Guidelines for below-grade,
under-floor spaces
"Ignorance never settles a question."
- Benjamin Disraeli
"There is no such thing as absolute certainty, but there is assurance
sufficient for the purposes of human life."
- John Stuart Mill
An inquiry initiated by the Tri County Health Department (TCHD) in March 2002
led to the development of an advisory document titled "Guidelines for
Design and Construction of New Homes with Below-grade Under-floor Spaces."
Riveting title, eh? But if you build on expansive soils along Colorado's
Front Range. you should snag a copy. For everyone else, the guidelines can act
as a reference as you design at-grade crawl spaces that address moisture issues.
The full document is accessible on E-Star's Web site (www.e-star.com).
A parallel document written in code language as proposed modifications to the
International Residential Code. is also available at the same site. Given the
demand for the latter document by building departments along the Front Range,
expect a number of local jurisdictions to propose adopting it within a few
months.
Background
After an initial health inquiry from TCHD into the effectiveness of codes in
addressing the potential for mold growth in new homes, a group called the
Moisture Management Task Force was assembled in May 2002. The group of 14
included Leo-technical engineers, building scientists, builders and building
officials. The task force's process, ably moderated by the Denver HBA's Kim
Calomino, was painfully long and demanding (ask me how I know this). Bimonthly
meetings through February 2003 led to draft guidelines. After a peer-review
period that elicited extensive comments, the task force revised the document.
Version LO of the guidelines was released on October 30.
The 23-page document presents potential causes of water infiltration/moisture
build-up in below-grade. Under-floor spaces. Those potential causes were broken
down into three generic categories: surface drainage, house design and
construction practices. and occupant behaviors. Then the document lays out alternative
solutions. Along the way the task force provided background commentary,
explaining rationales for both issues and solutions. Building officials within
the group used the report and other information to provide model code language
that was also reviewed by the task force.
A critical caveat: the guidelines are just that - guidelines. They are only
advisory in nature. They don't constitute an exhaustive compendium of
alternative and potential solutions. They don't provide the only viable measures
and methods for effectively managing moisture in and around crawlspaces and
foundations. They don't render judgment on other possible methods. But they do
provide a systems-engineering approach that describes workable techniques
already successfully used in the field by some builders.
Two additional notes: the original document will likely he modified over
lime, and the document only addresses options for new homes. A substantial
number of the measures itemized below are simply not practical as modifications
to existing homes.
Bullet points
Highlights from the guidelines are bulleted below. Realize that this
itemization lists key items in overview fashion while leaving out much essential
information. All readers involved in design, construction and inspection of
below-grade, under-floor spaces should print out from www.e-star.com
both a complete copy of the guidelines version 1.0 plus the parallel proposed
code document.
A key point to consider up front: the rate of water vapor intrusion to a
below-grade, under-floor space may be considerably greater than that for an
at-grade crawl space. These environments can be Mississippi-like humid, hence
the TLC needed during design and construction.
Surface drainage
- All projects need area grading plans that consider pre-existing drainage
patterns.
- Area grading plans should call out drainage swales to convey water away
from foundations and should, where possible. exceed the 2 percent-minimum
slope required by code.
- Plans for each lot should require platted drainage easements along lot
lines.
- Area underdrain systems, to help control subsurface water, should be
designed to allow maintenance (with cleanouts) and should be inspected
during and after construction.
- Lot-specific drainage plans should specify top of grade-beam/foundation
elevation for the foundation.
- The lot design should anticipate grades (5 percent minimum slope)
necessary to drain water away from foundations.
- Where lot lines, walls, slopes or other physical barriers prohibit 6
inches of fall within 10 feet, drains or swales should be provided to ensure
drainage away from the structure.
Drains, backfill
- Perimeter drains, whether interior or exterior, should have a positive
slope to a sump pit or gravity discharge.
- Sump pits should he equipped with pumps unless the drains are provided
with a gravity discharge (to daylight) or into an underdrain system.
- Soils report identifies degree of backfill compaction plus minimum slope
to achieve adequate drainage.
- Excavations for more than one house require special backfill
considerations.
- The ramp backfill should be compacted to avoid the ramp becoming a conduit
for surface water flowing towards the foundation.
Construction
- Remove standing water/snow/ice in the basement excavation prior to
installation of ground cover and floor deck.
