8/22/2019 Seal Out Water Vapor_tcm45-347057

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BY T. R. JOHNSON

CONSULTANT

DUNDEE, ILLINOIS

Loose floor tile, musty odors,peeling paint, moist floors, rust-

ing metal and other problems of dampness are not limited to leaky b u i l d i n g s. They occur in some of the otherwise best designed build-ings and the cause is likely to be wa-ter vapor moving through the con-crete floor and basement walls.

 All concrete used for floors per-

mits some vapor to pass. Theamount of moisture leaving the sur-face of a good quality concrete slabis about a quart per day per 1,000s q u a re feet when the slab has itsbottom surface in contact with wa-ter vapor but not liquid water. Theexact amount of vapor that can passt h rough a floor is related to the con-c rete quality. The factors that con-t ribute to strength and density of ac o n c rete floor also contribute to low p e rm e a b i l i t y.

 All concrete slabs on or below g rade are exposed to ground mois-t u re and will transmit water va p o runless they are protected by a va p o rb a r ri e r. The slab may appear to bed ry and the surface may look dry but below the surface the concre t emay hold a considerable amount of m o i s t u re. This moisture contri b u t e sto all the bad effects already men-tioned as well as mildew growth, de-t e ri o ration of stored documents, ef-f l o rescence and the swelling and warping of upper floors made of  w o o d .

The practice of total air condi-tioning and air flow control has ag-g ra vated the moisture problem. Al-though air conditioning re m ove sm o i s t u re from the air it often re-duces the tempera t u re in the air-conditioned areas to a level that is

b e l ow the dew point tempera t u re of the outside air and surrounding soil.The result is that moisture condens-

es on the walls and floors unless ane f f e c t i ve vapor barrier is provided tos e p a rate the two are a s.

Troubles from damp air and thosef rom condensed moisture in air-conditioned environments can bem i n i m i zed if not eliminated by anadequate vapor barrier installed be-l ow the concrete slab and aro u n dany walls below gra d e.

The vapor barriers used for thispurpose are commonly re f e r red toas membra n e s. To be most effectivethe membrane must be placed onthe pre p a red grade and continuedup the ve rtical or foundation wall tothe height of the floor so that thefloor is isolated. Protection for them e m b rane during constru c t i o nshould be detailed to pre vent them e m b rane from being damag edand re n d e red less effective.

Types of vapor barriers

T h e re are four types of vapor bar-riers for use below concrete floor

For the best floor performance and the most habitable space above the floor 

Seal out water vapor

Asphalt sheets, sealed together and turned up at t he edges to form a tightsaucer will isolate t he floor slab from the soil.

Asphalt membrane sheets on e t he

exterior complete t he isolation ! of thestruct ure from the soil.For best result s the sheet also runsover the footing and is sealed to t hesheet below the floor.

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slabs: roofing felts, plastic films,rubber sheet membranes and pre-f a b ricated asphalt panels. The se-lection of a vapor barrier should bemade on the basis of the moistureconditions of the ground and the in-tended use of the building, ra t h e rthan on the basis of initial cost.

The perm rating of a vapor barri e r

is a measure of the rate of va p o rt ransmission; the lower the permrating the better the barri e r. Fo l l ow-ing are some typical ra t i n g s :

Material Perm rating

15-pound roofing felt....0.6 to 2.0

4- and 6-milp o l yethylene film................... 0 . 1 0

55-pound roofing felt..0.03 to 0.08

butyl rubber sheeting.......... 0 . 0 0 2

1/8-inch asphalt

p a n e l s. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .0 . 0 0 0

Characteristics ofvapor barriers

Roofing felts probably we re thefirst moisture barriers used below c o n c rete slabs. They are available int h ree-foot rolls which are lappedfour to six inches and sealed withhot asphalt. The 55-pound felt ismuch superior to the 15-pound feltbut neither lasts ve ry long.

Po l yethylene film is ve ry low in

cost and easily installed but is ve ry easily punctured and not a com-plete barrier at the joints. It is ava i l-able in widths up to 32 feet; for larg ea reas the sheets are lapped six inch-es but not sealed. Po l ye t h y l e n e -

coated kraft paper, which has ac o m p a rable perm rating, is mored u ra b l e.

Butyl rubber sheeting is a goodm o i s t u re barrier that is rugged andlong-lasting; but it is expensive andmust be pre f a b ricated for the job.

 W h e re splices are necessary they aresealed with an adhesive.

Pre f a b ricated asphalt and as-phalt-plastic panels have the lowe s tp e rm ra t i n g s. They are rugged and,like butyl rubber sheeting, are lesslikely to be damaged by constru c-tion workers placing the concre t e.The panels come in 1/2-inch sheetsof four by eight feet. They are in-stalled either by lapping or by butt-

 joining and their joints are sealed by ove r-laying eight -inch-wide stri p scut from the panel. These are sealed with hot asphalt or an asphalt mas-

t i c.

Additional measures help

The use of a vapor barrier doesnot negate the need for pro p e rd rainage of ground water at the baseof the footings. If the water table isa b ove the level of the floor slabsomething more than a vapor barri-er is needed; here it is best to use am e m b rane in a sandwich constru c-tion between a base slab and thec o n c rete slab. (See CO N C R E T E CO N-

S T RU C T I O N, Oc t o b e r, 1973).

PUBLICATION #C730550Co py right © 1973, The Ab e rdeen Gro u p All rights re s e rve d