references to unvented attics with sloped roofs, but not many for flat roofs. Is this practice common for flat roofs? Does this practice work well for Tucson? Anybody recommend blown in cellulose? Any advice would be much appreciated.

Thank you,
Dennis

[alarsen1]

Originally Posted by Dennis Kenman

I'm planning to build a house in Tucson with a flat roof. I recently discovered the claimed benefits of unvented attics and I'm intrigued; unfortunately, my builder isn't familiar with this practice.

I've seen many references to unvented attics with sloped roofs, but not many for flat roofs. Is this practice common for flat roofs? Does this practice work well for Tucson? Anybody recommend blown in cellulose? Any advice would be much appreciated.

Thank you,
Dennis

Blown-in cellulose is a good product, so long as you have adequate soffit and ridge ventlation.

I don't believe unvented attics is common as it's a relatively new and more expensive option.

Arne

[David P. Valley]

Originally Posted by Dennis Kenman

I'm planning to build a house in Tucson with a flat roof. I recently discovered the claimed benefits of unvented attics and I'm intrigued; unfortunately, my builder isn't familiar with this practice.

I've seen many references to unvented attics with sloped roofs, but not many for flat roofs. Is this practice common for flat roofs? Does this practice work well for Tucson? Anybody recommend blown in cellulose? Any advice would be much appreciated.

Thank you,
Dennis

Dennis,

I like blown-in cellulose, but venting this flat roof will bring many varying opinions.

The typical flat or near-flat roof, with no intentional air space, is used mainly for commercial and industrial buildings. I don't see many single family homes with a flat roof. Flat roof cavities are often plagued by moisture problems. From the inside outward it consists of a structural roof deck, in most cases a near-impermeable vapor barrier, insulation, and an impermeable roofing membrane. A water and vapour trap can be created by such a system with insulation sandwiched between two membranes. If any water is present in the materials from which the roof is constructed of or moisture enters through the top by roof leaks, or from below as vapor, it may be trapped.

Such moisture will reduce the thermal resistance of the insulation and may contribute to its physical degradation and that of the membranes if they are moisture sensitive. Better performance can be assured if the designer recognizes the possibility of wetting and makes allowance for the escape of moisture from all possible sources. He can usually encourage this by roof drainage and venting.

Unfortunately, such allowances are often not designed, and an owner can be faced after a few years of service with a roof where the insulation is wet and the water is leaking into their building. Even if it is not, the wet insulation no longer provides adequate heat flow resistance. The home owner now has the drastic and expensive option of complete removal and replacement with a new system, or he may leave the roof as it is and make temporary repairs to prevent any further wetting. In this case he must attempt to dry the wet insulation in some manner.

In flat roofs, It has been a common practice (in recent years) for roofers to install breather vents in an attempt to remove moisture from moist insulation. Such vents usually consist of vertical pipes or stacks, open to the outside air, shielded from the rain by a cover and penetrating the roofing membrane to provide a path for moisture to reach to the outside. Where a rigid type of insulation is involved it is usual to cut it away below the vent and replace it with a loose form. This is intended to facilitate lateral movement of moisture through the insulation to the vents. Usually a number of such vents are aligned in one or more rows.

Reports on the effectiveness of stack venting vary widely. Some roofers and consultants claim great success in drying wet roofs; others have experienced little if any beneficial effect, and some owners insist that breathers make a bad situation worse. I like roof vents either way, I don't care what the situation is.

[Kenton H. Shepard, CMI]

Originally Posted by bking

That method is more common further north I believe.

I wonder what the r-value is of that expanding foam.

=

3.7-6.5 per inch depending on the type of foam. Looks a little on the thin side.