Glued down floor is popping and creaking

Q: Doing a remodel about 18 months ago, we installed bamboo flooring over concrete. Going with the advice of our contractor, we painted the floor with Kilz Primer then glued the bamboo directly to the concrete. We placed weight on the floor and let it dry overnight. Now it is popping and creaking in the areas that are most often walked on. Is there any way we can fix this problem short of pulling up the floor and replacing it? Thanks for any suggestions.

A: I’m wondering if the adhesive has not bonded to the primer. What does the manufacturer say about using their adhesive over paints and primers?

Related Q: The guy doing my home improvement installed Schon 3/8″ x 3.5″ Brazilian Cherry engineered wood on my bathroom vanity floor. He installed it directly to the concrete with some type of silicone glue gun (the style for caulking). Is this a good or recommended method? The bathroom sounds a bit clicky and spongy in places. Is silicon any good for this? I live in So California.

A: I’ve never heard of anyone taking this approach. I would have suggested Bostik’s Best or some other polyurethane adhesive. You might want to look up the product online and see what their recommendations for adhesive would be.

Popping and footstep noises

Q: My house makes noises. It’s coming from the upstairs area between the kitchen and the first room. The entire upstairs is wood floors. It makes 2 distinct noises, a stomp and a pop. Is it possible the noise is coming from the wooden floor? Because sometimes after the stomp a sound like footsteps occurs. I’m an Islander and live in Seattle,WA and my family believes that the dead are trying to communicate. Please help. It’s driving me nuts. Thanks.

A: I don’t know how old the house is or what the dimensions of the floor are nor what sub floor it is sitting on. It could be possible part of the sub floor is loose and that spot will make a noise. It might be good to check the relative humidity in the home and also the floor itself with a moisture meter to determine if there is excessive expansion in your floor.

Related Q: Twice there has been a loud pop noise in my house, any idea what it is?

A: Possibly expanding hardwood floors. Any moisture or humidity issues? Everything expands and contracts. Even steel roofing. It can make quite a bang.

Floors popping after nearly 5 years

Q: My Shaw engineered floors have been down 4.5 years, and one area recently started to pop when walked on. It spread to 4 boards. Then there are two pops under my oriental rug. Then one board in the bathroom started, and finally an area of 3 or 4 boards in a hall. The floor guy is mad at me because this happened! Why, you might ask, as I ask? Apparently he doesn’t know what to do. The floor under the pops feels spongy when walked on. The floor is on concrete, glued down. Having not been educated on laying floors of any kind, I’m at a loss also.

I read a lot on your website, will that injectable glue work on these floors? An independant inspector came out and just said it was not the manufacturer’s fault. His report was so vanilla, that one could make it mean anything or nothing. The installer paid the 150, and he thinks it means that he is in the clear. Can I expect this to continue to happen in other spots? This occurred about 3 months ago, and I’m wanting to put my house up for sale, but wondering if no one will want it due to these 4 areas of popping wood. Can you help me? I live in upstate South Carolina. We did have heavy rains this past winter,but there is no smell of moisture in my house and I use no humidifier. The report said that the inside conditions were 55% RH and 70 degrees F. Moisture presence in the flooring measured with a Tramex moisture meter averaged 15.6% with 24 total checks made.

A: Clearly something has changed in the environment. It sounds like your floors have swelled and this side pressure is causing the floor to raise in these areas. 15.6% moisture content in the wood I consider high and not normal. Is it possible you have a slow, hidden leak someplace and moisture is creeping under the floor? The moisture level in your floors will need to be corrected before you can think of going to other measures such as injectable adhesive. Did the inspector think this reading was fine?

Follow-up Q: Thank you so much for answering. Do you mean a leak in a water pipe under the floor? What do I do to find out? The areas where the popping occurs are far apart. Each place involves 1-3 planks. There are 4 places, rooms apart. The inspector was not useful in my opinion. He was working for the guy who laid the floor, so it seemed to me he thought it was the installer’s lack of preparing the floor, but didn’t have the courage to say so. He kept speaking about how an installer had to prepare cement because it was never even, but put nothing of the sort in his report. What do you think should be my next move? I want to sell in a few months, and need to have everything in good condition. Thank you again. There is no change in the appearance.

A: I would contact The National Wood Flooring Association and see if they have an inspector near enough that he would come out for an inspection. They will give you an impartial, honest appraisal. It could be that for some reason he didn’t get real good grab between the adhesive and floor. But why is this all happening now after 4 years? The spongy feel in those areas and the what I consider a high moisture level reading tells me to suspect something else.

