Category A HOUSE

Like shingles, siding, and insulation, drywall amounts are calculated by the square footage of the area to be covered (in this case, the walls and ceilings). Rather than measuring the ceiling and walls in every room, experienced drywallers use a shortcut calculation. They simply multiply the total square footage of a house by 31/2 (3.5). For instance, a 24-ft. by 36-ft. house has 864 sq. ft. of floor space, and 864 times 3.5 equals 3,024 sq. ft. of drywall coverage. A 4×12 sheet of drywall covers 48 sq. ft. of wall. Dividing 3,024 sq. ft. by 48 proves that you need 63 sheets of drywall for this particular house.

Your drywall order

For the modest-size houses that Habitat builds, it’s best to make up most of your drywall order with 12-ft. drywall pan­els. A 4×12 sheet of drywall is more difficult to carry than a 4×8 sheet, but it covers more area and often eliminates the need for butt joints on a wall or ceiling. To fine-tune your drywall order, subtract any greenboard you will be using in the bathroom. Also, if you decide to go with 5/8-in. drywall on the ceiling, subtract the floor area (864 sq. ft. in our example) from the square-foot total, then order that amount of 5/8-in. drywall for the ceiling.

Have the drywall delivered several days before you plan to hang it. If you’re using any 5/8-in. drywall, stack those sheets on top of the V2-in. sheets. Storing all the drywall in one room creates a lot of weight on a few floor joists. Therefore, make a neat pile in each room, with the drywall flat on the floor, finish side facing up, or lean the sheets against the wall.

Screws and nails

Professional drywall hangers rarely use drywall nails. Screws hold better than nails, and a screw gun automati­cally drives the screws just the right distance, dimpling the drywall surface without breaking the paper.

If you’re not a seasoned drywall hanger, you’ll probably find it useful to drive a few nails to hold a panel in place against the studs or ceiling joists. Then you can finish installing the panel with screws. A 5-lb. box of drywall nails and a 50-lb. box of 1i/4-in. drywall screws should give you all the fasteners you need for a 1,200-sq.-ft. house. If you’re hanging 5/8-in.-thick panels, order 1 г/2-in.-long fasteners.

Joint tape, corner beads, and drywall compound

You can order these finishing supplies when you order your drywall. Joint tape comes in rolls; order 400 ft. for every 1,000 sq. ft. of drywall.

Every outside corner covered with drywall requires a cor­ner bead. These steel or plastic trim pieces are typically sold in 8-ft. or 10-ft. lengths. When estimating the amount of bead to order, make sure you account for corners where drywall wraps around window and door openings.

As far as drywall compound goes, the typical Habitat house requires about nine 5-gal. buckets. For the Charlotte house, we used an all-purpose compound called Durabond®, which comes in powdered form and is mixed with water at the job site. Other folks prefer to buy premixed compound, which comes in buckets or boxes.

Remove fasten­ers that miss the framing. It’s easy to tell when a drywall screw or nail misses a stud, joist, or other framing mem­ber. When that happens, remove the fastener and make a dimple (a concave mark with a drywall ham­mer) at the spot so the hole can be filled and hidden with joint compound.

Cut the drywall panel to length. First score the sheet with a sharp utility knife. A large T-square, held to the measurement mark, guides the cut (see the photo at left). Once scored, the drywall breaks right along the cut line (see the photo at right). Cut the piece free by slicing along the crease on the back.

the gypsum core—about ‘/s in. or so. There’s no need to force the blade deep into the panel. Once the panel has been scored, snap it away from the cut, as shown in the photo above right. Running a utility knife along the crease on the back of the panel will separate the pieces. If the cut edges are rough or uneven, smooth them with a Surform rasp (see the bottom left photo on the facing page).

CUT ACCURATE HOLES IN PANELS

Holes for electrical outlet boxes, heating vents, and pipes must be laid out and cut accurately. Take your measurements from a wall, ceiling, floor, or sheet of drywall already in place. I like to transfer these measurements to the drywall panel with a T-square. For electrical outlets and heating vents, use a T-square to outline the hole, then make the cut with a small drywall saw. Plunge the point of the saw into the panel from the “good” side and saw along the cut line
(see the top right photo on the facing page). The finished cut should be within l/s in. of the outlet.

For a dryer vent or a round electrical outlet, measure and mark the center of the cut. Then use a compass or another round electrical box as a template to outline the hole. To make the cut, use a small drywall saw, a utility knife, or a circle-cutting tool made specifically for this job (see the bottom right photo on the facing page).

Another method for marking the location of an electrical box, regardless of its shape, is to rub the face of the box with chalk or a keel, place the sheet in position on the wall, and press the sheet against the outlet. The chalk will show you where to cut. Cut gently so you can avoid tearing the paper facing on the “good” side.

Use a drywall router to save time

Most of the time you can drywall right over door and window openings, attic access holes,
and heating vents, then cut around the outlet boxes with an electric drywall router, as shown in the bottom photo on p. 220. (Get a feel for this tool by making some practice cuts on scrap drywall.)

Make sure the electrical wires are shoved to the back of all the boxes, and double-check to be sure there isn’t power at any of the boxes for which you’re routing holes. Tack the sheet on the ceiling or wall, then mark on the sheet the location of each outlet with a line noting the edge of the box and an “X” showing the side the outlet is on. Don’t nail too near the outlet or you could break the drywall, but be sure to drive enough nails or screws into ceiling panels so they won’t fall down.

Set the router bit to extend about /4 in. past the base plate. With the router running, insert the bit into the center of the box and gently move it until it hits the side of the box. Pull

the bit out and reinsert it just to the outside of the box. Cut in a counterclockwise direction, maintaining slight pressure against the box.

The router generates some dust, so wear a good mask. A router or a large drywall saw can be used to cut larger openings as well.

It’s best to drywall the ceiling before you do the walls. This way, the top edges of the wall panels can butt up against the ceiling panels, support­ing them along the edges. The long edges of ceiling panels run perpendicular to the joists or joist chords. In bedrooms and other small rooms, you’ll probably be able to cover the full length of the ceiling with 12-ft. panels. If the ceiling is more that 12 ft. long, stagger the end joints where the panels butt together, just as you do on floor and roof sheathing panels. Try not to have a drywall joint land on an electrical or heat outlet, because this makes it harder to tape and hide the seam.

Measuring and cutting drywall panels

If you watch professional drywall installers measure and cut panels, you’ll be impressed with the speed and accuracy of their work. Although you may not achieve speed right away, accuracy is possible from the start if you use some of the tips explained here. With accurate cuts, you’ll have a much easier time mudding and taping the panels.

MAKE STRAIGHT CUTS. Instead of cutting a panel to the exact dimensions you measure on a wall or ceiling, cut it ‘/4 in. short. This leaves a ‘/s-in. gap on both sides of the panel, allowing you to fit the piece without binding on neighboring walls or panels. Make a straight cut by scoring along the cut line; snap the cut open so the panel folds back on itself and slice through the resulting crease on the back. Mark and cut on the “good” side of the panel.

If you have a drywall T-square and need to make a square cut, guide the knife against the edge of the square (see the top left photo on p. 21s). Take care not to let the knife slip and cut the hand that’s holding the T-square. Just cut through the paper and slightly into

Drywall is made by sandwiching a gypsum core between two sheets of paper. The “good" side of the panel is faced with smooth, white paper that takes paint easily. The “bad" side is darker in color, with a rough, porous paper surface. Panels (also called sheets) of drywall are packaged in pairs; to open the package, simply pull off the strips of paper that extend along each end.

The standard width for drywall panels is 48 in. For houses that have 9 ft. ceilings, use drywall sheets that are 4 ft. 6 in. wide. Different lengths are available, but for affordable hous­ing the most commonly used lengths are 8 ft. and 12 ft. The most common thickness for drywall is V2 in. However, 5/8-in.- thick panels are often used on ceilings where the joists are spaced 2 ft. o. c. because they are less prone to sagging. Most
codes require 5/8-in. panels between the garage and the house for fire resistance. If you use 5/8-in. drywall on the walls, be sure to order wider door jambs.

Water-resistant drywall is often used in high-moisture areas, such as bathrooms. Called “greenboard" because of its green-paper facing, it is treated to resist moisture damage but is not waterproof. It’s most often used to cover wall areas above tub and shower enclosures. Greenboard can be taped and painted just like regular drywall. It should not be installed on the ceiling unless the joists are spaced 12 in. o. c. to keep the board from sagging.

The short (48-in.) ends of a drywall panel are cut square, leaving the gypsum core exposed. The long edges of the panel are faced with paper and tapered so that the seams between panels can be leveled with the surrounding drywall during the finishing process.

to following the advice explained here, see the sidebar above and on p. 217 for information on sizes and types of drywall and how to order and store the material.

Make sure the studs and joists are dry

Framing lumber used today often arrives at the job site with a high moisture content. Over time, it will shrink—sometimes quite a lot. When the studs and joists shrink after the drywall has been installed, the fasteners can work loose. A loose nail or screw can create a noticeable and unsightly bump, or nail pop, in the drywall surface.

To reduce the chances that nail pops will mar your drywall work, you may need to close in the house and turn on the heat for a couple of weeks. Leave a couple of windows cracked open to allow moist air to escape as the wood dries. You can ignore this advice if you’re working with dry wood or if you’ve had the good fortune to frame your house in clear, warm weather.

Otherwise, make sure the wood dries out. You can even run a dehumidifier inside, if necessary.

Clean and mark the floor

Take time to clean up any scraps of wood or trash on the floor. Once the floor is clean, use a piece of keel (I use red because it shows up well) to mark the stud, trimmer, and cripple locations on the floor and the joist locations on the top plate. Knowing the location of studs and joists makes it easier to nail off drywall and, later, baseboard trim.

It’s also a good idea to mark the locations of electrical outlets on the floor. This helps avoid installing drywall panels over outlets, which can easily happen if you’re not paying attention. If it does happen anyway, at least there will be a mark on the floor telling you where the outlet is located. You can also mark the location of the backing placed in the walls to support towel racks, grab bars, toilet-paper holders, and so on.

To straighten a wall stud, cut a kerf into the stud at its most bowed point, pull the stud straight, then nail a 1x or 2x scab alongside it to strengthen the stud and keep it straight.

Dryall has delicate cor­ners and edges. When you store and handle sheets of drywall, make sure you protect the panels’ edges and corners from getting damaged.

Several specialized tools make it easier to cut and hang drywall on ceilings and wall studs. [Photo by Don Charles Blom]

Check and correct bad studs

Even if all the studs were crowned in one direction during wall framing, it doesn’t en­sure a perfectly straight wall. Sight down the length of the walls or lay a straightedge across them to locate bad studs. Replace any badly bowed studs, or fix a bowed stud by making a cut into the bowed area, forcing the stud straight, and bracing it with a 1x cleat (see the illustration on p. 215).

Tool up to hang drywall

The tools you need to install drywall are pretty basic. In addition to the chalkline and tape measure you’ve used for the work covered in earlier chapters, you’ll need the following tools:

UTILITY KNIVES AND SPARE BLADES.

Most straight cuts in drywall are made with a utility knife. Have a good supply of new blades handy. A sharp blade cuts cleanly through a panel’s paper facing, whereas a dull blade can tear the paper.

DRYWALL SQUARE. This large, aluminum, T-shaped square enables you to quickly and easily make straight, square cuts in drywall. SCREW GUN. A screw gun takes the guesswork out of fastening drywall because it sinks drywall screws just the right distance into the panel. This tool resembles an electric drill and holds a replaceable Phillips-head bit. DRYWALL HAMMER. This hammer looks like a small hatchet with a convex hitting surface. The curved face allows you to set the nail below the surface of the drywall without breaking the paper. The hatchet end is not sharp and can be used for levering or wedging drywall into place.

SURFORM® TOOL. Designed to function like a handplane, this shaping tool is very useful for trimming small amounts off the edge of a panel to improve its fit on the wall or ceiling. Avoid large Surform tools; the smaller versions are more maneuverable and fit in a pouch on your tool belt.

