GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Audio Play Icon Headphones Icon Plus Icon Minus Icon Check Icon Print Icon Picture icon Single Arrow Icon Double Arrow Icon Hamburger Icon TV Icon Close Icon Sorted Hamburger/Search Icon
Product Guide

A Vent for Every Roof

Ventilation is required by code, but not all building scientists think vents do much

Is air really this smart? Probably not, but roof ventilation is required by the International Residential Code, and manufacturers have plenty of vent offerings for builders to choose from. Illustration courtesy of Fine Homebuilding magazine.

A freshly immigrated Norwegian carpenter built our two-story house in 1931. It has a walk-up attic that can be aired out in the summer with a pair of windows in the gable ends. What the house doesn’t have—like many others in the neighborhood—is any apparent ventilation in the roof.

Houses like ours are no longer permitted by code. Builders usually install soffit and ridge vents on new houses, not only because building codes require ventilation but also because we’ve been told that vents carry away moisture and heat, lower the risk of mold and rot, and make the house more energy-efficient.

The current ventilation standard for residential roofs emerged in the 1940s, roughly 15 years after our house was constructed. The International Residential Code (IRC) provides a formula for calculating exactly how much ventilation a roof should have. It is described as the ratio of “net free ventilating area” to the area of the vented space (net free vent area is the unimpeded area of a vent that allows air to flow through it). That ratio should be no less than 1/300, and it carries a couple of caveats:

There are many types of roof vents on the market: louvered vents for gable ends, soffit vents, continuous ridge vents, mid-roof vents designed for buildings that don’t have soffits, vents for hip roofs, vents that look like sheet metal mushrooms, and electrically powered vents that can move as much air as the biggest kitchen range hoods on the market.

What brought on that fundamental shift in thinking about attic ventilation? Lots, says William Rose, senior research architect at the Applied Research Institute, University of Illinois Urbana-Champaign, and author of Water in Buildings: An Architect’s Guide to Moisture and Mold. As Rose explained in a 1995 article, a number of…

GBA Prime

This article is only available to GBA Prime Members

Sign up for a free trial and get instant access to this article as well as GBA’s complete library of premium articles and construction details.

Start Free Trial

7 Comments

  1. User avater Expert Member
    Carl Seville | | #1

    In Georgia power attic vents are no longer allowed by code, the only exception being those that are solar powered.

  2. Expert Member
    Malcolm Taylor | | #2

    Scott,

    A great comprehensive overview.

    The first illustration has me thinking: It shows a baffle at the exterior wall to keep the insulation out of the soffits - but also as recommended by GBA, it is air-sealed to minimize wind-washing of the insulation. Is there any data on how much of a problem this is? All I've seen was a link to some work by RDH that seemed to show it was negligible. Intuitively, especially with something like FG batts, wind-washing should be a problem, but is there real evidence it is?

  3. User avater GBA Editor
    Scott Gibson | | #3

    Thanks, Malcolm. As to your question, I just don't know what research is available to quantify the impact of wind-washing on insulation. Possibly another GBA reader can add a reference.

    1. Expert Member
      Malcolm Taylor | | #4

      Scott,

      All I've seen is this study by RDH that seems to show it isn't the problem we think it is. But it deals with ventilated walls, which are more protected than attics or cathedral roofs:
      https://www.rdh.com/wp-content/uploads/2018/06/Wind-Washing-Summary-Report-RDH-Tech-Library.pdf
      If wind-washing isn't significant, it fundamentally alters how baffles are installed - or in many cases whether they are even necessary.

      1. User avater GBA Editor
        Brian Pontolilo | | #5

        Hi Malcolm.

        I was just looking over that research and slides from a presentation given based on it. The testing was done for Roxul and shows that with mineral wool insulation, wind-washing has a negligible effect. However, the presentation had a couple of slides that seemed to indicate an 8% to 10% reduction in the R-value of fiberglass insulation due to the wind washing, as set up in their test conditions. I have a question in to them to see if I interpreted this all right and to see if there is any other information available on the topic.

        1. Expert Member
          Malcolm Taylor | | #6

          Thanks Brian,

          I guess another question would be whether it is worth sealing wall cavities to short circuit convective loops.

          I also hope I haven't short-circuited something you were planning to write about. If so, I'm happy to wait for a reply.

  4. User avater
    Nick Hayhoe | | #7

    I recently watched a video in which Lstiburek states that you want more NFA at the soffit than the ridge because, wait for it, “you don’t want your roof to suck.” If it sucks, and your attic floor isn’t perfectly sealed, it will encourage more conditioned air to infiltrate the attic. It seems like any powered (wind, solar, hard wired) vent would just suck even more. So, don’t use powered vents, and have more NFA at the soffit than the ridge.

Log in or become a member to post a comment.

Related

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |