The Case for Dry Sealed Attic Spaces
We have been discussing the problems associated with the current accepted practice of attic venting. The following sections summarizes the science of sealed roof/attic spaces in order to ensure that moisture does not enter the roof/attic spaces. This is what the code says.
"...Except where it can be shown to be unnecessary, where insulation is installed between a ceiling and the underside of the roof sheathing, a space shall be provided between the insulation and the sheathing, and vents shall be installed to permit the transfer of moisture from the space to the exterior…” (National Building Code (NBC) Section 9.19 Roof Spaces, 188.8.131.52.)
Clearly the code sanctions unventilated attics, but let’s look at the code requirements for ventilated attics. According to William Rose, a research architect at the Building Research Council at the University of Illinois, the 1/300 rule is an arbitrary number, selected in 1942, with no citations or references, by the writers of the Federal Housing Administration (FHA), an institute which is comparable to the NBC. In other words no science was employed.
The architect William B. Rose wrote the “Early History of Attic Ventilation” (the Rose Report), found at Rose report, which reviewed and summarized research literature with the objective of understanding the evolution of venting requirements in attics between 1930-52.
Rose’s findings can be summarized as follows:
- The attic ventilation ratio “1/300” is an arbitrary number selected by the writers of FHA (1942) with no citations or references;
- Tyler S. Rogers introduced the "condensation control" paradigm to the architecture press in early 1938;
- The asphalt shingle industry began to link installation practices to recommended and code-required venting practices in the mid-1980s
A review of the science and research that has been done to date follows. Richard Seifert writes in his Technical Paper 228 (NRC 9132), Ottawa
…If moisture from inside the house is prevented from getting into the roof cavity, and there is sufficient insulation to keep snow on the roof surface from melting, there is no reason for roof ventilation… (Seifert, Technical Paper 228 (NRC 9132), Ottawa.)
The design of a Passive House ceiling/roof starts with a very robust air/vapor barrier, 3/8" plywood or 7/16 OSB with an air leakage characteristic described in table A-184.108.40.206.(1), NBC as negligible-0.01, a perm rate of 40-57ng/(Pa*s*M2) perfectly acceptable as an air/vapor barrier. Plywood is also very stable because it does not change size over the years. All joints are then taped with special Air seal Tape. With over 40 years of experience, European tape systems uses no solvents or VOCs so the tape will not dry out or become brittle—for more info on tape, go to
The air/vapour barrier is protected by a 3.5 inch cavity. With only one penetration, the 3" ABS stack that is sealed with a neoprene gasket. The entire assembly is then tested to prove its air tight quality to below -.6AC@ 50pa.
We also employ a second line of defense, the vapor open air barrier under the metal roof. The plywood system on the bottom and the vapour open air barrier provide double redundancy and double duty in the event of metal roof failure.
As to the question is there "…insulation sufficient to keep snow from melting?..." In the case of a Passive House in Manitoba, there will be at least 30 inches of cellulose in the attic. Do we have enough? Of course the other aspect of snow is when it blows into the attic. CMHC had to develop a non-vented attic for northern homes because using the old vented system resulted in attics becoming packed full of snow. See http://prolineplus.ca/foam/PDFs/researchpapers/CMHC_Arctic_Hot_Roof.pdf .
In 2014, Morrison Hershfield prepared a report, “Attic Ventilation and Moisture Research Study, Final Report, June 2014” for the Homeowner Protection Office, a branch of BC Housing.
The report states
“…the general result of the analysis leads to the conclusion that the best way to minimize the potential growth of mold in ventilated attics in maritime temperate climates is to avoid using ventilated attics by selecting and designing assemblies that do not require ventilation, such as compact roof assemblies...” http://www.hpo.bc.ca/files/download/Report/Attic-Research-Study-Final.pdf
The experience in California is that homes that have soffit vents tend to burn down in wild fire events, while those that didn’t have vents, tended not to. That's why in many counties in California soffit vents are prohibited. See http://www.countyofplumas.com/index.aspx?NID=2107
While we do not live in a maritime climate or an arctic one, we can experience blowing snow, driving rain, humid conditions, and the possibility of wild fire on some sites.
The take away from all this is that the best approach to avoid moisture damage in an attic space would be to ensure that it cannot enter in the first place.
We spend a lot of time and effort to seal and test the Air/vapor barrier systems in high performance buildings. We add another air barrier in case the first one fails or in the event the roof leaks. All of the science is telling us not to go and chop holes in the attic space for vents.
Vents can be compared to a band aid used to treat a symptom (water in the attic/roof space) rather than curing the problem created by a poorly executed building envelope.
In summary, we recommend as per Richard Seifert, that “…The only effective way to reduce condensation in roof and attic spaces is to prevent it from entering in the first place…) (Seifert, Technical Paper 228).
If you don't have time to read all of these studies, Jeff Gordon sums it all up in a presentation http://energysmartohio.com/plan-your-job/attic-ventilation
If you follow the trail of science and research on roof/attic venting, you will find that most of the stated reasons as found below for venting roofs are false:
“Venting prevents condensation on roof sheathing”. False—see Forest and Berg, 1993, U of Alberta
“Venting prevents winter condensation problems”. False—see Buchan, Lawton, Parent, Survey of moisture levels in attics, CMHC, 1991.
“Venting cools the roof and protects shingles”. False—see Dutt and Harrje, 1978
“Venting reduces air conditioning bills”. False—see Burch and Treado,1978
“Venting is the best way to keep the attic dry”. False—see Richard Seifert
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Added: Thu June 18th 2015