Preventing warping from indoor relative humidity on hardwood plank floors

First off, I would have preferred if the customer had been sold a laminated (engineered) plank floor, which would have looked and sounded the same, only it would have minimal or no warping during wide indoor relative humidity (RH) swings. Proper indoor humidity level in winter The salesman’s job is to find out what “look” the customer wants and then lead them to the best suited product. Recommended humidity level for home in winter Anyway, what’s done is done.


It’s going to be ABSOLUTELY CRITICAL that you measure the EMC (equilibrium moisture content) of the hardwood plank, any underlay and the subfloor. Recommended humidity level for home uk Let this floor acclimatize (for weeks if needed) until all these components come to the same EMC. Recommended humidity level in a house Keep photographic and written records of this, or everybody will blame YOU when this floor fails. Recommended humidity level in home Find out what the EMC for indoor wood is in your area, any wood floor installer or cabinet shop will know.

So the first consideration is too beef up the subfloor. Recommended humidity level in home canada In your case the 3/4″ plywood is not quite thick enough for a plank floor. Recommended humidity level in home during summer I would like to see you staple 1/4″ waterproof underlay plywood with 7/8″ narrow crown staples (every 6″ square). Recommended humidity level in home during winter What you want to accomplish is a vapor barrier under the floor. Recommended humidity level in home during winter canada You understand that the 2″ flooring cleats you will be using will penetrate 7/8″ into this subfloor. Recommended humidity level in home in winter Without this extra thickness they will go right through the subfloor and expose the underside of the hardwood to possible dampness or at least a different humidity level from below.

What causes cupping and crowning in wide hardwood floors is the different EMC levels between the top face of the plank and the bottom face. Recommended humidity level in home summer And the waterproof plywood does a wonderful job of providing an unbroken vapor barrier (stagger those seams), to keep all the wood in the one RH level of the room you are laying the floor in. Recommended humidity level in home winter It’s also a good idea to put down 15 pound roofing felt, or rosin paper. Recommended humidity level in house But as this will get punched with a thousand holes, it’s not an effective moisture barrier, but gives the customer the feeling like you are doing the job according to NOFMA standards.

If you just cannot add even this much underlay you will need to apply at least one coat of polyurethane floor finish on the underside of all the boards. Recommended humidity level in house during winter This alone is a time consuming and messy project, and you really ought to let this finish cure for a week, so it doesn’t add an odor to the floor. Recommended humidity levels for home summer Shellac or lacquer would be a quicker choice for this undercoating, but these finishes are a bit less of a barrier. Relative humidity in a home Don’t use a water based finish for this.

Rack out about 6-8 rows, and really try to keep the joints staggered well, with no H’s. Relative humidity in the home uk Squiggle a bit of urethane adhesive on the ends, and on the middle of the long boards (4 feet and up). Relative humidity levels in a home Because these boards have stress relief grooves in the bottom, (it would have been better to have flat milled bottoms) you will be able to only squiggle several 4″ by 1″ spot of glue on the ends. Suggested humidity level in house Don’t apply too much glue, or the planks themselves may get glued together. What are normal humidity levels in a home This could tear the boards in half when they get stressed by excessive shrinking. What can cause high humidity in a house All the sides and ends of the planks need to be independent of each other. What causes high humidity levels in a house I hope this is clear. What humidity level should my house be in summer Stay with me Todd.

The glue is only meant to prevent cupping and crowning, and most urethane adhesives have some degree of flex to them. What is a comfortable humidity level in a home Yellow carpenter’s glue does not, and PVA glues in general are not used in the wood floor business anymore because of this. What is a comfortable humidity level in a house I would prefer you to use DriTac 7500 urethane adhesive, but I doubt you could find some before Monday. What is a comfortable humidity level in your home Lepage’s makes calking tubes of some pretty fair urethane adhesive, and I had good luck with it so far. What is a comfortable humidity level inside a house Also Bostik’s Best urethane adhesive is quite well known in the trade, and Bostik has urethane adhesives in calking tubes for your convenience. What is a comfortable indoor humidity level Be sure to pick up some urethane adhesive remover and some latex gloves, or you’ll be going home with black hands every day. What is a good home humidity level You’ll soon see what I mean.

