Construction Design-Wind Washing

Wind: The natural movement of air relative to the planet’s surface.

Washing:  A method of cleaning.

Wind Washing:  Cleaning using air?

Well, not quite.  Wind washing with regards to insulation is the ability of air movement to degrade the effectiveness of an insulation.  We will get to that in a little bit.  First a quick lesson on how fibrous insulations works.

A close-up of Rockwool ComfortBatt showing the fibers and small air pockets.

We use insulation to slow the movement of heat through our building assemblies.  Heat will move in three ways, conduction (heat moving when objects are in contact with one another), convection (heat transferred by moving air), and radiation (heat moving through spaces).  Fibrous insulations forces heat to move through fibers and small pockets of air, which slows heat movement in both radiation and conduction and prevents heat transmission by way of convection.

If air is allowed to move through or around a fibrous insulation, that’s the convection movement of heat that fibrous insulation is supposed to effectively stop, this moving air can strip away an insulation’s ability to function as designed.  A reduction in R-value can occur.  The effects of wind washing are most common in lower density fibrous insulation products (less than 1 pound per cubic foot).  Wind washing is most common in vented attics, but the phenomenon can occur anywhere insulation is improperly installed (where insulation is not in contact with all sides of a cavity) and where improper air sealing is present.  This thermal image shows the effects of wind washing through an attic that does not have proper air sealing.

the effects of wind washing in an attic. Air is being drawn between the insulation and ceiling finish, greatly lowering the effectiveness of the insulation.

So, how do we stop the effect wind washing can have on fibrous insulations?  First is to properly install the insulation.  For insulation to work in our walls, it needs to be in full contact with other building components.  We do not want gaps or areas of missing insulation that will allow air to move through the product.  In attics insulated at the attic floor, air sealing at the truss heal and the addition of air chutes to direct airflow above any fibrous insulation is needed.  The two photos below show how I’ve handled eave air sealing for vented attics on both existing and new construction.  The first photo is from a home built in the 1950’s, it has a hand framed roof.  (The earlier thermal image photo is of this home.)  To direct the air to move up the ventilation chute instead of through the insulation, I installed small pieces of rigid foam insulation and sealed to both the framing and plastic air chute using canned foam.  This project was then insulated using Rockwool batts.  The photo on the right is new construction using a raised heal rafter.  (A rafter design that allows for more insulation to be placed at the roof eave.)  The air chutes in this home are cardboard.  Closed cell spray foam was used to keep air from moving through the blown attic insulation in this home.

Another option to reduce the possibility of wind washing is to choose a more dense insulation.  The best and easiest sourced fibrous insulation products for reducing wind washing are dense packed cellulose and mineral wool type insulations, like ROCKWOOL.

One of the keys to a good insulation installation is for the insulation to perform as designed.  Eliminating air from moving through or around the insulation and “washing” out its ability to reduce heat flow is something that needs to be avoided.  Properly planning air sealing details is important in every home, whether the home is new or under a renovation.  Another important detail to get right is for the insulation to be installed correctly.  This is where I think ROCKWOOL shines.  The density of their fibrous cavity insulations tends to hold its full thickness and shape during installation, and completely fill the cavity, exactly what you want in a fibrous insulating product. When insulation is well installed and combined with an effective air control system, wind washing becomes a non-factor.

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