When the wind blows against your house and leaks air inside, it loses the battle with the wind.
The idiom “clean your plow” can refer to outwitting someone/something or winning a contest. Leaky buildings lose the wind-washing battle, and correcting wind-wash is challenging.
The most common type of wind wash a typical home inspector can report happens near the eave. It’s indicated by areas of thin or missing ceiling insulation. The wind blows through the soffit vent and pushes the attic insulation back away from the edges of the ceiling at the outside walls. The result is patches of exposed drywall, void of insulation. The effect in the home is hot and cold spots and cold rooms, especially along the outside walls.
Wind-washed loose-fill insulation is a thermal bypass on steroids. Here’s a common example:
Another adverse effect of wind washing happens when the displaced insulation piles onto nearby insulation. Loose-fill fiberglass, the most typical type of insulation in our area, loses its R-value when compressed. Compacted loose-fill insulation cannot protect you from heat transfer, leading to an increase in energy usage.
This photo is from the Energy Vanguard blog by Dr. Allison Bailes – courtesy of Erik Henson. Dr. Bailes talks about wind washing and its effects on the entire building. In this extreme case, note water running down the wall. (click/hover to zoom photo)
Have you worn a sweater with a light shirt underneath and experienced a cold wind blowing through the weave? You’ve been wind washed!
Award-winning architect Mark Siddall @MarkSiddallRIBA talks about wind washing and offers a very deep dive into the challenges. Jump over to Twitter and catch this thread that includes ten slides. Download his paper here – PDF.
The DoE estimates that average wind washing costs homeowners around $120 per year.1DoE Study: Investigating Solutions to Wind Washing
Start With Better Framing
To compound the issue, even without wind washing, many homes don’t have adequate insulation along the outer edges of the exterior walls. Why? Because the outside two feet of most ceilings in central Mississippi have poor insulation levels. It’s thinner there because it gets pinched by the roof decking. As a result, the wind doesn’t have to work too hard to displace it.
We can help solve wind washing at the eave by incorporating two solutions; we can build a more insulated house simultaneously. (click/hover to zoom photo)
1) Install a baffle between the rafters. Its job is to channel the air up and over the insulation. Most new homes have baffles. Baffles are included in our building code.2N1102.2.3 (R402.2.3) Eave baffle. For air-permeable insulation in vented attics, a baffle shall be installed adjacent to the soffit and eave vents. Baffles shall maintain a net-free area opening equal to or greater than the vent size The baffle shall extend over the top of the attic insulation. The baffle shall be permitted to be any solid material. The baffle shall be installed to the outer edge of the exterior wall top plate to provide maximum space for attic insulation coverage over the top plate. Where soffit venting is not continuous, baffles shall be installed continuously to prevent ventilation air in the eave soffit from bypassing the baffle.
2) Raise the roof by installing a raised plate. The extra room won’t pinch the insulation.
Most builders don’t or won’t raise the plate due to cost. In fairness, you’ll need to ask a builder why it’s not common practice.
…there is no simple test for it. That’s why careful installation and inspection matter: “In all cases good detailing and careful workmanship are required to prevent the formation of interlinking cavities, air gaps and joints which contribute to the risk of natural convection.” – Blog Article: What is a thermal bypass? Another thing for builders to worry about.
Lloyd Alter is Design Editor for Treehugger and teaches Sustainable Design at Ryerson University in Toronto.
Climate Matters
If you live in a cold climate, adequate insulation and ventilation over exterior wall top plates are especially critical. The heat gain in the attic through thin insulation can melt snow-covered roofs and lead to the risk of ice dam formation.
The Building America Solutions Center has detailed information about framing and battling wind wash.
Inspector Should Report
The biggest challenge to a home inspector is that wind-washing affects more building components than a typical inspector will see, such as cantilevered floors and open porch ceilings adjacent to second-story floor cavities. While in the attic, look for daylight penetrating the soffit. Compare the insulation levels at these points. You should also the effects of wind-washing using a thermal camera. Shoot the ceiling near the outside walls. Report it in a narrative format using as much detail as needed.
Other Good Reads
This Article's Footnotes/References
- 1DoE Study: Investigating Solutions to Wind Washing
- 2N1102.2.3 (R402.2.3) Eave baffle. For air-permeable insulation in vented attics, a baffle shall be installed adjacent to the soffit and eave vents. Baffles shall maintain a net-free area opening equal to or greater than the vent size The baffle shall extend over the top of the attic insulation. The baffle shall be permitted to be any solid material. The baffle shall be installed to the outer edge of the exterior wall top plate to provide maximum space for attic insulation coverage over the top plate. Where soffit venting is not continuous, baffles shall be installed continuously to prevent ventilation air in the eave soffit from bypassing the baffle.
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