Drywall and Painting

I’m not sure when drywall—also known as gypsum board, wallboard, and Sheetrock®—was first used in construction. I have seen drywall on pre—World War II houses, but we definitely didn’t have drywall in my old prairie home. It wasn’t until the late 1950s in California, where I was working, that drywall be­came the preferred wall covering in residential housing. “Knock on the Wall! Demand Genuine Lath and Plaster!” was the rallying cry of the once-mighty plaster industry, as they struggled against the newcomer—drywall.

Big plaster fought a losing battle. It took two or three weeks to cover walls with layers of plaster, and the process left the house frame waterlogged. In the winter, it could take a month or more for a house to dry out well. Cabinets installed after plastering often had sticky drawers. In addition, passage doors were hard to open, and hardwood floors expanded and buckled. It’s no wonder the construction indus­try switched to drywall. It allowed builders to complete houses in record time.

Drywall installation can begin once you’ve passed all your inspections— electrical, plumbing, heating, framing, insulation, and vapor barrier. Drywall is not difficult to secure to ceilings and walls, but it takes more skill to leave the finished walls straight and smooth. This chapter will tell you how to order and store drywall, which tools and methods you need to cut and “hang” it, how to tape and finish the joints, and how to paint the walls and trim. By the time you’re done with these jobs, your house will look a lot more like a home.

STEP 1 GET READY TO INSTALL DRYWALL

There’s some important prep work to be done before you take delivery of your drywall order and before any installation work can begin. In addition

While you’re thinking about insulation and ventilation, you should also pay attention to a few other details that relate to energy conservation. Taken together, these small improve­ments can make a big difference in how well your house works.

■ Locate the water heater near the kitchen and bathroom.

This avoids long runs of pipe that increase plumbing costs and dissipate heat between the water heater and the faucets or showerheads.

■ In cold climates, keep water-supply lines out of exterior walls.

■ Use an insulated wrap for the water heater. New water heaters typically come with built-in insulation, which helps keep your energy costs low. Another money saver is water heaters that come with timers. This allows you to activate the unit during specific times of day when heated water

is needed.

■ Insulate all hot-water lines, and insulate cold-water lines in a crawl space. Both foam and fiberglass-wrap pipe – insulation kits are available at building-supply outlets and home centers.

■ Install low-flow showerheads. A showerhead with a built-in shutoff valve provides even more savings, allowing you to shut off the water while soaping up.

■ Consider using a small solar panel to preheat your water. The sun’s energy is free. With a solar panel, you can reduce the energy used by your hot-water heater.

Habitat for Humanity has developed many guides to help homebuilders and homeowners save energy and money. They are available online and from HFH International (see Resources on p. 279).Take advantage of these resources and increase your understanding of how to build durable, energy-efficient houses with good indoor-air quality.

Avoid single-speed fans. You’ll appreciate having a vent fan that can operate at more than one speed. Multiple-speed and variable-speed models cost a little more, but they enable you to use a lower, quieter speed during extended operation.

inspectors) have some catch-up work to do when it comes to understanding house ventila­tion. You’re better off finding a knowledgeable and reliable HVAC (heating, ventilation, and air-conditioning) contractor with up-to-date knowledge of home ventilation requirements. That said, proper ventilation for small, afford­able houses isn’t all that difficult to obtain.

Source ventilation is the key to reducing moisture and odors

You can start by installing adequate spot, or source, ventilation wherever moisture or odors are created. Venting moist air directly to the outside prevents it from escaping through the walls or ceilings, where it can cause damage.

At a minimum, showers and stoves should have exhaust fans that are controlled by simple on-off switches or wired to come on automati­cally when a bathroom light is turned on or the stove is being used. For a stove installation, mechanical ventilation is usually provided by a vent hood equipped with a fan. In a bathroom, a variety of ceiling-mounted fans are available, including models with built-in lights.

Exhaust fans in moisture-producing areas should always be vented directly outdoors.