I’d be interested to know the results if you are able to get one of the NWFA inspectors out to look.

Second follow-up: Good morning, I have consulted with my water provider, and there has been no change in the amount of my water usage, thus no indication of a water leak under my concrete. That’s a relief. Still eager to hear where you think I should start.

A: I don’t know what you should do at this point. As I mentioned in my last email, if you received it, the inspector who was there is NWFA certified. His report seems to indicate concern with the high moisture readings in some areas, 15.6%. He mentions in his report that some boards should be removed to check the concrete. I think you should phone him and discuss his report. He is certified. He has inspected the job site.

Third follow-up: Thank you, so much. I’ll let you know how things turn out, as you requested.

A: How old is this house?

Fourth follow-up: It is a 20 year old, middle unit of 3, one floor, brick condo. I have lived here 12 years. Previously it had carpet. The other owner spent more time at the beach than in the home, so it was not used much the first 8 years. No one in the units next to me have had problems with moisture. One still has carpet, and the other one has some type of laminate (Pergo?) that has been there the whole 20 years. The floor is still in excellent condition.

Only person in our neighborhood who had problems is one lady who had her Mohawk floor, mine is Shaw, installed just before I did, by the same company who did my floor. She has had the same problem. They cut into her floor and replaced the boards. I haven’t seen it, but she says it is bad looking. I will try to go by to see it soon. No one else in our neighborhood used the same company that we did. Both of us are widows. The other lady is older than I, and not very well. I think she just gave up. I haven’t asked her what they told her was the reason her boards were popping.

A: Interesting. I can only do so much. I’m far away. It seems odd your floor was down without any issues for several years and now you have these areas with high moisture content readings. I don’t see the point of changing the boards unless the cause is identified. How is the floor being cleaned? Is a lot of water being applied? Do you have a cleaning person helping you?

Last follow-ups: I use only Shaw hard surface cleaner which is what Shaw company told me to use when I contacted them soon after my floors were installed. Thank you again. Since I don’t really have any faith in my installer and it appears not to be his error? Where do I start to get to the bottom of the issue. Plumber, builder or who?

Thank you so much for your time. This has been very upsetting to me, and the installer has been so rude. He is so scared that it is his fault, that he would not help me get to the bottom of this. I definitely will do as you have suggested. In case I misunderstood the anything in the inspector’s report, I am including it in its entirety. After your response, I will begin my quest, and will definitely let you know what the final verdict is.

I appreciate your help very much.

A: Okay, this inspector is NWFA certified. He basically is concerned over the same issue I mentioned. High readings for moisture in certain areas which would require removal of some of the boards to expose the concrete and then test the concrete. My question is why it took this long for this issue to appear. Perhaps something has changed around the building? A drainage issue? I don’t know. The excessive moisture appears to be the immediate problem.

Old floor popping sounds during the night

Q: I know that the question of wood floors making popping sounds during the night has been asked; however my white oak floors are original to the house built in 1948. I did have the floors refinished about 6-7 months ago with water-based finish and like the others, the popping only happens at night without anyone walking on them. I’ve had this house for 17 years and it’s never done this before. Do you think it’s still humidity related? I live in Southern CA so it’s pretty dry here. Thanks!

A: As companies have been forced over the years to develop water borne coatings they soon discovered they also needed to develop special sealers to prevent an issue called side bonding. These coatings are more brittle and rigid than oil based finishes and thus don’t stretch as much with movement in the floor. I suspect the noises you hear are the finish stretching and cracking along the board edges or between boards where it slipped into tiny gaps between boards. Once that has completed the noises should stop.

Hollow spot in floating floor

Q: I wondered if you could suggest a fix please, for our new kitchen. The original floor was concrete, and this had levelling compound put on top, a floor fitters request. All looked okay, but there is a hollow spot in floating floor (now it’s all fitted where it creaks when you step on it, you can see the wood dip when you apply weight). Obviously it’s a little hollow in that area. The floor is floating engineered wood, sitting on a moisture barrier/underlay.

As the floor is not glued down, can I still inject adhesive though the board and wait for it to cure effectively “filling” the hollow rather than gluing it down? The hollow spot is only a few boards in, so I may do it from side rather than drill down. Or is there a better way? (The fitter is no longer with us, floor is a year old now..)

A: I think you’ve got the right idea for an easy fix. I believe Bostich has such an ‘injectable’ adhesive. I’ve never used it but I would think it expands as it cures, thus filling the void. So, if you can do this without drilling a hole, all the better.

