I plan on having 3 1/2” x 3/4” ipe installed. I'm not sure if they are using cleats or staples. My house is 30 years old and I live in southern Ontario where we can have large fluctuations in temperature and humidity. My house is air conditioned and I have a humidifier on my furnace. The sub-floor is 3/4” x 4'x8' OSB. The long side runs perpendicular to the joists. Joists are 2x8's 16” oc. The sub-floor appears reasonably flush however there are a few high and low spots where the sheets meet or piecing was done. There is one 3”x3” soft spot unfortunately in a doorway and in another doorway the sheets don't intersect over the joist. I'm about half done screwing the sub-floor to the joists with 8 x1 3/4” floor screws. It feels solid but there are still some squeaks. My basement is unfinished. The installer says that the sub-floor is acceptable but I think that 1/4” plywood underlayment would give a better nailing surface. I would prefer to minimize costs and increase in height but I don't want to take the chance that I'll run into problems later. Another installer thought 3/8” ply underlay would be better for Ipe. I am a totally inexperienced DIY'er so I'm trying to learn how to do this properly.

Should I replace the sub-floor with 3/4” ply where it is a littly punky, ignore it or attach a patch on the basement side?

Should I put some adhesive between the joists and sub-floor before I screw it down?

Should I buy the cheapest 1/4” ply underlayment (it's called Miraply and looks like mahogany) or get poplar or fir?

Should the long side run parallel to the joists or to the OSB?

How should I attach the underlay to the subfloor? Glue, staple and / or screw?
We have had a warm, humid summer. How long should I acclimatise the hardwood in my house before I have it installed?

Any hints on how to insure that the installers are doing a good job?

Well I guess I'll throw my 2 cents in....

1) Some installers swear by cleats, some by staples. That's a debate that will go on after we're both gone. Bottom line , It shouldn't make a difference as long as the hardwood is fastened correctly over a solid approved substrate.
2) 4x8 x 3/4 osb over 2x8's, 16" oc ,is an approved substrate.
3) Re: High spots in subfloor where pieced & high spots at seams. The installers need to flatten these areas. I use an edger ( sander). I've heard of installers using belt sanders or similiar disc sanders, whatever approach they feel comfortable with.
4) Re: 3x3 soft spot, & where "sheets don't intersect over the joist". Both areas should be addressed . Depending on the situation and access, either by meens of cutting out, bracing, patching & screwing , or if possible, simply bracing and screwing.
5) Re: Squeeks. Their not going away on their own. Bottom line with a squeek is somethings loose enough to cause the friction of a squeek. Remove loose fasteners, brace, toenail, glue, screw, whatever it takes. Sheeting out with another layer MAY hide some, but they can re-appear.
6) I'm not familiar with Ipe, but I cant think of a reason why the installer feels more comfortable sheeting out with 3/8 ply, under that particular specie
7) Run the Hardwood perpendicular to the joists.
8) Acclimation- I go with a couple weeks minimum on 3/4,under normal living conditions, but nothing's written in stone. Moisture test the subfloor and hardwood,( with a pin type moisture meter), they should be within 2% before installation. Always keep your RH within range.

Sounds to me like you'll know if the installers are doing a good job.

We have installed some ipe prefinished floors in a couple new homes. Both homes had humidifiers and the wood was acclimated for several weeks. Both floors shrinkage problems, and on one prefinished floor the finish peeled of, luckily we found this out before installation. I would be very concerned about acclimation, especially with big swings in temp and humidity.

Thank you for your help Floorologist.

Now you have me worried, Mickey. I haven't paid in full but I left a sizable down payment. I really don't want to back out now. The Ipe is very beautiful. Oak (ebony stain) is probably the most popular hardwood in this area with Jatoba the most popular exotic.

Ipe is extremely hard. Would the 1/4" plywood underlayment do a better job holding the cleats or staples or should I have the Ipe installed directly on the OSB? I don't want to be penny wise but pound foolish if the extra cost and work is really necessary.

I will be installing a porcelain tile in the kitchen and would prefer no transition strip. What substrate would most people use under the tile so the heights were the same?

By all means, add the additional plywood. 3/8th inch spruce will have about the same holding power as 1/4 inch fir, but will be cheaper. And neither will cause significant height problems in most case.

I dont see the reasoning for the additional underlayment. the subfloor is 3/4 OSB. There is a 3 x3 soft spot, a few high seams, a doorway where the seam isn't over the joist, & some squeeks. All of these issues are in need of repair before installation,( underlayment or not).The Po states " The floor feels solid". If the OSB or joists have any one of a number of unmentioned issues, then sheeting out is a possible solution. Am I missing another problem?

Given that the house is 30 years old, and located in Ontario, I am willing to bet that the sub-floor is most likely a cheap grade of waferboard. You wouldn't believe what builders up here got away with during the last big building boom in the 70's - early 80's. At one time, 1/2 inch plywood was acceptable on 2 by 8 joists on 16 inch centres. We installed a ton of 3/8th inch strip on top of that trampoline, including in government housing complexes. Try sanding that without ripple marks from the bouncing.