That means out through a wall or up through the roof and not into an eave soffit or a crawl space. When we moved into our home in Oregon, I discovered that the clothes dryer was vented into the crawl space. Some pretty creepy looking stuff was growing down there in the dark. Even worse is venting moist kitchen or bathroom air into the attic.

Try to keep vent runs short—less than 10 ft., if possible. Avoid running vents through the attic, if possible; install them in interior soffits and dropped ceilings instead. If you can’t avoid running a vent through the attic, then make sure it is well insulated. This is cru­cial in cold climates, where heat inside the attic can cause ice damming along the eaves. This is serious business, so pay attention to the details.

Good indoor-air quality requires air exchange

We all need fresh air to stay healthy, and in a tightly built house, some form of mechani­cal air exchange is essential. You can provide air exchange fairly inexpensively by using a bathroom exhaust fan controlled by an auto­matic timer. Look for a fan that moves air at 80 CFM (cubic feet per minute) to 120 CFM.

Set the timer to run the fan about two-thirds of the time that people are generally home (it doesn’t make much sense to exchange air when no one’s home). The fan we have in our home is centrally located in a hallway, but check with your HVAC contractor to locate yours. Beware of bargain-priced fans. Those models are almost always noisier than other vent fans. An expe­rienced HVAC contractor can advise you on which models run quietly and reliably.

Whenever fans are blowing indoor air out­side the house, fresh air must come in to replace it. This ensures a healthy supply of fresh air and prevents negative air pressure from draw­ing exhaust gasses from the fireplace or furnace flues, which is a serious safety hazard. The simplest way to provide replacement air when exhaust fans are running is to open a window or two. It’s not necessary for the window to be fully open; just a crack will usually do. If it’s cold outside, open a window in a utility room, unused bedroom, or somewhere away from the main living area.

If you’re building a house where the winters are long and very cold (in Maine or Minnesota, for example), it may be necessary to have an HVAC contractor install a heat recovery ven­tilator (HRV) or an air-handling unit to bring replacement air into the house at a more com­fortable temperature. Those devices typically work by blowing warm indoor air outdoors while pulling an equal volume of outside air in­doors. Because only a thin membrane separates the passing airstreams, some of the interior’s warmth is transferred to the fresh incoming air.

Although it is not ideal in all climates or seasons, a whole-house fan can be a very attractive alter­native to air-conditioning. A simple fan is more reliable and less expensive than a single window – mounted AC unit, and it can effectively cool an entire house. As shown in the illustration at right, the principle is simple: A single, centrally located fan pulls in fresh air through open windows and blows hot indoor air outside. By turning the fan on in the evening and opening all major windows, it’s possible to quickly cool a house that has become hot during the day.

These fans work best in dry climates, or at least when the air is cool and dry outdoors. In the winter, when the fan is not being used, it must be protected with an insulated cover to prevent heated air from entering the attic space. I make a simple cover from plywood and then glue several layers of rigid foam to the top and sides.

stifr Habitat

"f 11 for Humanity

A PERFECT LITTLE NEST

Polyethylene sheeting is also commonly used as a vapor barrier. Thin (6 mil), clear “poly” sheet material is stapled to the face of studs, attic joists, and (on cathedral ceilings) rafters. The barrier is kept continuous by overlapping adjoining sheets and sealing the overlap with silicone or another sealant.

It’s much easier to describe what a vapor barrier does than to explain why it is essential in some situations but not in others. I once had to tear out an entire wall that had rotted because the house had no vapor barrier. There was so much water trapped in the walls that you could literally wring it out of the insula­tion. A vapor barrier would have prevented such damage.

To understand why and where a vapor bar­rier is important, imagine what happens when someone takes a long, hot shower in your home when it’s freezing cold outside. The bathroom is foggy with water vapor, and some of that warm, humid air makes its way into the attic and the exterior walls. The air can easily pass through openings around electrical outlets and light fixtures and even through the gypsum board itself, which absorbs moisture readily. At some point, the temperature in the attic and the exterior walls drops enough to cause condensa­tion. This dew point can occur in the middle of the attic or wall insulation or against roof and wall sheathing. Over the course of a cold winter, a steady supply of moist interior air can easily accumulate, causing soggy insulation, mold, and rotten wood.

