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A Brief History of Attic Ventilation For Residential Roofing

Writing & Credit Given to:

Early History of Attic Ventilation

William B. Rose

Research Architect Building Research Council

School of Architecture University of Illinois at Urbana-Champaign

Keywords: ventilation, moisture control, shingle durability, history, residential roofing, attic ventilation, roof replacement, new roof installation, best roofing practices, roofing code, best roof installs


Research literature and articles from professional magazines have been surveyed and reviewed with the aim of outlining the early history of attic ventilation between 1930-52. In addition, the question of the earliest use by the asphalt shingle industry of venting requirements has been addressed. The findings of this survey and review include the following:

Tyler S. Rogers introduced the "condensation control" paradigm to the architecture press in early 1938. The paradigm was based on work under way at the U.S. Forest Products Laboratory and on work recently funded by the National Mineral Wool Association under the direction of Frank Rowley at the University of Minnesota. The original 1938 Rogers article contained suggestions that the nascent insulation industry should be protected against claims of moisture damage. The two principal recommendations for moisture control under this paradigm were vapor barriers and attic ventilation. By 1952, when Rogers was in the employ of Owens-Corning Fiberglas, he more strongly stated that his effort at developing a condensation-control understanding was to defend the insulation industry.

The attic ventilation ratio “1/300” is an arbitrary number selected by the writers of FHA (1942) with no citations or references. One might speculate that it is based on Rowley’s 1939 research, which showed a slight performance difference between openings with vent ratios of 1/288 and 1/576. However, other evidence indicates it was not based on Rowley.

The asphalt shingle industry began to link installation practices to recommended and code-required venting practices in the mid-1980s.

Author Biography

William Rose is a research architect with the Building Research Council, University of Illinois at Urbana Champaign. His research is on water and its effects in buildings. He is the Handbook Chair of the ASHRAE Handbook chapters related to thermal and moisture effects in building envelopes.

Aim and background

The aim of this paper is to describe and assess the early research and professional literature that underpins current attic ventilation practices. The period covered in this paper is 1930-52. In addition, the aim of the paper will be to determine when the argument that attic ventilation enhances shingle service life first appeared. It is not the aim of this paper to review actual attic ventilation practices nor is it the aim to provide professional guidance regarding attic ventilation. It is hoped this paper will be useful in the future review of attic ventilation requirements.

For purposes of this paper, an attic is the unoccupied space above a ceiling plane and beneath a steep roof system. It may include cavities in cathedral ceiling construction. Cavities in low-slope roof systems are outside the scope of this paper.

Vent devices on roofs first appeared as steeples, towers or cupolas, which assisted buoyant flow upward through a building. This was common in barns, mill buildings or any buildings subject to buildup of odors or contaminants. It was also common in buildings in hot climates to assist comfort by increasing air speed across the skin. Buildings designed for such flow are outside the scope of this paper. Nevertheless, flow upward from living areas or foundation areas in buildings into an attic cavity may play a predominant role in hygrothermal performance of attic systems (see Britton, below).

Most early roofing material, such as wood shingle, slate or tile, was applied to spaced wood lathing. Continuous roof sheathing began to appear in the late 1800s in some construction because it allowed the application of asphalt felt underlayment for additional protection against rainwater. In quality construction in northern climates, nails were sized so the points did not penetrate the underside of the sheathing. The use of longer nails that penetrated through the sheathing probably represented low-quality construction when it appeared in the first decades of the 20th century. During the depression of the 1930s, the use of asphalt shingle roofing materials on continuous 1-by-6 sheathing became the norm for one- and two-family construction. To what extent was ventilation practiced prior to the 1930s? It is difficult to provide an answer because reroofing often involves changes in reconfiguration of venting. Thus it may be necessary to state only that roof systems with and without ventilation were both used up to the 1930s.

Paul D. Close

Paul Close* was one of the early writers on preventing condensation on building surfaces in insulated assemblies. He wrote in Transactions of the American Society of Heating and Ventilating Engineers (ASHVE now ASHRAE). His comments were only indirectly about attic venting.

*Close, Paul D. 1930. Preventing condensation on interior building surfaces. ASHVE Transactions no. 854, January 1930.

Where should insulation be applied? From the theoretical standpoint, the most effective results are obtained by applying the insulation to the interior surface of the wall or roof, or as near in the wall or roof to the interior surface as possible, especially if the building is allowed to cool at night and is heated quickly in the morning.

He then gave five reasons for placing insulation inboard, all based on specific heat and lag-time considerations. But he followed by saying:

There are other factors, or perhaps even greater importance than the foregoing, which make it advisable to apply the insulation as far as possible from the interior surface of the wall or roof. Probably the most important is that of providing the necessary vapor protection to the insulation, for no insulation will function satisfactorily if it is not properly vaporproofed.