- Gutter downspout extensions minimum 5 feet perpendicular to the
foundation, with tip-up hook removed prior to occupancy.
- Ground surface below floor must have positive slope to drainage trenching:
no "dished out" sections that could allow ponding of water.
- At the time of construction, the minimum clearance below steel floor
components (beams, joists. etc.) and plumbing lines (where practical) should
be equal to the void height plus four inches; the soils report may specify a
different clearance figure.
- If trenching for sewer lines for clearance below beams, those trenches
should slope to the perimeter drain system.
- When basement floor decking is wetted after placement, it should be
allowed to dry. One suggestion: leave off or remove two 4x8 sheets at
opposite corners of the floor and place a walkable metal grate over
openings.
Ground cover
- All dirt on under-floor spaces. once sloped for proper drainage and
cleared of debris, should be covered with a 10-mil un-reinforced
polyethylene vapor retarder (or material with equivalent puncture
protection).
- The ground cover should be continuously fastened to all foundation walls.
footing pads, plumbing lines. etc.. with laps continuously sealed.
- Alternative systems may be considered based upon engineered design.
Insulation and ventilation
- Don't insulate between floor joists beneath the wood deck, or against the
interior surface of the foundation wall.
- Recommend exterior insulation of foundation walls vs. interior vinyl-faced
fiberglass batts.
- Recommend using indoor (conditioned) air to ventilate below-grade
under-floor spaces.
- Transfer floor openings should be designed (e.g.. one per 250 square feet
of basement floor area) and sized (refer to table in document) to allow
sufficient conditioned air to flow from the conditioned basement to the
under-floor space.
- The ventilation system should he designed to provide the home with fresh
air to meet the ASHRAE 62.2 ventilation standard (see document).
- Ventilation fans should be designed to last at least 44,000 hours (5
years) and should he equipped with a trouble light or alarm to alert
homeowners of the need to repair or replace the fan.
- The requirements for ventilation of below-grade under-floor spaces that
rely on concrete decking systems vs. wood or wood/steel floors alas vary
from those described above.
Miscellaneous
- When appropriate, structural components in below-grade under-floor spaces
should be design to tolerate corrosion or should he protected against
corrosion that may impair the strength or serviceability of the structure.
- Provide comprehensive flashing systems that drain water away from the
building. Pan flashing for a door in a walkout basement may require altered
design of the foundation height.
- Dryer vents should always extend to the exterior.
- All drains for water heaters, humidifiers and condensate should terminate
directly above floor drains with the required air gap and a turn-down in the
floor drain.
- Avoiding negative pressure in basements, by careful sealing of ductwork
and elimination of return ducts that use building cavities, should help
reduce the transferring of any moisture problems in below-grade spaces to
above-grade spaces.
Landscaping
- No landscaping requiring irrigation should be located within five feet of
foundation walls.
- Landscaping edging should not interfere with the discharge of the roof
drainage system.
- No valve boxes or drain valves should be placed within the backfill or
within five feet of the foundation. Wherever possible, sprinkler lines
should be installed with the same guideline.
- Don't plant trees in swales or place retaining walls that would change
drainage pathways.
Homeowners and maintenance
- Homeowners need to improve their knowledge of the moisture control system
and their own important role in maintaining that system.
- Watering around foundations should be limited; no watering within five
feet of the foundation.
- The installation of rain/moisture sensors is strongly recommended.
- Maintain gutters, downspouts and extensions.
- Landscaping should not interfere with the discharge of roof drainage.
- Proper soil preparation below sod is critical to create water storage
capacity, which reduces watering requirements.
- To protect the long-term durability of the structure, humidifiers are not
recommended.
- Maintain fans that exhaust air from under-floor spaces.
- Unvented combustion appliances, especially gas fireplaces, are not
recommended.
- Periodically check under-floor spaces for leaks, integrity of the ground
cover, condensation, standing water. etc.
- Do not use the under-floor space for storage, as this can damage the
ground cover.
- When finishing a basement, be aware of the problems, accept the associated
risks. and carefully design, construct and maintain the basement finish
"system" to control moisture.
Steve Andrews consults with builders for E-Star
Colorado and writes on energy issues (sbandrews@att.net).
E-Star (www.e-star.com),
is a nonprofit home energy rating system that works with both new and
existing homes statewide.
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