Crawl Spaces: How to Avoid Trouble with the Foundation

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Crawl Spaces: How to Avoid Trouble with the Foundation

Ser

NA7110

~ 2 1 h 8

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8

BLDG

National Research Council

Canada

Division of Building Research

H O U S I N G N O T E N O . 8

CRAWL SPACES: MOW

TO

AVOID TROlVlRllE

by

A. T.

Hansen

Reprinted from Canadian Builder,

Vol.

XII, No. 10, October 1962, p. 63-64, 89

Ottawa, February 1963

SLOPE GROUND AWAY

FROM FOUNDATION 9 :

-I-- VAPOUR BARRIER

BEHIND INSULATION TO L

C n a n r I n . r o

LDRAINAGE

OF CRAWL SPACE FLOOR

FIGURE 1

-

Crawl spaces:

How

to

avoiid trouble

with the

foundation

By A. T. HANSEN Housing Section,

Divisicm of Building Research, N.R.C. During the past several years, the construction of basementless houses with crawl spaces has enjoyed a revival in Canada. This has been due partially to the demonstration house projects of the National House Builders Association which featured this system as a low cost foundation.

There are a number of points one should consider before deciding on the most suitable type of foundation system to use. These include storage space, site conditions, and construction costs. The purpose of this paper, however, is not to discuss the pros and cons of any par- ticular system, but to outline the most important principles to be followed in the construction of a crawl space foun- dation.

Crawl space foundations can undoubt- edly be built considerably cheaper than conventional basements of similar over- all dimensions and when properly con- structed there is no reason why crawl space foundations should not perform quite satisfactorily. When not con- structed properly, however, they can be the source of serious problems which can lead to the premature deterioration of the entire house.

A number of examples have been brought to the attention of the Division of Building Research of improper con- struction in crawl space houses that re- sulted in serious problems. In one in- stance the deterioration of the house had progressed to the extent that the house, only two years old, was declared unfit for human habitation. On the other hand there are many examples of prop- erly constructed crawl space houses which have performed quite satisfactor- ily. The main principles of successful crawl space construction are as follows. Foundation Depth

Foundations for crawl spaces need not extend below frost level if the crawl space is adequately heated during the winter. The minimum depth of footings for heated crawl spaces depends on the type of soil on which the foundation is placed. If the soil is of the type that displays significant movement with vari- ations of soil moisture content, as with some clay soils, the foundation should be at least 4ft. below grade since move- ments due to volume change are greater near the ground surface. Rock or granu- lar soils, such as sand, gravel or silt, are not sensitive to moisture changes and the foundation can be any depth if the crawl space is adequately heated. If the crawl space is vented in winter,

however, the air temperature within the space may drop close to outside air temperature and all foundation footings must be below the depth of frost pene- tration unless the footings are on rock c r on well-drained sand or gravel. There is also a possibility with some wet soils that the soil will freeze to the side of the foundation wall and further freezing may heave the foundation even though the footings may be below the frost level.

Crawl Space Clearance

Sufficient clearance should be pro- vided in crawl spaces so that any equip- ment located there such as pumps or plumbing cleanouts, can be serviced and to ensure some circulation of air beneath the joists. The deeper the excavation, however, the more danger there will be of flooding, especially in low-lying sites. The absolute minimum clearance one should provide beneath framing mem- bers to the ground surface is 12ins. over those areas where access for servicing is not required, and 24ins. in those areas where servicing of equipment is necessary. Any access trenches to serv- ice equipment should be at least 3ft. wide and should not be below the level drained by the footing tile unless a method of draining these areas is pro- vided. As a gcncral practice, a crawl space with a 24-in. clearance beneath joists is the most convenient and per- mits access through the space without the necessity of service access trenches. Heating System

Warm air perimeter heating is most commonly used with crawl space con- struction. The warm air may be ducted from the furnace to the floor registers in the same manner as for conventional houses, or the entire crawl space may be used as a warm air plenum. In the latter case, the furnace, preferably centrally located, blows warm air into the crawl space through several stub ducts usually about 6ft. long and directed to the cor- ners of the house. Floor registers are located in much the same manner as for the ducted system. The warm air in the crawl space, which is pressurized by the furnace blower, is forced up through the registers although the registers and furnace are not connected by ducts. A relatively tight crawl space is essential, therefore, for efficient operation. Fur- ther information on the different meth- ods of crawl space heating may be found in Manual 4 of the National Warm Air Heating and Air Conditioning Asso- ciation.

Insulation

The method of insulating the crawl space will depend to a large extent on whether the crawl space is to be venti- lated during the heating season.

If the crawl space is not to be venti-

terior foundation walls should be insu- lated. The insulation should extend from at least 12in. below finished grade up to the bottom of the subfloor (Fig. 1).

Insulations whose durability or ther- mal properties are unaffected by mois- ture, such as some of the foamed plas- tics or cellular glass, may be installed either on the interior or exterior of the foundation. (Figs. 1 and 2).