Close makes no direct recommendations regarding attic ventilation, but his recommendation to ensure that insulation is applied tightly against the exterior roof deck, along with (bitumen) vapor protection was followed in the 1930s. Frank Lloyd Wright’s Wingspread in Racine, Wis. exhibits exactly this construction with excellent results.2

Forest Products Laboratory

During the 1930s, the problem of paint peeling became widespread. It seemed to occur primarily on insulated buildings. The U.S. Forest Products Laboratory (FPL) was the first U.S. organization to write about the occurrence.

F.L. Browne* senior chemist with FPL cited two types of circumstances that have been observed to cause abnormal conditions of exposure leading to paint peeling. The first type was rainwater seeping through leaky joints left by poor carpenter work or faulty design. The second type was “moisture originating within the building and carried by air circulating within the hollow outside walls. When moisture laden air comes in contact with surfaces at sufficiently lower temperature, water condenses.” He cited five conditions of this second type:

Attempting to hasten the drying of wet plaster Designing parts of buildings in such a way that stagnant air spaces are enclosed by wood walls (i.e. porches or hollow columns) Lack of ventilation in unused attics Failure to secure a watertight basement 2

*See Rose, W. 1997. Control of moisture in the modern building envelope: the history of the vapor barrier in the United States 1923-1952. APT Bulletin, Vol. XVIII. No. 4, October 1997.

*Browne, F.L.1933. Some causes of blistering and peeling of paint on house siding. US Forest Products Laboratory No. R6, Madison WI. 11 pp.

Activities within the building that humidify the air

Item 3 reads in full:

Lack of ventilation in unused attics. During cold weather water may condense beneath the cold roof and drain down toward the cornice. If the top course of siding is placed below the frieze board the water is directed between siding and sheathing, coming directly in contact with the backs of the painted clapboards.

This is a rather explicit form of failure. In fact it is hard to imagine water running down toward the cornice along the underside of cold sheathing. Nevertheless, the observation of water behind siding at the top of the wall must have needed some explanation.

In 1937, Larry V. Teesdale, senior engineer with FPL, published “Condensation in walls and attics.”4 Regarding attics, he states:

Roof condensation is reported far more frequently than sidewall condensation, not necessarily because it occurs more frequently but rather because it is more likely to be seen by the occupants. For example, in a pitched roof house having, say, fill insulation in the ceiling below the attic, condensation may develop during a severe cold spell on the underside of the roof boards, forming as ice or frost. When the weather moderates, or even under a bright sun, the ice melts and drips on the attic floor, leaks through and spots the ceiling below. Often such spots are assumed to be roof leaks and cause owners and contractors considerable unnecessary expense in attempting to waterproof a roof that is not leaking. If the attic has adequate ventilation little or no trouble will occur but adequate ventilation is sometimes difficult to attain, and tends to increase the heat loss.

On page 6, he explains that attics under pitched roofs can be ventilated either through windows or louvered openings or by separating roof boards 2 feet or more. The article contains no other mention of attic ventilation until the final page under General Recommendations: “For new construction it is recommended that a suitable vapor barrier be installed on the side wall studs and below the ceiling insulation and that some attic ventilation also be provided.” The overall emphasis of the article is the importance of reducing indoor humidity.

Teesdale’s recommendation for attic ventilation was quite clear, but the support for this position was his personal experience, which did not make it strongly into the research record.

*Teesdale, L.V., October 1937. Condensation in walls and attics. U.S. Department of Agriculture, Forest Service. Madison WI. 12 pp.

Tyler Stewart Rogers

Tyler Stewart Rogers was a writer on technical issues in the architecture press through the 1930s. By 1950, he was director of technical publications for Owens-Corning Fiberglas, but when his affiliation with OCF began is not known at this point.5 He had become prominent as a contributor to “Timesaver Standards,” a regular feature of American Architect and Architecture magazine. In November 1936, he published “Insulation: What we know and ought to know about it”6 that promised that research was getting under way. The article concluded with: “It may confidently be expected that this new phase of building science will soon become as well established and as familiar as carpentry, masonry and steel work. Standardized practices are the objective.”

He delivered on his promise in March 1938. “Timesaver Standards” had moved to Architectural Record magazine where the “standardized practices” were described, this time for “Preventing Condensation in Insulated Structures.”7 Rogers cited two sources: “Condensation in Walls and Attics” by L.V. Teesdale of the Forest Products Laboratory and “Condensation Within Walls” By Prof. F. B. Rowley and others. Rowley’s work had been presented in January 1938, but had not as yet been published.

Rogers’ article paints a picture that is surprisingly complete and up-to-date. That is, things have not changed much since their first appearance. The article begins: “Architects, owners and research technicians have observed, in recent years, a small but growing number of buildings in which dampness or frost has developed in walls, roofs or attic spaces. Most of these were insulated houses, a few were winter air-conditioned. The erroneous impression has spread that insulation ‘draws’ water into the walls and roofs...Obviously, insulation is not at fault.” Note the hint that a new industry-insulation needs to be defended against a perception that it leads to moisture problems. Teesdale, as well as Rogers, had sought to counter the notion that insulation “draws” water into constructions. It is worth noting that insulation does in fact “draw” water into walls and roofs in that it ensures colder temperatures for exterior materials during cold weather, leading to higher surrounding relative humidity and higher moisture contents for those materials. This can be confirmed using the ASHRAE dew-point method, which was about to make its appearance.

