[Editor's note: This is the first in a series of occasional blog posts by Foster Lyons, a Connecticut-based engineer with over thirty years of experience in the architecture/engineering/construction industry. These posts will be based on his popular Building Science Sunday newsletters, which Lyons says started with a simple email he sent to a couple of summer intern architects to help them learn more about construction, constructability, building science, and the like.]

Most wood finish flooring installed in the past 150 years has had some sort of paper underlayment installed between the subfloor and the finish material. Not always, but most of the time.

Very often it was/is rosin paper. Very often it was felt paper – not so much anymore since oil became evil. These days we have a variety of newfangled, expensive papers.

There is no great reason for putting paper beneath wood floors if you’re building to modern standards – modern enclosure standards. You can skip the paper and be totally fine. In fact, if it’s a wide plank floor that needs to be glued down you can’t install paper. But it’s still commonly used and if you speak with builders or wood flooring installers or architects, you hear all sorts of reasons for the purpose of this paper:

  • It prevents floor squeaks
  • It’s a vapor barrier
  • It cuts down on dust
  • It makes the floor installation easier

Once in a while you hear a real wanker [1] slinging some serious bull excrement say something like “it acts as a moisture reservoir and prevents the floorboards from warping.” Not.

The funny thing is, you almost never read or hear anything about the real reason it was first used: to reduce breezes of air from going through floor assemblies. This, BTW, is the same reason the building industry started using paper in our exterior walls: to reduce breezes of air from going through wall assemblies. It was a comfort improvement.

A couple hundred years ago paper was expensive and difficult to manufacture. Today paper is made from wood pulp and it’s cheap, but that’s a somewhat recent development in the manufacturing of paper (+/- 1844, by a Canadian, eh!). Prior to the use of wood pulp the most common paper fibers were recycled from used textiles. Rags.

Today Ragman is a mystic vigilante in comic books but that wasn’t always so. In my lifetime there were “rag men” and they all worked in the clothing/fashion industry. This isn’t such a common use of the term anymore, but it’s not totally gone. [2] In my parent’s lifetime “ragman” was what you called people that purchased all sorts of things for recycling – textiles, metals, paper, etc.

During my great-grandparent’s lifetime, prior to the use of wood pulp in paper manufacturing, “ragman” was a steady job and mostly involved the purchase and re-selling of used textiles for recycling into paper. It was done all over the world. Imagine that, a successful recycling campaign that didn’t need government promotion.

Anyway, before wood pulp, there wasn’t so much paper around. Then, suddenly, when wood pulp made it cheap, we had paper everywhere. So much of it that we even started wrapping fish in it (yeah, I saw that in my lifetime also). Like all products that suddenly find themselves in abundance, pulp paper products were put to use in all sorts of new ways. One of those ways was more newsprint. We suddenly had a lot more newspapers around.

Then one day – there is no record of who, when or where – some wood flooring installer decided to install some newsprint beneath the new floor they were installing. Then it was done more and more until it became the standard. It’s not uncommon to rip out an old floor and find old newspapers between the subfloor and the finish floor. There’s no doubt this was in an effort to prevent breezes through the assembly. There were breezes through the floor assembly because all buildings in that era (mid 1800’s) were breezy – there were holes all over the place and lots of air breezing around.[3] That builder knew carpets stopped those breezes, now they had a cheap alternative - paper. And that was the birth of wood flooring underlayment.

For many, many years newsprint, rosin paper and felt paper were the underlayment of choice. And, people started to forget why it was there at all.

I can’t remember the word to describe the human characteristic of assigning a purpose to those things that we do but don’t understand why but that’s what’s going on with this paper underlayment. Mostly, we do it because it’s been done for a long time, even though we don’t know why it was done in the past and we don’t know why we do it now. It’s not a code requirement, it’s not a manufacturer’s requirement and it’s not even an industry association recommendation. It’s just a thing we do, and specifiers think “Well, there must be a reason, so, I’ll just specify it even though I have no F’ing idea why I’m specifying it.” That happens a lot. I’ve even heard contractors claim that some architects just copy old specs for use on new projects – no idea, but I hear that’s a thing.

