Wood Handbook, Wood as an Engineering Material

need to work in concert with flashing; they are not a substi-

tute for flashing.

Vent clothes dryers, showers, and cooking areas to outside,

not to the crawl space or attic. Place an air barrier in exterior

walls and top-floor ceilings, and flash penetration through

exterior walls (doors, windows, and vents). Vent to soffits if

possible. Avoid using humidifiers. If the structure contains

a crawl space, cover the soil with a vapor-retarding material

such as black plastic or tar paper.

Do not seal the overlap of lap siding.

In northern climates, use an air barrier–vapor retarder on the

interior side of all exterior walls and top-floor ceiling to

prevent condensation in the walls and attic. In buildings

that are air-conditioned most of the year, place the air

barrier–vapor retarder on the exterior side.

General Technical Report FPL–GTR– 190

Table 16–3. Moisture-excluding effectiveness of
various finishes on ponderosa pinea

Finish

No.

of coats

Moisture-excluding

effectiveness (%)

1

day

7

days

14

days

Linseed oil 1 12 0 0
2 22 0 0
3 33 2 0
Water repellentb 1 12 0 0
2 46 2 0
3 78 27 11
Latex flat wall paint 1 5 0 0
(vinyl acrylic resin) 2 11 0 0
3 22 0 0
Latex primer wall paint 1 78 37 20
(butadiene–styrene resin) 2 86 47 27
3 88 55 33
Alkyd flat wall paint 1 9 1 0
(soya alkyd) 2 21 2 0
3 37 5 0
Acrylic latex house 1 43 6 1
primer paint 2 66 14 2
3 72 20 4
Acrylic latex flat 1 52 12 5
house paint 2 77 28 11
3 84 39 16
Solid-color latex stain 1 5 0 0
(acrylic resin) 2 38 4 0
3 50 6 0
Solid-color oil-based stain 1 45 7 1
(linseed oil) 2 84 48 26
3 90 64 42
Semitransparent oil-based 1 7 0 0
stain (commercial) 2 13 0 0
3 21 1 0
Alkyd house primer paint 1 85 46 24
(maleic-alkyd resin) 2 93 70 49
3 95 78 60
Urethane varnish 1 55 10 2
(oil-modified) 2 83 43 23
3 90 64 44
4 91 68 51
5 93 72 57
6 93 76 62
Polyurethane paint, gloss 1 91 66 44
(two components) 2 94 79 62
3 96 86 74
Aluminum flake pigmented 3 98 91 84
varnish (oil-modified) 4 98 93 87
Paraffin wax, brushed 1 97 82 69
Paraffin wax, dipped 1 100 97 95
aSapwood was initially finished and conditioned to 26 °C (80 °F)
and 30% RH, then exposed to the same temperature and 90% RH.b
WRP would be about the same.

Figure 16–14. Decay and paint failure in wood railing

fully exposed to weather.

Figure 16–13. Change in moisture content of pon-

derosa pine sapwood finished with three coats of

aliuminum-pigmented alkyd paint and exposed to

90% and 30% RH at 26 °C (80 °F), compared with

moisture content of unfinished wood.