A vapor barrier prevents the movement of vapor from a warm area to a cold surface. In cold climates, it should definitely be installed right underneath the drywall, paneling, or other interior wall finish material. Instructions for installing a poly vapor barrier are provided in the next section.

Vapor barriers are often eliminated in warm climates, especially in areas of low humidity, such as the Southwest. But you may want to consider installing a vapor barrier beneath the exterior siding if the house will be exposed to

warm, moist air outdoors and frequent air­conditioning indoors.

In mixed-climate zones—the region that extends from the mid-Atlantic states through the Carolinas and west by southwest to northern Texas—the need for a vapor barrier is minimal. In those regions, where mild winters are the rule, any moisture that does enter the wall cavities can dry from the outside in during the summer and from the inside out during the winter.

Installing a polyethylene vapor barrier

To work effectively, a vapor barrier must be installed with care. Even the smallest holes in a poly or kraft-paper vapor barrier must be sealed with housewrap tape or its equivalent. Use a durable, high-quality tape; neither duct tape nor packing tape will hold over the long run.

A friend of mine is a carpenter in Fairbanks, Alaska. They’re serious about vapor barriers up there. They cut sheets of poly from rolls that are 10 ft. to 20 ft. wide and 100 ft. long, covering the entire ceiling and all the exterior walls (on the inside). They even make sure to put poly behind a bathtub installed against an exterior wall.

In any given room, there are two steps to installing a poly vapor barrier. This isn’t a job you want to do solo; have helpers so that some can spread the sheet out over framing members while others staple it fast. You can begin as soon as all the insulation is in place.

1. Install the ceiling poly. Cut a piece of poly to fit the ceiling. If you have to use several pieces, make sure they overlap by at least one joist (or rafter, if you’re working on a cathe­dral ceiling). Seal overlaps with a layer of mastic, acoustical sealant, or housewrap tape. At the edges of the ceiling, the poly should lap at least 3 in. down onto the walls. Begin stapling the poly to the joists or joist chords in the center of the room and work out toward the walls. My friend staples about 12 in. o. c. through small, precut squares of heavy paper. This keeps the poly from tearing. Fit the poly

tightly into all corners so the drywall will go on easily. The drywall holds the poly tight against the studs and insulation.

2. Install poly on the walls. Make this sheet continuous so that it laps over the ceiling poly along the wall’s top plate and extends past the bottom plate to lap about 3 in. onto the subfloor surface. First staple the sheet along the top plate, working from the upper center of the wall and down and out to the edges of the wall. If you need to join one sheet of poly to another, overlap them by at least one stud and seal the lap as described previously.

You can sheet right over door and window
openings, then cut openings in the poly after it’s

Glass fibers can irritate your skin and damage your eyes and lungs, so safety precautions are very important when working with fiberglass insulation. Cover your body with a loose-fitting, long-sleeved shirt and long trousers, and wear gloves and a hat, especially while insulating a ceiling (see the photo below). It’s best to wear a pair of quality goggles, too, because eyeglasses alone don’t keep fiberglass par­ticles out of your eyes. Make sure the goggles fit properly; goggles that fit well don’t fog over. Wear a good-quality dust mask or, better yet, get yourself a respirator. Don’t scratch your skin while you’re working (you’ll just embed glass fi­bers), and be sure to wash up well when you are finished. CUTTING BATTS. Cutting fiberglass batts to size is straightforward. The best tool for the job is a sharp utility knife. Note that I said “sharp." A dull blade will tear paper­faced batts, and torn paper doesn’t work as a vapor barrier. A sheet of plywood or OSB makes a good cutting table. Place the insulation batt on the worktable, with the paper side down if you’re using faced batts. Measure where the batt should be cut and add at least V2 in. (it’s better for a

batt to be a bit snug than to have a gap at the edge or the end). Compress it with a straight board, then run the knife along the board, as shown in the photo above. Be careful with the utility knife. If it’s sharp, you don’t have to exert a lot of pressure. Keep the hand that is holding the board out of the blade’s path.

