I joke with contractors that they sometimes march in like Caesar’s legions, ‘Veni Vidi Vented’. I came, I saw, I ventilated.
One of the stickiest and most muddily understood questions about homes is that of a venting. What is the difference between a vented attic and an unvented attic? And what is the advantage of one over the other? How does this impact your insulation and what types of insulation are most affected by the air flow?
This questions comes up frequently with homeowners and more so with contractors. Vented attics have a strong performance history in most US climates and ably deal with a wide spectrum of temperature and moisture conditions.
Vented attics are time tested and work great in the right circumstances. Issues occur when venting is adopted as a cure-all without considering all the factors. Is there a clear path for air flow from the soffit to the peak? Are the soffits vents properly installed to prevent wind washing (and since you haven’t read the post, what the heck is wind washing)? What if the ceiling structure is rife with ells, dormers and architectural detailing? Can it be effectively ventilated? (That’s a highly conditional, uh, maybe).
What is a Vented Roof (And Why Do It?)
A vented roof is one where air flows into the soffits and is (hopefully) connected to a ridge or gable vent with attic baffles/channel vents. Here’s an illustration of a vented attic system:
A vented roof serves two purposes: Controlling the roof deck temperature, reducing the possibility of ice dams in the winter, preserving shingle life by lowering their operating temperature in the summer (with many caveats, see “The Problems with Venting Attics”) and reducing summer A/C cooling load. A second function is moving attic space moisture out of the house.
Vented Attics and Your ‘Breathing’ House
Ventilation and vented attics are usually synonymous terms. Confusion can arise when ‘breathing’ enters the conversation. The term is very relatable and homeowners often conflate ‘breathing’ (which in building science terms means a certain minimum level of fresh air moving through the structure) with ventilating.
As an aside, when folks talk about a house breathing does anyone else imagine a hybrid of modern colonial and Frankstein’s Monster? No? Just me? Moving on then.
The image is that homes breathe in some organic, human way. When discussing vented attics, it is often connected in people’s minds with breathing. And there is a minimum desired level of fresh air within a building for occupant health. ASHRAE, the American Society of Heating, Refrigerating and Air Conditioning Engineers, has established minimum standards for fresh building air and ventilation. It’s just that this has VERY little to do with your attic ventilation.
A more accurate phrase would be ‘People breathe, houses are ventilated’.
A Minor Historical Detour That Will (I promise) Eventually Return to Vented and Unvented Attics
Time for a minor and entirely unresearched detour; I will compensate for the over-simplifications by grossly generalizing.
The United States was for a long period (and this is very hard for modern Americans to believe) the largest exporter of oil the largest exporter of oil in the world. The oil fortunes of Texas and the California basins were born in this early oil boom with the good fortune continuing for decades. But as the suburban commuter lifestyle spread and the most readily accessible oil fields petered out, domestic production steadily lost ground against domestic demand.
Comparable scenarios played out in most of the industrial world. Countries like the U.S., England and France were attaining levels of energy consumption that were beginning to surpass production. Into this breech stepped the countries that eventually formed OPEC.
OPEC (originally comprised of Saudi Arabia, Iran, Kuwait, Iraq and Venezuela) imposed oil embargoes in the 70s to control the supply of oil and subsequently prices. Once organized, OPEC controlled the majority (at the time) of exportable oil with Saudi Arabia (and their behemoth Ghawar Field) leading all exporters. Prices predictably spiked, making oil-based home heating a burdensome expense for the first time.
Insulation in some form has existed practically forever but had never been implemented with any urgency in modern construction. Why spend any money on insulation (which pre-oil price spike was not a very mature technology) when heating was essentially free? This changed with the oil embargo (and later OPEC related spikes). Homeowners, stunned by actually large heating bills, began insulating like crazy. Fiberglass, cellulose (for the good) and urea formaldehyde spray foam (for the bad) insulation were rushed out into homes nationwide. Assisted by government sponsored weatherization programs, almost all homes in colder climates enjoyed some stab at improving insulation.
Problems began sprouting up (shocker!) almost immediately. When you add thermal resistance to a structure without addressing air and moisture movement, problems inevitably arise. Mold on basement joists, mold on the roof sheathing, rotted floor boards just to name a few. Here’s a favorite photo of mine. This was taken just three months after attic air sealing and insulation.
There is mold growing everywhere after only three months. Yech.
Ventilating attics as a means of controlling building moisture has been explored since the 40s. Increasing the thermal resistance of buildings without reducing moisture or air flow exploded the number of mold and rot problems. Roofers and contractors explored different methods of preventing the emergent problems, including added ventilation. After several years of integration into modern construction, building ventilation was standardized and added to the uniform Building Code.
When done properly (cue ominous music), venting an attic works quite reliably. It is considered standard for residential construction…maybe a little too standard. Problems arise with ventilated roofs when builders don’t account for air and moisture or when the roof structures grow so complex that a proper vent system is difficult, if not impractical. More complex roof assemblies make an unvented approach more effective. But it is 3am and that’s for another post.