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Use Tyvek to cover attic insulation

Question - Blown-in cellulose insulation in my attic floor is dusty, and I'd like to use part of the attic for storage, putting plywood on the floor where needed. What can I put over the cellulose to keep the dust down - poly or Tyvek? Eric Young, Concord, Mass.

Answer - Use Tyvek, or Typar, similar materials that allow water vapor to go through it but not air, making it ideal as a dust cover. Staple it loosely on the joists to keep it from billowing. The plywood can go on top of the Tyvek. Polyethylene is a vapor barrier, and in that position, could trap moisture under it. Nail the plywood lightly, to keep it from moving as you walk on it.

Question - My brick fireplace has white, chalk-looking stains on one side, mostly where the grout (or whatever you call the stuff between the bricks) is; not so much on the bricks. I sort of painted over it with black fireplace paint, but now the white is back. What would cause this and how to I get rid of it permanently? Julie Orofino (email), Wilmington, Mass.

Answer - The white stuff is ef-fluor-escence, the leaching out of lime in the grout - we call it mortar - by water. It is unlikely to occur on bricks because there is no lime in bricks. It is caused by water, and it's virtually impossible to avoid or get rid of permanently, unless you eliminate the water, which is also difficult. Painting over it, as you discovered, does no good. It is harmless; you can sweep it up and throw it way, but it will come back. Live with it.

Question - A snowplow got rambunctious and knocked two pieces of concrete off the top of my retaining wall. They are about 7 1/2 inches wide, 2 inches deep and 46 inches long, and look as if they were a part of a cap poured on top of the wall. How can I restore the wall? Can I pour new concrete? Joe Baran, Arlington, Mass.

Answer - You sure can, and pouring concrete will give the best look. Set up plywood forms against each side of the wall; use 5/8- or 3/4-inch plywood, and secure them by wedging wood posts from the ground against the plywood. Concrete is heavy, so the forms should be secure.

Make the top of the forms even with the top of the original wall, so that when you pour concrete to the top, you can strike it off, or screed it smooth, by drawing a board across the top, using the forms as a guide. After 15 minutes or so, can float the concrete; that is, rub it with a wood float (a wood trowel, really) to make a rough but even surface. You're not done yet, either. Now, run an edging tool (inexpensive in hardware stores) between concrete and forms, to round off the edge. If you don't round off the edge, it will be very sharp and will tend to chip.

For this small job, use Sakrete concrete mix, which you simply mix with water and pour in place after dampening the top of the wall. Don't make the mix too soupy; just a little crumbly, and it will work nicely.

Or, you can mortar in those chips. Buy mortar mix at the same store you bought the concrete mix, mix it with water to a wet, crumbly state, dampen the wall, place the mortar about 1/2 inch thick and set the chip in place. The chip may be a little higher than the top of the wall, but that should be OK.

Still another way: Instead of mortar, use thin-set mortar, also sold in hardware and building supply stores, and apply it thinly. It's called thin-set because it can be applied thinly and will hold better than standard mortar. Thin-set and standard mor-tar are waterproof.

Question - About half of the ceramic tiles on my tub surround fell off because the plywood wall had warped. I fixed the warp and scraped the old glue off the tiles. How can I get the tiles to stay? The plywood is over plasterboard. Barry Stearns, Waltham, Mass.

Answer - The tiles failed because of the warp, and also because the surface they were applied to is wood. The plywood is not stable; it will expand and contract with intake and loss of moisture. This movement, in addition to the warp, worked off the tiles. Use a little more adhesive when putting the tiles back.

This happened to the handyman years ago; he simply used more adhesive in resetting the tiles, and they've held for years. Ideally, it is better to put ceramic tiles on cement board (WonderBoard is one brand), which is really concrete reinforced with fiberglass. But you don't have to do this.

Question - The sills of my basement windows are touching the ground and have rotted out. I am replacing the sills and will put in window wells to keep the sills away from the ground. How deep should the window wells be? Earl Doggett, Watertown

Answer - Make the wells 12 inches deep and put in 6 inches of crushed stone for a neat look. You could add a plastic cover to keep the wells from filling up with water. Make the sills of pressure-treated wood, which you can paint or stain or leave bare.