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Community and Q&A

Windwashing through loose-fill cellulose

Alan B | Posted in Energy Efficiency and Durability on

We have some high winds, 40km/h, gusts up to 105km/h. I pulled some of the plugs in the wall (I have not plastered them yet) and noticed the wind gets through the cellulose with no problems, the wind blew in a few puffs of cellulose during strong gusts.

Essentially my house has an air barrier at the interior plaster, wall stack up is:
plaster/lath/plaster/barn board/cellulose-true 2×4/wood siding/vinyl siding.

I could not read the wind washing article since i’m not a premium member but i’m guessing it found wind washing is real if my experience is any indication.

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  1. GBA Editor
    Martin Holladay | | #1

    Windwashing is real. Cellulose insulation is not an air barrier. Ideally, whenever a fluffy insulation (like cellulose, fiberglass, or mineral wool) is installed in a wall, the insulation is protected on both sides (or, as some sources explain, "on all 6 sides") by an air barrier.

    Good examples of an exterior air barrier would be taped Zip sheathing, taped plywood, or taped rigid foam. Old barn board sheathing is not an air barrier.

    My guess is that the cellulose in your walls was not installed according to the dense-pack method. The denser the cellulose installation, the better it is at resisting air flow. (That said, cellulose will never be an air barrier.)

    If you want to improve your wall's thermal performance, you can either hire a cellulose contractor to try to add more cellulose to your stud bays (thereby increasing the density of the cellulose) -- an approach that may or may not be feasible or advisable, depending on the density of the insulation you have -- or you can remove the vinyl siding and install an exterior air barrier.

  2. Alan B | | #2

    I don't know how long vinyl siding lasts (its about 10 years old) but i was very disappointed, it means that my house has no effective insulation at all right now :(
    How much better would dense pack cellulose do, and do i have to remove the loose fill or can it be dense packed?

    Does this also mean that cellulose in an attic is very wind washed?

  3. GBA Editor
    Martin Holladay | | #3

    The cause of your problem is not the vinyl siding.(Just so you know: if you remove the vinyl siding temporarily to drill holes in your sheathing for insulating purposes, or if you remove the vinyl siding so that you can install rigid foam as an air barrier, the vinyl siding can usually be re-installed.)

    You are not alone. Most older homes are leaky and poorly insulated.

    You don't have to remove the existing insulation to add more cellulose, but I'm not sure whether or not you want to add more insulation to your walls. Whether or not to go that route depends on (a) the density of the existing insulation, (b) whether or not you want to pay to have a bunch of holes drilled in your wall, and (c) whether or not there is a skilled insulation contractor in your area who understands dense-packing methods.

    Windwashing in attics is usually only a problem at the perimeter, where the ventilation air enters at the soffits. Windwashing in attics can be addressed by installing windwashing dams above the top plates of your exterior walls, and by installing ventilation baffles under your roof sheathing. Details here: Site-Built Ventilation Baffles for Roofs.

    In most cases, windwashing is less of a concern than leaky ceilings. Ideally, someone performed air sealing work at your ceiling before the cellulose was installed on your attic floor. If you think your ceiling is leaky, you can hire a home-performance contractor to seal the leaks in your ceiling. More information here: Air Sealing an Attic.

  4. Alan B | | #4

    Hey Martin,

    I actually have no sheathing on the outside of the house at all, after the true 2x4 i have wood siding thats painted, a cloth mesh/blanket then the vinyl siding. The cellulose in the walls is loose filled.
    Something i have thought about for the future when the current siding wears out is to remove the vinyl siding, add exterior sheathing, dense pack cellulose the walls (i thought about Roxul but the packages are designed for 3.5in 2x4s) then rigid foam to R20 then new vinyl siding. I don't know if this will be a moisture problem since my walls are such thick plaster and barn board on the inside plus many layers of oil based paint (but no wallpaper in part of the house and old paper based wallpaper in another part).

    As for the attic how can there not be a wind washing problem, its only airtight on one side instead of six (if people are lucky)

  5. GBA Editor
    Martin Holladay | | #5

    Adding exterior rigid foam to your walls will not cause moisture problems, as long as the rigid foam is thick enough to keep the wall sheathing or the interior face of the rigid foam above the dew point in winter. The minimum R-value of the rigid foam depends on your climate zone. (You still haven't told us your climate zone or geographical location.)

    For more information on determining the minimum thickness of rigid foam installed on the exterior side of a wall, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

    Q. "As for the attic, how can there not be a windwashing problem, since it's only airtight on one side instead of six?"

    A. For windwashing to be a problem, the wind needs to have access to the fluffy insulation. The only portion of the attic insulation that is vulnerable to the wind is at the perimeter of the attic, near the soffit vents, as I explained in my last response.

    The reason that builders don't install a top-side air barrier (for example, Tyvek) above insulation installed on an attic floor is because (a) installing an air barrier above attic insulation is difficult, (b) the Tyvek wouldn't stay in place for long, and (c) research has shown that adding a thicker layer of insulation is less expensive (and more effective from a thermal perspective) than installing a top-side air barrier. So if you are worried about convective currents in the fluffy insulation installed on your attic floor, you can assuage your worries by making your insulation a little bit thicker.

  6. Alan B | | #6

    I'm near Toronto Ontario, climate zone 4 i believe. Will many layers of oil based paint (plus latex on top) an inch thick of plaster, lath and 2 inches thick wood in the plaster be moisture open on the interior?
    Is there any research on wind washing not being an issue in attics, i was surprised at the amount of wind washing i have, it was completely unexpected.

  7. GBA Editor
    Martin Holladay | | #7

    Toronto is in Climate Zone 6, not Zone 4. Here is a link to the map: Climate Zone Map Including Canada. If you want to install rigid foam on the exterior side of a 2x4 wall in your climate zone, the rigid foam needs to have a minimum R-value of R-7.5.

    My response on windwashing in attics is unchanged, even when you ask the question three times. The issue has been extensively studied by building science researchers. The solution near the eaves is to install windwashing dams, which are described in this article: Site-Built Ventilation Baffles for Roofs (as I already explained in Comment #3).

    GBA advises the installation of above-code levels of attic insulation (R-60 would be good in your climate zone). GBA also reminds readers that attic insulation must extend over the entire top plate of your exterior walls. Follow these guidelines, and you'll be fine.

  8. Expert Member
    Dana Dorsett | | #8

    Not to put too fine a spin on it, but the Ontario Riviera around Toronto is really the cool edge of zone 5, as measured by 25 and 50 year heating degree-days, but you can practically spit to zone 6 from there in a high southerly wind. Only in the coldest of years does it (barely) edge over 4000 HDD-C, (the 5/6 boundary), whereas the 25 year average is under 3750 HDD-C:

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