When fitting batts around a window, you’ll need to cut pieces to fit above and below the window. To speed the pro­cess of insulating walls, I measure both spaces, mark their lengths on the cutting table, and cut as many pieces as I need. Don’t be sloppy with your cuts. Even small holes or gaps in fiberglass insulation can dramatically reduce its effectiveness.

INSTALLING BATTS. Batts faced with kraft paper have a foldout tab that should be stapled to the face of the studs or ceiling joists. The most common method of attaching faced batts to wood is with a hammer-type stapler and 1/4-in.-long staples. Make sure the staples go in all the way, so that you won’t have problems hanging drywall later. Unfaced batts are held by friction between studs or joists until the vapor barrier or drywall is in place.

Insulate around electrical boxes. First, divide the batt into two layers instead of compressing it. Slide the back layer behind the outlet box (see the photo above), then cut out the front layer to fit around the box (see the photo below). This technique also works for installing fiberglass batts around electrical wires and plumbing pipes. [Photos

by Steve Culpepper, courtesy Fine Homebuilding magazine © The Taunton Press, Inc.]
uninsulated space around them. Don’t use these fixtures. It’s much better to choose models that require no insulation gap. You can insulate right up to and on top of those fixtures. Some states require that fixtures be airtight, too, so check with your building inspector.

Insulating between floor joists in crawl spaces

Floor insulation is important in a house with a crawl-space foundation. Often, it is not enough

To keep cold air from entering on top of the floor insulation, ensure that insulation is right up against the subfloor (above, left) or roll the insulation up the inside of the rim joist to the subfloor (above, right). Either of these techniques will help prevent a cold floor.

SUPPORTING INSULATION BETWEEN FLOOR JOISTS

just to put insulation under the floor, because cold can pass through the rim joist. Unless batts fill the entire joist space, cold air can seep in through the rim joist and over the top of the batts, making the floor uncomfortably chilly.

To prevent this, you can either hold the insulation high or roll it up to cover the rim joist (see the top left illustration). Better yet, use a thicker batt with a higher R-value to fill the entire joist space and butt up against the rim joist.

When insulating between I-joists, make sure the insulation is wide enough to extend all the way from web to web. If you live in a cold part of the country and you’re using kraft paper-faced insulation, the paper should face toward the floor. This may seem backward, but the paper acts as a vapor barrier (more on that later) and must face the heat, so to speak. If you live in an area where cooling (air-conditioning) is an issue for a majority of the year, staple the kraft paper to the under­side of the joists.

It can be a pain to install batts of insula­tion under a floor, because there is often not much space between the ground and the joists. It’s not a lot of fun to lie on your back and install fiberglass batts! Sometimes, especially in dry climates, it’s possible to insulate the floor before you sheathe. The drawback with this technique is that subcontractors (plumb­ing and heating, especially) may not treat your work with TLC. In rainy Oregon, we wait to insulate until after the shingles are on and the house is closed in. Either way, take your time, and make sure that underfloor insulation batts are installed properly and securely around all pipes and conduits.

There are a number of ways to hold under­floor batts in place (see the bottom left illustra­tion). In Oregon, it’s common to nail strips of lath every 12 in. to 16 in. o. c. across the bottom of the joists once the insulation is installed. It’s a lot of work, but it holds the batts securely without compressing them. Another way is to staple polypropylene (not cotton) twine or

mesh to the bottom edges of the joists. I’ve also seen people staple chicken wire or hardware cloth across the joists. Still another option is to use wire supports designed specifically for the job. These wire supports, called lightning rods or tiger teeth, clip between joists and bow up against the batts, holding them in place. Installed about every 12 in. or so, they do a good job of keeping the batts in place for years to come. Just take care not to compress the batts when installing the rods.

STEP 3 INSTALL VAPOR BARRIERS (IF NECESSARY)

Unlike housewrap, a properly installed vapor barrier is supposed to be impermeable. Vapor should not pass through it. Different materi­als are used as vapor barriers. The kraft-paper facing on fiberglass batt insulation is designed to function as a vapor barrier. To form a continu­ous barrier, the paper flanges must overlap on the stud face, where they are stapled in place.