Good evening everyone. I have read through many helpful threads on this forum in my preparation to lay my floors, however I am hoping someone can help clarify a question I was unable to find an answer to. I have roughly 1900 sq ft of 3/4" x 3 1/4" prefinsihed hickory to lay on my first and second floors.

When is a moisture barrier needed/required? My first floor is entirely over my basement which stays bone dry. The bonus room sits directly over the garage and is much warmer then the rest of the house in the summer and much colder in the winter. I am thinking a moisture barrier would be necessary here. Am I correct in thinking this? However, I am also using an the LL dream home underlayment.


Thanks in advance for your help.

A moisture barrier (not to be confused with a moisture retarder) is typically needed when installing a floating floor over a cement or concrete slab.it's even more necessary when a floor is being layed below grade where moisture is even more of a problem. A moisture barrier might consist of a urethane or epoxy product that is applied to the slab to help slow down moisture transmission and then a layer of 6 mil construction plastic can be layed over that when it is dry to act as a 2nd line of defense. A floating floor can then be layed on that and if done right should last a long time .

For more normal floor installations, something more common like 15 lb felt paper, aquabar or sometimes rosin paper can be used on the sub floor and then hard wood can be fastened over this with nails or staples. In these cases , the underlayment is only a moisture retarder. Some moisture can move through .

Knowing what system to use in each home is tricky. You have to test RH of the rooms as well as moisture content of the slab or sub flooring. Also, most flooring experts will pay attention to the season and know what changes to expect in the home over time. In some installations, we know that the floor will expand and we plan for that and vice versa.

Thanks for the information Paul. My bonus room for example, the RH is at, or near peak currently. I would expect that after the wood is acclimated to the room it will shrink, if anything, in the colder months.

None of my flooring is going over a slab, but I am going to take extra precaution in the bonus room and lay 15 lb felt paper as well. What's a few more bucks in the grand scheme of things. I'd rather overdo then cut corners and run into problems down the line.

Almost ready to go! Sub floor has been secured using deck screws, high spots have been taken down, and I removed about 1/2 of drywall at the bottom of the walls to fore go bullnose or quarter round. Going to level off the low spots using 30lb felt paper, lay my underlayment and get going this week

I think I am really going to enjoy this. Time to order the M111!

Paul, or anyone else...

Given the circumstances for my project, laying 3/4" on wood sub floor, would you recommend using 15 lb felt paper throughout the entire house as a moisture retarder? Or is is not necessary?

thank you

15lb felt is a great under layment so feel free to use it everywhere as long as it isn't going over any radiant heat.Radiant heat will warm it up and cause it to give off a tar smell in the house. If you mean use felt instead of rosin, I would also say "yes". Rosin paper is pretty mediocre and can easily tear as you move pieces of flooring across it .if your mallet hits it , it tears immediately where Felt paper holds together much better ,especially over time.

Thanks again for the response. I apologize for not explaining this in detail in my previous post. I am using an insulayment from LL for sound absorption so is felt underneath the insulayment necessary? No radiant heat.

it isn't necessary but probably can be done if you want more of a moisture retarder/barrier.

Hi
Newbie to this forum. Good question, may I add another on top of it?

Would you consider waterproofing a subfloor (plywood on top of 1x4 decking) when there is a dirt crawl space involved?

Wondering because in spring/fall the humidity levels from the crawlspace can be quite high, even moreso if I open the foundation vents.

Thoughts?

Thanks,
Michelle
Central Ontario

i would never use red rosin paper. might as well use news paper, it has no value for retarding moisture at any level.