78 FINE HOMEBUILDING

inished-basement projects usu-ally begin with visions of agame room for the kids or of asecluded spot for Dad to watch

Sunday football games with his cronies. Justabout as frequently, these projects end badlywith black spots of mold, crumbling drywall,and a smell reminiscent of a dungeon. Whatgoes wrong? In most cases, water becomestrapped behind a wood wall or floor and nur-tures a bloom of rot.

However, it probably isn’t a flood thatcauses the problem. Yes, bulk water, the kindthat flows across the floor, needs to be elimi-nated before an attempt is made to finish abasement. (Some common measures includeexterior waterproofing, functioning gutters,and/or an internal drain system.) But even ifyour basement looks dry, you easily can haveproblems when you enclose the concrete witha framed wall. The real villain here is watervapor, the invisible moisture that keeps con-crete damp and makes cold-water pipes dripwith condensation in the summer. This water

BY ANDY ENGEL

Rigid-foam insulation is the key to a comfortable living space

F

Finished BasementThe Stay-Dry, No-Mold

A rec room that will last. Built with conventional wall fram-ing, a plywood subfloor, and rigid-foam insulation, thisbasement remodel doesn’t trap moisture that can causeproblems later. (By the way, the stair railing isn’t finished.)

After

Before

Drawings: Toby Welles @ Design Core

FEBRUARY/MARCH 2005 79

In summer, warm,damp air infiltratesand condenses onthe concretefoundation.

Vapor barrierscan cause damageby sealingmoisture in thewall cavity, whereit promotes rot.

Expanded-polystyrene (EPS)rigid-foam insulationis semipermeableand won’t trapmoisture; it alsoisolates the woodframing from contactwith damp concrete.

Don’t blockmoisture withfinishes such as vinylflooring or oil-basepaints. Instead, usecork or woodflooring, carpet, andlatex-base paints.

A waterprooffoundation that’sadequately drainedprevents floodedbasements.

2x4 stud wall

2-in.-thick rigidfoam glued tofoundation wall

Subfloor (two layers of 1⁄2-in.-thick plywoodsecured with concretescrews)

1-in.-thick rigidfoam laid overconcrete floor

Understanding the nature of basements and water

Moisture moves from wet to

dry and from warm to cool.

In the summer, damp soils

and warm air outside make

the moisture drive mostly

inward. Humid outside air

enters the basement and

condenses on anything below

its dew point: cold-water

pipes, concrete walls, and

floors. In particular, carpeted

concrete floors can be a

problem because they easily

can become wet enough to

support mold and dust mites.

Most basements dry out

only in the winter when inte-

rior heat sucks the available

moisture out of the base-

ment and drives some mois-

ture outward through the

exposed portion of the foun-

dation. There’s also some

drive-out through the foun-

dation itself because the

basement is warmer than

the surrounding soil. The

trouble is that the soil tends

to be wet, and so has a lim-

ited capacity for drying.

There’s a significant energy

cost in moving this water

through the foundation. The

traditional response has

been to frame walls next to

the foundation, fill them

with fiberglass, and seal them

with a plastic vapor barrier.

But a basement vapor barrier

can trap moisture and pro-

mote rot. Basement floors

built with a similar system

fare no better.

Finish wallsand ceilingswith 1⁄2-in.drywall.

Seam tape sealsjoints betweenpanels.

1x3 strapping, 16 in. o.c.

Plywood stripblocks air.

2-in.-thick rigid foamglued to rim joist

80 FINE HOMEBUILDING

is always present. To reduce moldgrowth, water’s contact with cel-lulose (paper, wood, etc.) has tobe limited, and the water has tobe allowed to escape.

Through research published byBuilding Science Corporation(sources below), I’ve found thatrigid-foam insulation both ther-mally protects the basement andbreaks the contact between fram-ing and concrete. To avoid trap-ping moisture, I never install avapor barrier. Instead, I use mate-rials and finishes that allow mois-ture to diffuse. You can get rid ofthis diffused water by installinga dehumidifier or by extendingthe air-conditioning ductworkinto the basement. I’m no expertin this area, so let an HVAC con-tractor figure out the specifics.

Isolate and insulate the concreteI use 2-in.-thick expanded-polystyrene foam (EPS, or styro-foam) on the walls and 1-in.-thick EPS below theplywood subfloor. Thisrigid-foam insulation is suf-ficient to make a noticeabletemperature difference in thebasement without crowding inthe walls or the ceiling height.EPS is cheap, effective, and vaporpermeable. Believe it or not, it alsohas the compressive strength tosupport a two-layer plywood sub-floor without the use of sleepers.

After insulating the rim joist, I cover the floor with a layer of

Use rigid foam instead of wood for sleepersTo isolate the wood from moisture in the concrete, full sheets ofEPS are laid on the concrete floor. Seam tape and expanding foamseal the seams against the infiltration of moist air.

The foam gun, canisters ofexpanding-foam sealant,seam tape, and low-pressure dampers areavailable from The EnergyFederation Inc. (800-379-4121; www.efi.org).