Having said that, there are some reasonably good reasons for it. First, and, in my opinion, most significant, it makes the flooring installation easier for the installer. That’s not a real reason to specify it, but it is a good reason to allow contractors to use it. Those new floorboards slide around much easier on the smooth paper rather than the rough subfloor. More productivity, that’s a good thing. Second, it does reduce dust – the dust on the subfloor that’s been getting ground in over the past months during the construction process is now trapped there. That’s a good thing and probably enough of a reason to specify it’s use.

Unfortunately, some of the reason’s you’ll hear for its use are partial or complete BS. One of the most common is that it prevents floor squeaks. Not so much. Most floor squeaks result from two side-by-side boards rubbing up against each other. Paper below the finish material isn’t going to solve that problem. The second most common cause is one board moving against a nail. Paper underlayment is no help there. Way down the list of reasons is floorboards moving horizontally against the subfloor. I’m not saying that never happens, it does, but not so much. The squeak thing is BS.

The vapor barrier thing is also commonly heard when floor underlayment is discussed. In fact, it is a thing that quite a bit is written about, even in the National Wood Flooring Association Installation Guidelines and Methods brochure.

First, if you are about to install a wood floor over a concrete slab then don’t spend one moment thinking about anything, I’m writing in this email regarding wood flooring installation. For that assembly you need a serious vapor barrier, not paper or felt paper. New concrete holds A LOT of excess water and it will exit over time and warp your new wood floor. Likewise, if you’re going to go out on a crazy limb and install a floor over earth without a concrete slab (yes, that’s a thing) you need a SERIOUS vapor barrier and this email isn’t about that. The more common condition – wood floor over a crawl space or unfinished basement – is, however, worth considering.

Old crawl spaces and unfinished basements have elevated concentrations of water vapor because there is a serious water source down there, the wet ground. Without this water source, the water vapor concentration would be the same in the crawl space and the living space above – the relative humidity would be higher because the temperature is lower down there, but the concentration would be the same. Concentration differences are the thing that moves gaseous materials (like water vapor) through gas mixtures. The high concentration in the crawl space will “push” water vapor up into the living space above. Other things, like stack effect are actually moving a lot more material, but the diffusion caused by concentration gradient is a real thing and not insignificant. So, it is worth wondering about installing a vapor barrier as a finish flooring underlayment above a crawl space.

Of course, that sort of thinking is ridiculous. The problem is the water getting into the crawl space, not the water then moving from your crawl space up into your building. Fix the crawl space! Then condition it! Then you don’t have to worry so much about what sort of vapor barrier you need in your finish floor underlayment.

Now, if you, or your client, are cheap and you can’t or don’t want to fix the crawl space water problem then you can try to mitigate it with a vapor barrier in the floor assembly. But that’s also ridiculous. First of all, it really needs to be an air barrier, not a vapor barrier (refer back to that stack effect thing). Second, if the vapor barrier is installed anywhere other than directly on the ground, it’s in the wrong place. If the vapor barrier is installed between the subfloor (plywood) and the finish floor, then the plywood and structure below it will likely be trapped in a very high (80+%?) relative humidity condition and will likely rot pretty fast. That’s not good.

Trying to control water vapor transfer with a membrane between two slabs of wood is not a good idea. Control it on one of the sides or let it through.

Unfortunately, there’s all sorts of written stuff about “vapor barriers” in framed floor assemblies above crawl spaces, but I don’t think it should be there. I think all that writing should be reduced to one sentence: fix the crawl space. You can’t fix your crawl space problems with a vapor barrier between the subfloor and finish floor. Stop thinking about it.

However, if your crawl space or basement doesn’t have a water problem but is unheated that paper underlayment will help for a different reason: it will help maintain the finish wood floor temperature at room temperature, not basement temperature. That “prevent the breezes” thing helps. This is a good thing because wood movement is caused by changes in relative humidity (a bit of a mind bender, but true). If the temperature of the wood is different on its top and bottom surfaces it will cup. If the temperature of the wood is the same on its top and bottom surfaces it will remain flat. The paper underlayment helps maintain a constant temperature on its top and bottom surfaces – it keeps the warm air on the warm side and the cool air on the cool side; air control.

That’s about it for paper under wood flooring.

  1. A wanker is not necessarily one who wanks; it is a stupid, foolish or unpleasant person.
  2. www.chicagoragman.com
  3. My father related to me many times that he grew up in a vintage 1860 row house that had a crack so large in the rear wall that he could put his hand through it to the exterior as he lay in bed at night. And, the foundation was rubble stone. “Air-tight construction” was not a thing in the 1800’s.

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