When applied to the exterior, insula- tion must be protected with a covering such as asbestos cement board. When conventional mineral wool batts are used, however, they must be applied to the interior of the wall and protected with a vapour barrier on the warm side.

Batts carelessly installed so that they permit circulation of air around them lose a great deal of their effectiveness and condensation can occur as this air contacts the colder foundation wall. If this condensation wets the insulation it will further reduce its effectiveness.

To reduce convection, the edges of the batts should be installed tight against the foundation or furring and applied horizontally to reduce the number of vertical joints (Fig. 1).

If the crawl space is to be vented during the heating season, the floor above the crawl space must be insu- lated, as well as the water and drain pipes and the heating ducts if these are unprotected by the floor insulation. At least 3in. of mineral wool insulation should be provided. The batts should be installed with the vapour barrier facing up, and supported on galvanized mesh or wooden strips nailed across the bot- toms of the joists.

Ventilation reduces the temperature within the crawl space to that approach- ing outside air temperature so that wall insulation has little or no effect on re- ducing hcat loss from this space. Floor insulation of course is ruled out when the crawl space is used as a plenum. Moisture Protection

It is of prime importance in crawl space construction to restrict the en- trance of ground moisture into the dwell- ing. Ground moisture can raise the rel- ative humidity of the air in a crawl space to the extent that wood members exposed to the humid environment will

eventually rot. If the moisture has ac- ccss to thc habitable part of the house, as would be the case if the crawl space is used as a warm air plenum, high humidities may be built up throughout the house, causing condensation not only on window surfaces but even on exterior walls. When such conditions persist mil- dew growth may appear on the affected walls.

Because of rapid air circulation and fairly high temperatures usually associ- ated with crawl spaces used as warm air p ! e n ~ ~ n ~ s (sometimes over 100 deg F), this type of heat distribution system

EXTERIOR APPLICATION INTER13R APPLICATION FLASHlllC 7

%' ASBESTOS

-

,

-,!

,

,

i * l -

SLOPE GROUND AWAY 1:1,

FROM FOUNDATION

-

INSIOE WALL TO GRADE 1 BEHIND IHSOLATIUM

f .-.

r 4 MIL POLYETHYLENE OR

-

GROUND COVER

<-.

-DRAINAGE TILE BELOW LEVEL -'

OF CRAWL SPACE FLOOR

FIGURE 2

-

tent than the ducted systems. Because of this, it is of particular importance that all necessary prccautions be taken to ensure a dry crawl space with this system.

When protected with a ground cover such as 45-lb. roll roofing or 4-mil poly- ethylene, the evaporation from the crawl space floor is reduced to only a small fraction of what it would be without the ground cover. The vapour barrier should be lapped about 4 in. at joints and the laps weighted down with stone or gravel.

The crawl space walls may also trans- fer significant amounts of moisture since the walls are in direct contact with the moist ground. These walls should be damp-proofed in the same manner as basement walls, or with polyethylene film (Figs. 1 and 2).

Ventilation

Crawl spaces used as heating plenums do not ordinarily have to be vented to the outside. The opcration of the heating system ensures air changes during the heating season to prevent a build-up of water vapour in the crawl space. Even during the non-heating season, the crawl space is vented to the habitable area through the floor registers, and in any event, the furnace blower may be oper- ated periodically during the non-heating season should conditions in the crawl space warrant it.

Even crawl spaces in which the warm

ventilation. The vents must be equipped with tight fitting closures which can be closed during the heating season. Drainage

Of equal importance to providing ade- quate vapour protection is the provision of adequate drainage, both surface and subsurface, since the effectiveness of the ground cover is largely defeated if the crawl space is subject to periodic flood- ing.

There are many cases on record where adequate drainage was not provided and the crawl spaces flooded, acting as large sumps during spring run-off or heavy rains.

Surface drainage is provided by sloping the finished grade away from the foun- dation. Wide eave overhangs or the use of roof gutters and down spouts to di- rect roof drainage away from the foun- dation may also be used in addition to proper grading.

Drainage tile should be placed around the perimeter of the footings in the same manner as for conventional basements to collect watcr at the footing level and drain it to a sewer or ditch by gravity flow if possible or through the use of a sump pump.

The surface of the crawl space floor should be graded and left reasonably even. This will provide a suitable base for the ground cover and ensures that if water does enter the crawl space it will not collect in pools which will not dry up rapidly or drain away.

air is ducted to the registers may be _Summary safely left unvented during the heating

Experience indicates that crawl space season because of the efficiency of the

foundations will provide acceptable per- ground cover in controlling evaporation

formance if constructed properly with from the ground surface. It is still con-

attention given to moisture protection, sidered good practice, however, to vent

drainage, and insulation, and if construc- such crawl spaces during the non-heating

tion details are according to good prac- season as an added precaution against

tice. If attention is not given to these moisture build-up. Vents should consist

items, however, serious problems can be of louvered openings with a total area of

at least 1/500 of the floor area. Thev expected.

[This paper is a contribution to Canadian should be equipped with a _{Builder from the Division of Building Research,} prevent the entry of insects or vermin. National Research Council, and is published