Your going to have to install the new floor at a right angle over the existing wood floor.

No there is nothing that I'm aware of, besides Bostik's MVP, or Mapei's WFM that you could use, that has some self leveling properties, but at $40 a gallon........

Thanks Floor Guy.

I was hoping to get away with breaking the right angle rule for our foyer, which has a 1x12" diagonal subfloor covered with 1-1/2" x 1/4" oak strips. It looks pretty smooth, but I thought skimming it with some Mapei WFM (a friend gave me half of a 5 gallon can) might further reduce the chance of cupping.

I wrote to Mapei, but they responded that none of their products can be used with nail-down floors. I can see the reasoning behind not using a cement-based self-leveling product, but I don't see the harm in a thin urethane-based layer.

Feel free to sternly warn me if you think it's a bad idea! I really think our hardwood floor would look wrong if I follow the 90 degree rule (which would have the floor running along the room's short dimension) but I don't want to ruin the project over a minor detail.

I know the kind of construction you have with your existing foyer floor. Just fasten down any loose spots with a screw-type fastener or hardened spiral flooring nail prior to installing your wood and it should be fine. However, expect some seasonal movement of your maple, no matter what you do to prevent that from happening.

I've used standard cementious (portland cement based) floor fillers to float out dips and valleys before installing nail down wood and it's been just fine. However, you must wait for the filler to cure before installing any wood product over it. A good filler for such a use would be Ardex SDP (self drying patch) or a similar product. Self leveling cementious fillers may have too high a PSI rating (pounds per square inch) for you to easily nail through the cured product.

Mapei is not going to tell you to use their product for a purpose for which it was not designed and tested. You are on your own when using using a manufactured product when you are "inventing" an unapproved use for it.

Moisture in a crawlspace below a suspended floor system of wood construction can affect that floor system. Flooring contractors have devised many ways of dealing with crawlspaces and preparing wood flooring to be installed over them. There is no stock answer as each house and the soil it is built on is a unique construction. You need to make the best guess you can after researching all your options.

Search "moisture in crawlspaces" and you'll get an idea of what you need to consider. Best of luck.

Just got a response from Bruce floors. They only recommend 15 lb felt, up to six layers thick.