Hygrometers - Test Relative Humidity
Hygrometers serve to check the relative humidity on jobsites. Maintaining a stable reading throughout the period in which flooring is delivered, acclimated and installed will provide some peace of mind. You can get by without the use of one of these tools should the residence have an HVAC system that monitors all these elements, providing it's up and running properly.
Why Use A Hygrometer?
Solid flooring will gain and lose moisture content throughout its life cycle even though the material itself is no longer living. During periods of high humidity, the cells in the hardwood collect or gain moisture. Conversely during dry periods it will be lost.
For a successful installation, monitoring the jobsite before the delivery of material should be considered. Let's say two weeks prior, an unusually wet period is encountered and it has rained for days on end. A few windows were on back order along with the front door for your new house. You don't think twice about the effect of all that moisture is having on the subfloor, after all it looks dry so what's the big deal?
Trust me, it has gained some moisture. Now the door and windows arrive and get installed. They were the only things holding up the floor guys, so the contractor calls them to bring in the flooring to acclimate for a week. Wait! Acclimate? Acclimation is a good thing, but the hardwood is now being acclimated to high moisture conditions. It's not going to help if the relative humidity after the installation is significantly lower. In fact you're likely to get a gapped floor eventually. This is a very common occurrence in areas that use dry heat during colder winter months.
Builders - Good & Bad
Let's look at a similar project. Another builder down the street has a similar situation. Relative humidity was monitored two months before the flooring was considered. The builder waited for all windows and doors, and finished most of the other work that can affect wood floors such as painting, concrete work and the like. The Rh reading remained in a safe range or under 60% (most manufacturers suggest 40-60% depending on climate norms). The flooring is brought in, acclimated to proper conditions, and finally installed.
Without monitoring, your builder has made two poor decisions. Aside from a gapped floor in the winter months, chances are also good you could get some cupping of the new floor. Reasons being the subfloor moisture content could have been too high to safely install the floor.