Expanded-polystyrene(EPS) rigid foam isavailable at mostlumberyards and homecenters.

Much of the information inthis article was obtainedfrom the consulting firmBuilding ScienceCorporation; its Web site(www.buildingscience.com)has a wealth of informationon building technology.

SOURCES OF SUPPLY AND INFORMATION

PLYWOOD SUBFLOOR IS SUPPORTED AND

FEBRUARY/MARCH 2005 81

Two layers of plywood go down with screwsAfter drilling and counter-sinking pilot holes (1, 2),the author attaches thefirst layer of 1⁄2-in.-thick ply-wood with 21⁄2-in.-long con-crete screws (3). To allowfor expansion, 1⁄8-in. gapsare left between each sheetand around the room’sperimeter. Laid at right angles to the first layer, a secondlayer of plywood is fastened with15⁄8-in. drywall screws and spansthe joints between sheets to makea stronger floor. (4).

INSULATED WITH RIGID FOAM

2

4

3

1

82 FINE HOMEBUILDING

1-in.-thick EPS. On top of this, Ilay the subfloor, then build a reg-ular stud wall against the foamon the walls.

Keeping wood from contact-ing concrete is critical. Fail here,and you’re inviting water inthrough capillary action. Youcould use pressure-treated ply-wood and framing lumber, butI think that’s false security andan unnecessary expense. If you’vegot enough moisture in the wallor the floor to cause rot, thenyou’ve also got the right condi-tions for mold growth, some-thing that pressure-treatedlumber won’t prevent.

It’s also possible to skip the studwall and to screw furring stripsto the concrete through thefoam, but I don’t like that approach for two reasons. First,I haven’t seen many basementwalls that are as plumb or asstraight as I can build a stud wall.Unless you want to spend daysplaying with shims, the furringstrips will mimic the defects ofthe foundation. Second, furringstrips don’t have the depth thatallows easy installation of elec-trical boxes. �

Andy Engel is a builder andwriter who lives in Roxbury,Conn. Photos by CharlesBickford. continued

Air quality and fire safety in the basement

An insulated band joist stops thermal bridgingInsulating the joist bays with 2-in.-thick EPS will keepair leaks and cold spots to a minimum. Minimalamounts of expanding foam applied around the edgesof each piece act as both a sealant and an adhesive.

Blocking air infiltrationA strip of plywood is screwed to the undersideof the floor joists. Extending from the edge ofthe mudsill to the inside of the wall plate, thisplywood creates an air barrier. Gaps are filledwith expanding foam.

Outsideair

CombustionairOne of the effects of finishing a basement is to cut off leaks

that may have been supplying combustion air for the boiler,furnace, or water heater. Failure to replace this air supply couldcontribute to backdrafting and the possible buildup of lethalamounts of carbon monoxide.

Unless the appliance manufacturer provides specs that sayotherwise, the rule of thumb is to provide makeup-air ductsleading to the outside that are twice the size of the combinedflues. In this basement, the boiler and water heater share a Fabric damper

FRAMED WALLS ARE ISOLATED

Make the foam on the walls as tight as possibleAfter the floor is done, thewalls are insulated with EPSsheets trimmed for a frictionfit and glued to the foundationwith expanding foam (1, 2).Seams and gaps are filled withthe foam and taped. Plywoodscraps keep the sheets in placeuntil the glue sets (3). Unlikefurring strips, a stud wall goes in plumb and straight,and allows room to run anyutilities normally (4).

6-in.-dia. flue. I provided two 6-in. supplies, one that ended at theceiling level and one that ended near the floor in the mechanicalroom. To prevent these ducts from chimneying nice, warm air tothe outside, they were fitted with fabric dampers (photo facingpage; sources p. 80) made for this low-pressure application.

One other safety consideration is basement egress. Most build-ing codes require habitable basements to have two exits in case offire. This basement already had two doors, so that requirementwasn’t an issue. Lacking the second door, I’d have had to providea code-approved egress window (a 5.2-sq.-ft. opening within 44 in. of the floor, leading to a 36-in. by 36-in. well with ladderrungs leading to grade). Last, if your home isn’t already soequipped, install hard-wired smoke and carbon-monoxide detec-tors in the basement.

1 2 3 4

FROM THE FOUNDATION’S MOISTURE

84 FINE HOMEBUILDING

Reader Response

Get the foundation drainin the right placeIn your article “The No-MoldFinished Basement” (FHB#169, pp. 78-83), the drawing onp. 79 shows the bottom of thefoundation drainpipe above thegrade of the finished concretefloor. In soils with a high silt orclay content, that basementwould be wet because the waterlevel has to get as high as thebottom of the perforated drain-pipe before it exits the crushed-stone well created in the soilsurrounding the foundation.We always put our drainpipe atthe bottom of the footing toprevent seepage from the jointsin the concrete. Also, after theexterior of the foundation hasbeen waterproofed, we put fil-ter fabric only on the top and onthe side of the crushed stone be-cause it will most likely be cont-aminated from backfill sandwashing vertically down fromabove rather than from thehard-packed surface below.

—LONN M. L IVENGOOD

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