Nailing and gluing would be done in addition to controlling your indoor temperature and humidity, and ensuring that your subfloor is in the same moisture range as your wood floor, now and in the future. Solid hardwood is generally nailed. The glue is added insurance but might not be a cure all if you have something else that would cause cupping, like moisture migrating through the subfloor.

Johnson Flooring Co- would sealing the crawlspace and adding a dehumidifier help with the moisture problem of the subfloor? What width would you recommend in the 5-7” range we are considering? Thanks

Yes it would, though testing would be in order first. An inexpensive thermo-hygrometer will tell you wnat your levels are and many of them have a memory feature so you can check it periodically and see what your extremes are. Even cooler is one with remote sensors that you can put in various areas and observe. My mindset is based on south Florida conditions so yours will be a bit different.

We glue and nail anything 5" and over. The wider you go the more likely you are to get and see cupping.Some people out beads or stripes of adhesive on the backs of boards but we generally do a full spread with a urethane adhesive, then nail or staple.

You will need a whole home dehumidifier to get this to work with wide plank. Even with a narrow plank (like 2.25" wide) you will need humidity control.

Engineered hardwood will 'help' but it won't fix the household issue of humidity (too high is just as bad as too low). You will need to seal the crawl space (properly) and add in the whole home dehumidifier. The HVAC system *might be able to handle an 'add on' or you might need a full upgrade (how old is the home?).

Once you have the house all happy and healthy and the dehumidifier working/drying out the house for a few weeks, you can then find out where the house likes to 'sit' re: humidity levels. You will need to find the homes equilibrium. Once you have that figure out (could take a full summer/winter/spring cycle) you can then plan the best way forward.

Lets pretend you take the time and the effort to seal the crawl space AND add on the whole home dehumidifier (this can be several thousand all by itself...little FYI). Let's pretend you get everything sitting around 45% humidity (swings between 40%-50% would be the perfect range....nothing bigger than a 10% swing in humidity please). Now you start testing the subfloor moisture level. It must drop into the correct range. Once it is dry enough, you can bring in wood to acclimate in the house for a few weeks.

As you can see this is going to take some time. And it is HIGHLY dependent on what your home is capable of. My suggestion is to work with the extra expensive rift and quarter sawn product white oak. This can be 30%-50% more expensive but it is worth it because of it's stability.

If a 5" wide (which is considered 'wide' = glue assist) you will be looking at a $8/sf product to begin with. Now add in the rift/quarter sawn price and we can sit at $12/sf. Labour is going to be $3/sf (at least). Subfloor preparation is extra (I always tell people to plan $2-$4/sf for subfloor prep....especially if you are removing carpet).

If you move into the 7" rift/quarter sawn you could be looking at $15-$20/sf. The 7" DEFINITELY requires the 'full glue spread'. You can nail it too if you like...but the full spread is DEFINITELY needed. That adds another $1/sf for glue and another $1/sf for labour. Are you ready for a $25/sf total cost?

Oh...and you are not guaranteed 'no cupping'. It is greatly reduced...but not guaranteed. 'Cause nothing says trouble like a 3 day electrical black-out due to hurricanes or tornados.

Remember: pay the extra price for rift and quarter sawn. You will be thankful that you did. The width is going to be up to your home's ability to offer 45% humidity AND your ability to finance the whole deal.

Deumidify, roll on moisture barrier, full spread glue (no beads), nails and keep at the smallest width possible.