Q: I am interested in your recommendations on using lightweight concrete for a super-insulated house in a wet, cold Pacific Northwest climate (76 inches of precipitation; 30 days of snowfall per year; lots of freeze-thaw cycles; nice warm summers (75-85). What is your general advice. I am particularly undecided about the use of a vapor barrier. I anticipate using additional insulation on the outside of the exterior walls (hard rock wool?) and then another thin wall with a stucco finish. Should I be concerned about condensation in the rock wool? How would that affect the rock wool? Should I put a vapor barrier between the wall and the insulation, or between the inside finish and the wall?

A: (Bruce Schundler)Water can lower the thermal insulation values of any kind of product. As a result, you should be concerned about water penetration, vapor barriers, etc. Most lightweight concretes are permeable (e.g. water can pass through them) and they have to be treated with water proofing sprays (silicone for instance) when used in exterior applications. There are a number of exterior wrapping materials that would be good, and polystyrene board would probably be better than hard rock wool (rock wool can hold the moisture and eventually develop problems like mold if it is never allowed to dry.) These house wrapping materials allow moisture to leave the house while preventing water from getting in. The problem with absolute vapor barriers like plastic sheeting, for instance, is that they will not let any water in or out, and this wouldn't be good.

Q: I have completed the the frame of my house(columns & beams) with steel I- section...I,m now trying to make a big panel from lightweight concrete using perlite, lime & cement for the walls (which have no loads on them) what is your advice? Can I use the panels (with reinforcement) in the roof? Do I need to plaster the walls and paint them to keep rain from them?

A: (Kelly) Perlite can be used as a lightweight concrete aggregate but there are many variables involved. Test mixes should be done and the thickness of the wall is important.

A: (Bruce Schundler) Perlite concrete could be used in tilt up construction---but it should have some reinforcement (both fibers and metal rods--depending on the spans). The roof panels could also be manufactured from perlite concrete, but these would need to have reinforcement too. Years ago several companies made "pre-stressed" roofing panels, and others have made less engineered panels. BUT, these do have to be supported or engineered so as to minimize the spans between beams. Perlite concrete does have to be waterproofed (as does all concrete.)

Q: I am designing a custom design build house in Los angeles west side area and the main structure of the house is an 2 stores high steel frame and wood stud in between. My questions are: 1) Is light weight concrete appropriate for this type of structure? 2) Can I just design the light weight concrete as a pre-cast concrete panel, or I can apply the concrete on the surface of a plywood on the exterior wall and make it look like an concrete finish building? Which way is better and cheaper, if I just want the finish to look like concrete finish?

A: (Bruce Schundler) Essentially you are asking both structural questions and some exterior design questions. On the structural side, you really need to check with an engineer or builder. To be sure, how much a building can or cannot hold is based on the kind of steel used, the thickness and style, etc. Stucco and stucco cements have been applied on the surface of buildings for years. It can be applied over masonry units or over wood. Usually the stucco cement would be applied over expanded metal nailed the the exterior surface. There also are "systems" like the Dryvit system which uses a stucco veneer. To be sure, there are many options. For more definitive answers four your area, however, you should contact both stucco plasterers, local masons or contractors, and perlite suppliers like the Redco company in N. Hollywood (www.perlite.net.)

Q: I am building a enclosed room with a flat roof patio/deck. Proposed construction of the roof is 3/4" ply with EPDM as water barrier and tiled top layer. I have been advised that I should have approx 2" of concrete between the EPDM and the tile layer, is lightweight concrete an option?.. does it have sufficient strength and how much weight (%) would I save?

A: (Bruce Schundler) We think perlite concrete could be used, but we aren't sure if it has to be used. The main thing would be for the floor (the 3/4 plywood) to be supported very well, and maybe two layers would be helpful. The problem with tile is that if there is any flexing or "give" in the floor, the tiles and the supporting concrete could be damaged. But no matter what, this is a question more for tile people than us. If perlite or vermiculite lightweight concrete is used, we would recommend a 1 to four mix with fibers or wire (for reinforcement.)

Q: About forty years ago my father built some apartments. For the second floor he used a lightweight concrete (type unknown). The floor is starting to crumble under the tile in the kitchen. Can you suggest a repair material? Can I use a standard floor repair such as Floorfix or similar product? Also, are there any special steps that I would need to follow to assure a proper repair procedure.

A: (Bruce) I think the answer is yes. As usual, the older material should be cleaned out carefully and completely.

Q: I have a concrete subfloor and bituminous/orangeburg piping for my plumbing. The Orangeburg pipes have deteriorated and I need to bust up the existing concrete slab, re-do the plumbing and then replace the concrete. My question is, could I use the light-weight concrete to replace the slab and if so, would this be cheaper and would it make the plumbing more accessible in the future if repairs were needed? I do not believe any load-bearing walls are involved, only a room-dividing wall (between kitchen and bathroom); the subfloor will have to support a bathtub.

A: (Kelly) I doubt that lightweight concrete would be any cheaper...most likely it would be more expensive, depending on what aggregate is used. Lightweight concrete would likely be easier to bust up in the future if this were necessary. If the concrete is a subfloor then, lightweight concrete might be a good option. As a final flooring material, it is not as hard and durable as ordinary concrete.