Similar Q: Why are there air bubbles in the glue that I’m using for our bamboo floor? I recently installed my living room and now when you walk on it, you hear popping.

A: It’s possible that you didn’t get complete grab of the adhesive between the sub floor and the bamboo. This could be caused by something as simple as a slight depression in the sub floor. It’s probably best with a glue down floor to go over the floor with a weighted roller as you work. If there are hollow spots here and there you can use an adhesive syringe after drilling a tiny hole. This will expand in the cavity under the floor and stop the movement. I believe companies such as Bostik Findley make products like this.

Related Q: We just installed engineered maple over concrete slab. There are some areas of “hollow” sounds, when you knock on boards, but no sounds when walking on floor. What do you think causes the uneven “hollow” sounds?

A: A possible small dip in the concrete and perhaps poor grab of the adhesive in those areas. Was the floor rolled with a weighted roller when installed?

Floor popping after refinishing

Q: Oh my goodness….. We just had carpet ripped up and prefinished ash laced in to an existing prefinished ash floor. The installer then sanded everything down and restained. I went to look at it tonight and every single step I take makes a very loud “popping” creaking sound. It sounds like the fourth of July! Interestingly enough, everywhere the original floors were made no discernible sound. Only on the areas where the new floor was laid. I have not yet paid the installer. I sent him a message asking about it and so far no response. Is this due to a faulty installation? Is it something that goes away after the floor “settles”? I have a whole series of other work scheduled to be done before I move in which is only 10 days from now so I would rather not have to redo the floor, but I cannot live with the sound it is making. Thanks for your help,

A: It seems clear to me this should not be happening. Did he nail adequately? Did he make sure the sub floor was secured before beginning the installation? Did he acclimate the new floor before starting the installation? Did he make sure the ash was within 4% moisture content to the sub floor? So many questions. So few answers. It is easy enough to find out how far spaced out his nails, cleats or staples are. Get one of those magnetic stud finders and run it down the edge of the planks. Any time you pass over a nail it should point downward. You should find a nail at least every 10 inches or less.

Follow-up Q: Thanks for getting back to me.We questioned the installer and he claims that it is because he did three coats of water based sealer and there is so much moisture. We made him put in writing that the problem will go away as it dries but I am very sceptical! Does this sound like a reasonable explanation?

Thanks for all of your help!

A: I don’t know what product he used and I haven’t used water borne coatings in quite some time. However, I have used them off and on for probably 30 years. Having said that, from my experience it is really pushing it to try to apply 3 coats of a water borne finish in one day. And my bigger question is: why did he use 3 coats of sealer except that it is cheaper? And did he give you one coat of the finish which is generally tougher and is meant to stand up to wear? If that is what he has done you can expect this floor to need refinishing not too long down the line. Applying multiple coats in one day can cause some swelling of the wood.

Bamboo floor lifting in doorways

Q: We just had bamboo floors installed over cement, and now I’ve noticed the bamboo floor lifting in our doorways. Any idea what happened?

A: Do you mean it is buckling, heaving up or the edges of the boards are loose on a few boards? If it is buckling, heaving the culprit is likely a water leak somewhere. If it is the latter it could be the adhesive was missed in those spots or the floor, if it was rolled with a weighted roller missed those spots and failed to get good adhesion. There are injectable adhesives for situations like this or when there may be a low spot in the concrete. Bostik makes such an adhesive.

Similar Q: I just had click bamboo floors installed 2 weeks ago and now in nine certain areas they seem to be lifting off the subfloor, and popping when stepped on. What can be done to fix this problem permanently and cost effectively?

A: Perhaps there is a small dip in the floor (concrete?) under that area. If this is the case, though it would require taking the floor apart up to this area, it could be filled with layers of roofing felt to make the area flat. Seems like it could be a real pain though.

Pilot holes through the sub-floor to stop creaking

Q: Our 3 year old 3/4″ maple hardwoods (nailed down with a nailer) are creaking all over and some boards actually feel loose. It was nailed down to OSB on top of I-joists. I’ve checked the sub-floor (our basement is un-finished) and it is secure to the joists. I have tried adjusting the humidity and that didn’t help.

I was wondering if I could pilot holes through the sub-floor underneath and pull the hardwood down secure to stop the noise. Will this cause damage, because of the natural movement of the wood through the different seasons and the wood unable be able to move because of the screws? I just don’t want to make things worse, but it’s really annoying. How to fix a creaking subfloor?