I was told that the Owens Corning Granville Ohio facility is planning to name a new building after Rogers. Nevertheless, employment records at Owens-Corning for Rogers are not available. *

*Rogers, T.S. 1936. Insulation: What we know and ought to know about it American Architect and Architecture November 1936. New York. 7 *Rogers, T.S. “Preventing Condensation in Insulated Structures” Architectural Record March 1938, pp. 109-119.

He has a section titled “Explaining to Clients” in which he presents a comparison of liquid and vapor transport.8 Note that he was selling this approach to clients, not to the construction industry; there is no corresponding section titled “Explaining to the Construction Industry.” Without going into the sociology of design and construction, it must be pointed out that Rogers, the architect, makes no effort to sell the theory to builders but rather leaves them with the responsibility of high-quality execution. “As with most other details of construction, workmanship has an important bearing on final performance. The most perfect barrier material, poorly installed, will fail to function at high efficiency.”

Rogers states later: “Absolute protection against occasional condensation in small amounts does not appear to be necessary. Wood with less than 23% moisture content is perfectly safe from dry rot and fungus growth.”

Toward the end of the article comes the section subtitled “Attic and Roof Insulation.”

Principles that apply to wall construction apply with equal force to ceilings, attics and roofs, but somewhat different techniques are needed to meet the conditions encountered. A vapor barrier undoubtedly should be employed on the warm side of any insulation as the first step in minimizing condensation; venting to the cold air is an equally desirable second step. Either one may suffice; both are desirable. Venting of roof areas above insulation may be accomplished by various means, according to the construction involved. Unoccupied attics or loft spaces, above insulation installed at the ceiling below, should be vented by louvers in gable ends or side walls at the highest possible point, or by ridge ventilators or false chimneys. Wood shingle roofs applied on spaced shingle lath without vapor resistant papers provide sufficiently free vapor movement to make additional venting unnecessary, but roof decks of any kind which are covered with vapor-resistive materials should have special vents.

He shows three diagrams of venting. See Figure 1.

The following month, an article “Condensation” appeared without author attribution in Architectural Forum magazine.9 It was similar to the Architectural Record article in most respects. It allows that wood may remain at 25 percent moisture content without fear of fungal damage.

The single exception to (rare frost formation) has been the poorly ventilated attic. Such frost often takes a curious form known in some sections as “walnuts”; balls of rust-colored ice which gather on nail-ends projecting

This image was repeated many times in the following decades. Unfortunately, it masks the role of temperature reduction in leading to high moisture contents of materials.

*Anon. 1938. Condensation. Architectural Record. April 1938. New York.

through the roof boards which-since they are colder than the wooden parts of the roof-attract the water vapor. Such ice or frost seldom damages the roof structure, but if quickly melted by sun shining on the roof or a sudden rise in temperature may drip on the ceilings below and cause discoloration and even disintegration of the plaster.

This article concludes, “Condensation in attics is best prevented by providing adequate ventilation, supplemented where necessary by a vapor barrier on the underside of the attic joists.”

Frank B. Rowley

Professor Frank B. Rowley was well-known to the ASHVE (ASHRAE) community. He had established his reputation by measuring R-values of materials in the laboratory and by showing how these values could be used to accurately estimate heat loss through enclosures. His research had been funded by the National Mineral Wool Association. It was normal that the funding would continue for his studies on vapor transfer and condensation. In 1934, Rowley had been elected president of ASHVE. His prestige and valuable contributions to heat transfer may have contributed to relatively uncritical acceptance of his work on vapor transfer. He wrote on theory and on practice.

“A theory covering the transfer of vapor through materials”10 laid out the theory of vapor diffusion. It begins:

There has been much speculation about the theory relating to the transfer of vapor through materials and the application of the theory to building construction. For convenience it has often been assumed that the laws for vapor transmission are similar in form to those governing the flow of heat through the walls of a building, and that coefficients of vapor transmittance may be developed for materials or combinations of materials which may be applied in the same manner as coefficients of heat transmission...Before accepting a complete analogy between the two problems an analysis should be made to determine those elements which are similar and those which may be conflicting.

In short, Rowley finds the analogy convincing, and it thereby became the principal explanatory tool for moisture transfer. The question of whether this form of moisture movement is actually of significance in actual building performance was not asked until much later.

*Rowley, F.B. 1938. A theory covering the transfer of vapor through materials. ASHRAE Transactions. No. 1134. July 1939. American Society of Heating Refrigerating and Air Conditioning Engineers, Atlanta GA.

In January 1939, Rowley, Algren and Lund published “Condensation of moisture and its relation to building construction and operation.”11 The study reported on five lines of investigation:

-A further study of vapor barriers

-Ventilation of walls through the exterior surfaces

-The effect of vapor barriers on the drying of wet plaster

-The effect of attic ventilation on the accumulation of moisture and frost within the attic and upon attic temperatures

-The effect of vapor pressures on the rate of vapor travel through materials