If you're planning to install new flooring, it's important to give some thought to the underlayment—an intermediate surface lying between the structural subfloor and the surface flooring. Virtually every type of flooring except for some types of vinyl requires an underlayment, and its purpose varies with the flooring material. For wood and laminate flooring, the underlayment may provides moisture protection; for ceramic tile flooring laid over concrete, it separates the tile from the concrete pad and prevents cracks damaging the tiles. In addition, underlayments can provide thermal and sound insulation as well as a cushion that makes walking on the floor more comfortable. Because underlayments are so important, you'll find a wide selection on the market, and the job of choosing the most suitable one can be challenging. The job becomes that much easier when you know what to look for.
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Underlayment for Wood Floors
When it comes to wood floor installation, moisture protection is paramount. The National Wood Flooring Association publishes specific guidelines for acceptable moisture levels in plywood and concrete subfloors, and failure to adhere to these can result in cupping, crowning and gapping of hardwood floorboards. In addition , the NWFA specifies a vapor barrier with a permeability between 0.7 and 10 perm. Among the materials that satisfy its criteria are asphalt laminated paper (tar paper) and asphalt-saturated kraft paper. Plastic sheeting is not a suitable material because it can trap moisture and doesn't allow the floor to "breathe."
You'll also find wood floor underlayment made from materials such as cork, rubber and felt. These are all suitable as long as they satisfy NWFA's permeability requirements, although they may be more expensive than tar paper or kraft paper. Their main advantage is that they provide a cushion, as well as a limited amount of thermal and sound insulation. These materials run anywhere from $0.50 to $1.50 per square foot, compared to about $0.25 per foot for tar paper. Cork, in particular, is becoming increasingly popular because of its sound-deadening properties.
Underlayment for Laminate Floors
The same moisture considerations apply to laminate floors as apply to wood floors, but here, cushioning and insulation are usually just as important. A foam underlayment specifically designed for laminate flooring is probably the best choice. You can buy foam underlayment with or without an incorporated vapor barrier. You probably don't need the vapor barrier when installing a floor in a second-story room, but you should always have one when installing on the ground floor or in the basement—provided the laminate is approved for basement use. Some products carry specific cautions about using them in basement locations.
If you're not sure, go with the product that incorporates a vapor barrier. Foam underlay costs, on average, about $0.30 per square foot.
Underlayment for Tile Floors
The traditional way to prepare for tile is to lay a smooth mortar bed. It's a painstaking procedure that takes skill, and many professional tilers still do it. However, it's easier—and now much more common—to install ceramic tile on a plywood or cement board underlayment instead. Like mortar, cement board is smooth and stable, and it's moisture proof. By itself, cement board is a suitable underlayment for tile.
If you're installing tile over an existing concrete pad or on a traditional mortar bed, cement board would be redundant. However, you still need an underlayment to isolate the tiles from the concrete subfloor and prevent cracks from telescoping through. The most popular underlay for this purpose is known by its brand name: Schluter Ditra. Although it's a German product, it's widely available in North America. One side is pocked with indentations that accept the tile mastic, and the other side is a fibrous material that sticks to wet mortar. It costs about $2.00 per square foot.
Underlayment for Vinyl Floors
If you're gluing vinyl sheeting to the subfloor, you should first install a layer of 1/4-inch or thicker plywood and glue the sheeting to that. Exterior-grade lauan or a similar A-grade hardwood plywood with a close grain that won't show through the flooring are the best choices. OSB and hardboard generally aren't recommended. Keep in mind that some manufacturers, such as Armstrong, do not recommend installing sheet vinyl flooring on concrete pads that are on or below grade unless the pad has been installed over a vapor barrier and the moisture content of the concrete is within acceptable limits. It's important to always check manufacturer's recommendations before installing any type of flooring.
You don't need an underlayment when installing vinyl plank flooring, since the planks are engineered with their own underlayment component. However, you should install one when using click-lock vinyl tiles or wood-plastic composite planks. Both of these products are vulnerable to moisture damage, so the underlayment is there primarily to function as a vapor barrier. You might choose an underlay that also provides some cushioning and insulation. That's up to you.
How to Install an Underlayment
Before installing an underlayment, it's essential to level the subfloor, particularly when replacing carpeting with wood, laminate, tile or a similarly inflexible material. Seal cracks between plywood sheets with patching compound or self-leveling floor compound. Use a cementitious floor leveler to fill cracks in a concrete subfloor prior to installing the underlayment.
The installation method depends on the underlayment material, but when installing paper or foam sheeting on plywood or OSB, it's common to use staples. The edges of the underlayment sheets should overlap by 1 or 2 inches, and you should secure them with cellophane or plastic tape. Extend the underlayment all the way to the walls. If you are using a paper or fiber sheet underlayment, leave an inch or two extra that you can trim off after you've installed the flooring. When installing this kind of underlayment over mortar or concrete, it can float, but the edges still need to be taped.
Installing Plywood Underlayment
When installing a plywood underlayment, you can choose to glue or nail it to the subfloor. Arrange the sheets of plywood so the seams are staggered, and leave 1/4-inch expansion gaps along the walls and between sheets.
If you glue down the plywood underlayment, choose a construction adhesive that won't stain the flooring, and make sure to clean away any adhesive that oozes up between seams. When nailing, avoid coated fasteners, because the coating can wear through the flooring and discolor it. If using screws, attach the sheets with 1-inch deck screws driven every 6 inches along the seams and edges, and at 8-inch intervals through the field. Make sure the screw heads are slightly recessed below the surface.
Once the sheets are secured to the subfloor, mix floor-patching compound and fill the seams between sheets, as well as the recessed screw heads, using a drywall knife.
Chris Deziel is a contractor, builder and general fix-it pro who has been active in the construction trades for 40 years. He has degrees in science and humanities and years of teaching experience. An avid craftsman and musician, Deziel began writing on home improvement topics in 2010. He worked as an expert consultant with eHow Now and Pro Referral -- a Home Depot site. A DIYer by nature, Deziel regularly shares tips and tricks for a better home and garden at Hunker and Family Handyman.