A: Screwing from below is certainly worth a try. I would suggest drilling pilot holes using a depth gauge on your drill. I don’t like installing on OSB even though it apparently has been approved by the NWFA. This being a new build I’d even guess the builder went minimum code and only used 5/8 instead of ‘¾. Little wonder the floor is moving. There isn’t much really holding the cleat or staple and maple can be sensitive to humidity changes as it is.

New floor popping loudly in middle of night

Q: My question is related to your posting on “popping” or “cracking”, wood floor popping sound Q&A.

In my 2nd fl bedroom I recently replaced the old carpet to hardwood floor (not the pre-finished kind). We have, several times, been awoken by the loud popping sounds from the wood floor around 2-4 AM. What is the cause of that? Could it be the old sub-floor is very thin? When we walk on it, we don’t hear any noises. Thank you for your help.

A: My guess, especially since it is a newer floor is that it is moving slightly according to changes in humidity in the house. Especially when the fit of the tongue and groove are a tight fit noises like popping sounds could be heard. This should stop in time. Try to control the humidity levels in the house.

Similar Q: On the 2nd floor of my house I recently had the hardwood flooring replaced with new. Now when I walk in a room the entire floor crackles like breaking ice. If I walk around a few times it will stop for a while. At night I can hear it pop even though no one is walking on it. What is wrong and how can it be fixed?

A: Did the installers acclimate the flooring before installing? It sounds like there is some movement happening. If the milling of the tongue and grooves is particularly tight as the floor contracts it can make such noises. Try to control your humidity levels. I would think given some time the floor will settle down. Unless it starts to cup or buckle I would consider it a temporary annoyance.

Another Similar Q: We recently moved into a home and refinished the hardwood floors which had been down for 5 years. Spring has come (in Sydney) and the floors are now starting to pop and separate at some of the seams. Could this be caused by the refinishing?

A: It sounds to me that the wooden floors are simply reacting to changing environmental moisture levels in the home. Wood can and does release moisture that is in the air. If it is very dry the wood can shrink. Solvent based top coats generally allow more film stretch than water born urethanes. The popping may be the sound of the finish cracking at the joints from board movement. Perhaps you can add humidity to your home.

Squeaky floor without a sub floor

Q: My wife and I moved into a rehabbed 2nd floor condo this summer. It was built in 1970, has hardwood floors throughout, and tiles in the kitchen and bathrooms. As we’ve been living here, the wood floors (red oak, I think) have become really squeaky. A little Internet investigation lead me to think that most people with this problem nail the offending board to the sub floor and are done with it. But our problem isn’t just one or two boards, it’s pretty much all of them. Also, I don’t think we have a sub floor.

I think this for two reasons: I installed a ceiling fan this summer and got a good look at the bottom side of my upstairs neighbor’s floor. It looked a lot like the top of my floor, with about 2′ boards with the ends offset. Since the bottom of his and the top of my floor look the same, this makes me think, that both of our floors are just one layer of boards running perpendicular to the joists. The second reason I think there’s no sub floor is that there are rows of nails (in every third floorboard) spaced 16 inches apart running parallel to the joists.

So, how do fix my squeaky hardwood floors? I noticed that some regions of the floor don’t have the rows of nails, and so I assume aren’t nailed to the joists. And the few places where the floor is quiet is where there are lots of nails. Since nails seem to make it quiet, I was thinking of putting more nails into the floor, like maybe at every joist.

Since it seems like a simple solution, I’m a little nervous about adding more nails, because there may be a reason that the original flooring guys didn’t put nails everywhere. Did they nail every third board (and not every board) to let the wood move a little bit? If I nail more regions down, should I also nail every third board?

A: I have seen a few floors like this installed without a sub floor. Sometimes they did not stop the end of the plank at the middle of the joist, but between them. Not a good thing. This floor may be quite old but should have been nailed through the tongue and into the joist. They likely didn’t have a nailer but used simple 2′ common finishing nails. These would tend to work loose over time, especially if there was seasonal movement. You could try a few nails through the face and see if it helps. Counter sink the head and use colour match wood putty to fill the hole. Use spiral nails, not the ones with the smooth shank.

Edge of floor coming unglued

Q: We had wood floors installed in our kitchen. They put in a transition piece between the dining room and kitchen. The edge of the wood floor unglued and you can push it down. The company drilled a couple of holes and filled it with glue but it is coming up again. What can be done to fix this problem?

A: I would think the drill and insert glue method is a good approach. Did they weigh the strip down after doing that? If this is a nail down installation, could they fire a couple of nails through and fill will colour match putty?