We use urethane adhesives for flooring these days, because this glue contains no water or solvents. What is a good house humidity level It uses the moisture in the air to cure itself, and has a bond much stronger than maple wood. What is a good humidity level for a home If the EMC of the subfloor and the hardwood is below 8% this winter you should mist a little water on the subfloor area and this will speed the curing. What is a good humidity level for my house Especially since these are prefinished boards, the trapped glue may never fully cure otherwise. What is a good humidity level in a home It always says this on the tube label, but I thought I would mention it here. What is a good humidity level in a house Oh, and don’t get any of this glue on the finished face of the hardwood. What is a good humidity level in your home Keep the urethane cleaner handy, and get it right off, or you will be replacing that board when the glue dries permanently on the surface, and the customer complains.

Next, once this is done use a regular nailing schedule (I’m so glad you are not using staples) but 6″ would be better and make sure that each board has at least one nail about 4″ from each end, but no closer. What is a good humidity level in your house Keep the pressure on your nailer so that the nails just nicely tuck into the little groove on top of the tongue, check often. What is a good humidity level indoors Take your time so that you don’t get glue everywhere. What is a good humidity level inside a house The glue sets up in about 30 minutes, so if you have some stubborn boards, get them in fast, because once the adhesive has set, it’s almost impossible to remove.

Oh, and that’s another reason for the 1/4″ underlay. What is a good humidity level inside during summer If this floor fails you will be able to remove the plank from the underlay (destroying the underlay, not the subfloor).

For the first few and last three rows that your nailer cannot reach, use a nail spinner available at: http://www.leevalley.com/wood/page.asp?SID=&ccurrency=1&page=32273&category=

Use the nail spinner with spiral (ardox) finishing nails instead of the lousy thin pneumatic finishing nails, that I’ll bet you use. What is a good humidity level inside during winter Don’t try shortcuts here.

After the job is done tell the customer that this type of installation, only works when they keep their indoor humidity levels, within a narrow 20% range. What is a good humidity level outside Try to find the MEAN indoor RH that this house experiences, and have them keep the house + and – 10% of this figure. What is a good indoor humidity level in summer And strongly suggest that they buy a wet/dry bulb hygrometer to accurately measure this. What is a good indoor humidity level in the winter The cheap metal ones at the hardware stores are not accurate. What is a good inside humidity level A cheap but good one is available though:

This assures only minimal seasonal movement of the planks. What is a good level of humidity in a house Less gaping, crowning and cupping. What is a good level of humidity indoors I hope this wood has a satin finish on it. What is a healthy humidity level for a home With a high gloss or semi gloss finish, the home owners will see the crowning effects right away once the spring hits, as the wood gets slightly damp from below. What is a healthy humidity level in a home Especially if there is only a crawl space under the floor. What is a healthy humidity level in a house I would prefer not to install such floor over an unheated crawl space, this is just asking for trouble.

Again Todd, I do not advocate this type of installation. What is a healthy humidity level in your home Basically the wood becomes trapped in it’s own glue. What is a healthy humidity level indoors And when the home owners subject the wood to extremes in RH changes (believe me they will) they will blame the store, or you, or the manufacture of the wood when this floor starts bucking in humid weather or gaping in the winter.

And then I will finally hear about it when they (or someone like them) write to me complaining of shoddy workmanship and material. What is a healthy indoor humidity level What they need to understand is the hygroscopic nature of wood, but I doubt they will. What is a healthy level of humidity in a house I never tackle jobs like this.

Any more questions you may have on this subject or clarifications of your original question feel free to write again at no cost. What is a high humidity level in a house I hope you have enjoyed this personal service, real human responses are the best.

P.S. What is a high humidity level outside If you are interested, I have a long (12 screen page) article on how to get your own better paying customers. What is a normal humidity level for inside a house It’s at the bottom of the Quick Links.