Insulated, Enclosed, Unventilated Attic Question

I haven’t ran into foam insulation yet, but a lot of foam insulation companies state that attic vents are not necessary.

Here’s a link to an article in the current issue of Journal of Light Construction
titled “Insulating Unvented Attics with Spray Foam”. A different way to do things.

Or you can use the Arkansas chicken farm method!

Closed attics have been around for a few years and the foam has been around even longer. New ways to reinvent the wheel and find a new market niche. I’d like to see them catch on as soon as possible. The last one I did it was 100F outside a balmy 80 in the attic. Quite the opposite of what is normally encountered.

“Does any of that make sense, or did I stay up too late again?:shock:”
Wendy, you know the answer to this, just think a little harder.
NO, wait, not that hard I can smell them burning from here.

Great article! I grew up on a farm and my dad had a 100 ton walk-in freezer that was completely insulated… floor, sides, and ceiling… with about 10" of sprayed-on foam.

I watch Holmes on Homes once in awhile and he’s gone to insulating basement exterior walls with this same method as it’s an insulation and vapour barrier all-in-one… and because the foam bonds directly to the concrete, with no “pockets of air” behind, there’s no chance for mould to grow in cavities.

I’m glad you thought so highly of the article. For those who are unfamiliar with the Journal of Light Construction, its a GREAT magazine and I continue to find its archived articles a great help explaining technical dwelling matters to the average home owner. I’d recommend all members give jlconline a test drive and see for themselves.

Starting to this in north florida with SIP wall construction. Very strong roof, great insulation, vapor barrier all in one. Keeps the AC ductwork inside the house envelope, saves alot of money on cooling bills. Needs to have an Air to air exchanger to let fresh air in and moist air out, or the house will stew in its own juices.

Hi to all,

I agree with you Todd, I believe this is the future of insulation, I attended a very thought provoking presentation by Mark Cramer (ITA Tampa) a couple of weeks ago where he was discussing the benefits of foam in place insulation. I also wonder if this would also cure some of our wind uplift and building pressure issues down here, in particular I was wondering what the shear resistance of that foam is, as it connects the rafters to the underside of the sheathing I am guessing that it offers a lot of wind lift resistance.



Right here Gerry on page 8.

Marcel :slight_smile: :slight_smile:

Great site thanks Marcel

Roy Cooke

Thanks Marcel,

The link you sent was to the closed cell foam, it is my understanding that it is the open cell that is normaly used for this application (at least down here in the sun drenched south :mrgreen: ) I would imagine that the open cell version would be weaker in this area.



Sorrry Gerry, you did not specify. :wink:

Maybe this will help,
and in anycase the closed cell contributes to structural uplift as you were concerned about. Or should say curious about?

Marcel :slight_smile: :smiley:

Exactly. Why they wanted to pay to heat the attic’s a bit of a mystery. Maybe that’s the only place his wife lets him ferment the beer. Looks like they didn’t know what they were doing.
They’d have paid for less foam if they’d insulated the ceiling instead of the roof structure, since there’s more sqaure footage with the sloped roof.

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,

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.



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.

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