Follow-up: No they did not weigh it down and it is not a nail down. It’s in a very open area that is quite noticeable. The company now thinks they will have to take out the transition strip and re-glue the floor underneath.

Wood planks pop when you walk on them

Q: My house is almost two years old, concrete slab, wood floor B****-Manufacturer (glued down). There is a section of my living room where the wood planks pop when you walk on them. All the pops seem to be in a line (where the concrete floor had a small crack due to settling). How do I fix these pops? I’ve heard that I have to drill a small hole and inject wood glue or a putty, to fill the gap between the concrete and wood.

A: The only thing I can suggest is to use an injectable adhesive. Bostik makes just such a product.

Floors making a cracking/popping sound

Q: We have Bruce hardwood floors in the downstairs. The house is about 4 years old. When you walk on the floors they make a cracking/popping sound. I heard the hardwood floors popping somewhat from day one, but it is really bad now. Is there any way to fix this problem? Just wondering if the floor is too tight or uneven?

A: Since I don’t know if the floor was nailed down well and what the sub floor is, my best guess is that it is likely environmental. This time of year the furnace goes on and with generally lower humidity levels in the house some contraction will likely occur. Especially if the milling of the tongue and groove happened to be very tight you could get a popping sound from the pressure the board is under as it pulls slightly away from the matching rows. I would check the relative humidity in the home with a hygrometer (you can buy an inexpensive one at most electronics stores) and try to maintain RH in winter between 35-40%. You don’t want it to drop below 30.

Related Q: My tongue and groove flooring makes a popping sound in certain spots. The boards seem to have warped in some areas to the point the floor looks raised. These type of noises didn’t happen before, when I had carpet on the floor, even though there was no padding underneath.

A: It would appear your floor is under pressure from expansion of the boards. The only thing that can cause this is excess moisture in the wood. You either have a leak somewhere or extremely high humidity. You can check humidity with an inexpensive hygrometer. A dehumidifier may help.

Similar Q: We have an 11 month old home in Ohio. Our pre-finished oak hardwood floors pop and crack when we walk on them. We had the builder out, and they placed some nails in the seams of the worst areas, but it seems to have made it worse. This is a problem during all seasons, not just when the humidity is higher or lower. Is this the nature of the product or is there a flaw in the installation?

A: I could only make a guess at this point, not knowing what the floor was installed on top of. This can be important because some types of sub flooring material do not do a good job of holding nails well. It may well be the case here. Also, if not enough nails were used and there is a small dip in the sub floor, walking over that spot can cause some flex. This is particularly important near board ends. There is suppose to be a nail not more than 3 inches from the ends of each board.

Creaking wood floor

Q: My creaking wood floor creaks and pops when I walk in certain spots. But it also does it when I am not walking on it. How can that be?

A: It’s probably a humidity issue. Particularly if the flooring has tightly milled tongue and groove. As it shrinks slightly it can make this cracking sound. Is this floor fairly new? In winter I would try to keep the RH in the home between 35-40% in cold climates. It should settle down in time.

Popping noises after 6 months

Q: We had wood flooring installed in several rooms in late May – early June 2007. The carpet and tile were removed and the surfaces prepared by the installer, glue applied and the planks installed. After a few months and now more time, we experience popping noises throughout the house as if the glue under the plank is loose. Can the floor be repaired without removing all the planks? Does glue need to be reapplied? What is the suggested solution with minimal disruption to the existing flooring?

A: Your floor is likely reacting to climate changes in the home. These popping sounds are aggravated when the milling of the tongue and groove joints is very tight. Once it has gone through a couple of seasonal cycles it should settle down. Try to keep the relative humidity in the home to near 40% in winter to minimize shrinkage. Buy a hygrometer and/or humidifier which will have a digital readout of room temperature and RH.

Floors sound like bubble wrap

Q: I just bought a new home in WI. We have hardwood (oak) floors thru out the home. When we walk on the floors it sounds like we are walking on bubble wrap. Would yoy have a solution to this problem?

A: I suspect this sound will settle down after the floors have become more accustomed to the home. I am going to make a guess that these are pre finished floors. I have installed one such which had such tightly milled tongue and groove that it also made sounds for a time. Good climate control will ad stability to the environment and you will have less movement from the floor.

Noisy floor

Q: I have bought an old house. The hardwood floor makes so much noise, and it’s increasing day by day. Earlier it was not that much, but now it’s bad. Any solution?

A: If you have access to the sub floor from the basement ceiling, perhaps you could screw support pieces across sections of the sub floor to lessen its movement. But don’t use such a long screw as to go right through the floor.

Popping or cracking sound in hardwood floors

Q: We just had new floors installed in a 1 year old home, on the entire first floor, with a crawl space underneath. This is a redo job since the original floor had to be ripped up, because of major cupping due to moisture in the subfloor. Now we have new, prefinished floors (on a dry subfloor). In various locations we hear a popping or cracking sound in hardwood floors. Any ideas, before we go back to the installer?

A: This would likely be the floor shrinking. Especially if the tongue and groove are a very tight fit, any movement is likely to make a sound like that. Nothing to worry about yet. Try to keep the RH in the home as constant as possible, or avoid sudden, dramatic swings in humidity.

Follow-up Q: Thanks for the reply. What’s odd is that this started right after the installers left. We’ll keep an eye on it.

A: I’ve only had that happen on one of my installations. The tongue and groove of the pre finished floor were so tight, I could barely pull them apart when taking the boards out of the box. With a tight fit like that, any movement is going to make a noise. Hope for the best.

Related Q: We are having a new home built and the builder has installed white oak hardwood. It has not been sanded or finished yet. We currently do not have the HVAC system installed. (We have a crawl space.) We have noticed that the floors are popping and we are concerned. We want to deal with this before they continue with the hardwoods and rest of the house. Is this something that needs to be corrected before continuing? We don’t know what to do and would appreciate your input. Thank you and I will be awaiting your response.

A: At this early juncture, since the floor has only been installed I think I would have someone come in and check the condition of the floor since installation. Someone needs to put a moisture meter on the floor. If there is room to get under this crawl space, I think I would cover the dirt if that is what is there and make sure there is good ventilation so that any moist air has a way to escape besides heading up through the floor. Depending on your location, I would also consider insulating the floor. Spray foam would be the best option I’m aware of.

And for another culprit / Floors popping after installed on incorrect underlay:

Related Q: Our solid white oak wood floor was installed over the wrong underlay. I hear lots of popping when I walk on them! My wife weighs half as much as I do and they do not pop as much for her. Half of the steps I take pop. The floors were installed a few months ago and were acclimated. I was told by the second guys that came in and fixed the top coat that the other guys used the wrong underlay. They used a thick felt underlay that was supposed to be an upgrade but has made it uneven. They put a bunch of nails in it, but that has not solved the problem. It is so beautiful, but it drives me crazy to walk around my house. We spent a lot of money on it!

I was thinking of buying the squeeek no more kit or buying some screws as I think it needs to be put down securely. Thoughts? Suggestions of the best screws to use? Thank you so much for any help.

A: I haven’t heard of the ‘Squeak No More’ in quite a few years. Home Depot used to carry those once upon a time. They use a thin gauge screw which has the head snap off at a certain point leaving just a tiny hole. Whatever fastener is used you really need to find and go into the floor joists. Also if the ceiling below the floor is open you may be able to do some shimming or sub floor securing from below.

Follow-up Q: Thanks Craig. I’m pretty certain it is not the sub floor squeaking but the wood planks on top of the too thick underlay. Does that make sense? It is a brand new house but we used an underlay we should not have.

A: How thick was this underlay? If they used long enough cleats or staples and enough of them, every 7-9″ the floor should have been fine. Unless this underlay was something like, for example, a manufactured sound barrier, some of which can be about ¼” thick. If they used something like that and overlapped the joints then that would be a big mistake. If there are so many voids between the finished floor and sub floor then there may have been an issue with the sub floor.

Follow-up: Thank you for the help. I think it was this underlay:

I don’t believe they overlapped it but I also think they used the standard length staples and did not put enough down. The guy came in and threw a ton of long finish nails in a lot of areas but there are still a bunch of squeaks. Long story short, I fired the guy that did the install after he couldn’t figure out how to sand and stain correctly. The second guys that came in did a good job sanding and staining, but said all of the creaking was from the underlay. They saw it under there for a few spots they fixed and a vents they added.

Related Q: I would like to know what the long term consequences are of just putting up with crackling floor boards versus a rip up and securing of sub floor. Will the life of the floor be shortened?

A: If the floor has been sanded too many times movement like this would cause splinters to break off the edges of the boards. If you don’t have that happening and can live with the noise you are good. This doesn’t affect how long the floor will last. It sounds to me you have an old, 3/8″ thick floor. 3/4″ is much more rigid and less prone to flex.