# Seal MDF-type flooring before carpeting?



## VOC breath (Sep 4, 2011)

Any thoughts on whether I should seal medium density fiberboard flooring before I put down carpeting? This is my own house, and it's one of those "I might as well" type projects. I have the carpet torn out. This is a quick afternoon's work, and after the carpeting is in, the chance to seal the flooring is gone forever. I am thinking a coat or two of polyurethane is a good idea. Anyone else disagree with sealing or have a better idea for the sealer? Let me know! Thank you.


----------



## journeymanPainter (Feb 26, 2014)

VOC breath said:


> Any thoughts on whether I should seal medium density fiberboard flooring before I put down carpeting? This is my own house, and it's one of those "I might as well" type projects. I have the carpet torn out. This is a quick afternoon's work, and after the carpeting is in, the chance to seal the flooring is gone forever. I am thinking a coat or two of polyurethane is a good idea. Anyone else disagree with sealing or have a better idea for the sealer? Let me know! Thank you.


I would remove the fiber board and replace it with plywood. I would worry about it's strength to hold up to everyday household traffic and weight, etc


----------



## VOC breath (Sep 4, 2011)

Yeah, thought of that, but the rooms with the MDF are bedrooms on the second floor, and the house is 35 years old. The MDF has held up okay until now, and I don't think those rooms get much traffic.


----------



## RH (Sep 7, 2010)

MDF used over a substantial support base (ex. 2×8s on trusses) is just fine for under carpeting. Our house was custom built in the seventies and has it wherever carpet is located and there have been absolutely no issues.

The only reason for doing anything to it prior to recarpeting would be if pets have urinated in areas causing an odor to set in. Then spot coating those areas with a shellac based primer would be a good idea.

From experience I will add, recarpeting is a good time to locate and eliminate any floor board sqeaks.


----------



## Wildbill7145 (Apr 30, 2014)

Wouldn't MDF blow up if you ever had to bring in a carpet cleaner? It doesn't fare very well when it gets wet. I've never heard of it being used for subfloor.


----------



## RH (Sep 7, 2010)

Wildbill7145 said:


> Wouldn't MDF blow up if you ever had to bring in a carpet cleaner? It doesn't fare very well when it gets wet. I've never heard of it being used for subfloor.


We have our carpets cleaned pretty regularly (dog and cats). Never been an issue. With a good pad in place, and with the water extracted almost immediately, I don't believe much water would even contact it let alone sit long enough to soak in. Now a flood would be another story. Still, if it really concerns the op, well then seal away. But IMO it would be overkill.


----------



## Gough (Nov 24, 2010)

Is it really MDF? At least around here, that's rarely used for underlayment. I think the only time I've seen it is as a finished (painted/stained) floor. Generally, what is used is PBU, Particle Board Underlayment, a very different critter.


----------



## RH (Sep 7, 2010)

Gough said:


> Is it really MDF? At least around here, that's rarely used for underlayment. I think the only time I've seen it is as a finished (painted/stained) floor. Generally, what is used is PBU, Particle Board Underlayment, a very different critter.


That could very well be the case since I've never had occasion to take it up nor ever seen any identifying information on it. MDF tends to be a generic label.


----------



## Gough (Nov 24, 2010)

RH said:


> That could very well be the case since I've never had occasion to take it up nor ever seen any identifying information on it. MDF tends to be a generic label.


You really don't need any identifying marks on it, the difference is pretty obvious. PBU is made up of much coarser pieces, basically coarse sawdust. MDF is composed of very small wood fibers, almost wood flour. Sheets of MDF are designed for milling into cabinet parts, etc., so it comes in bigger sheets (49x97). More importantly for the builders, MDF is substantilly more expensive. I think the latter is probably the biggest reason it doesn't get used for underlayment.


----------



## RH (Sep 7, 2010)

Gough said:


> You really don't need any identifying marks on it, the difference is pretty obvious. PBU is made up of much coarser pieces, basically coarse sawdust. MDF is composed of very small wood fibers, almost wood flour. Sheets of MDF are designed for milling into cabinet parts, etc., so it comes in bigger sheets (49x97). More importantly for the builders, MDF is substantilly more expensive. I think the latter is probably the biggest reason it doesn't get used for underlayment.


You know far more about them than I do. Whatever is down now in our home has never been a problem. 

The contractor doing our addition used plywood since most of it was a bathroom and the proximity of water was major factor.


----------



## Gough (Nov 24, 2010)

RH said:


> The contractor doing our addition used plywood since most of it was a bathroom and the proximity of water was major factor.


Yeah, good call. PBU is OK under carpet, but not under much of anything else. I had the great good fortune to see a builder's reaction to a hardwood floor installer telling him that all the PBU had to be replaced with ply before the hardwood was installed.


----------



## Mike2coat (Nov 12, 2013)

Definitely seal it. When it gets wet it swells up. It also holds pet urine smells.


----------



## RH (Sep 7, 2010)

Gough said:


> Yeah, good call. PBU is OK under carpet, but not under much of anything else. I had the great good fortune to see a builder's reaction to a hardwood floor installer telling him that all the PBU had to be replaced with ply before the hardwood was installed.


I was doing a place not long ago where the new owners were having some engineered hardwood floors installed. The floor guys took up all the "MDF" (apparently not good for under that material) and the quality of sub flooring under that was abdominal. Trusses on 36" centers and non tongue-in-grove 1x8. Walking on it in some spots felt like a trampoline. They put down OSB because the extra financial hit to the new owners to put down plywood (as was recommended) would have been pretty high. Since the new flooring isn't something that would be replaced very soon I would have opted for the plywood.


----------



## I paint paint (May 4, 2014)

RH said:


> I was doing a place not long ago where the new owners were having some engineered hardwood floors installed. The floor guys took up all the "MDF" (apparently not good for under that material) and the quality of sub flooring under that was *abdominal*. Trusses on 36" centers and non tongue-in-grove 1x8. Walking on it in some spots felt like a trampoline. They put down OSB because the extra financial hit to the new owners to put down plywood (as was recommended) would have been pretty high. Since the new flooring isn't something that would be replaced very soon I would have opted for the plywood.


A blow to the gut for sure!


----------



## straight_lines (Oct 17, 2007)

Gough said:


> You really don't need any identifying marks on it, the difference is pretty obvious. PBU is made up of much coarser pieces, basically coarse sawdust. MDF is composed of very small wood fibers, almost wood flour. Sheets of MDF are designed for milling into cabinet parts, etc., so it comes in bigger sheets (49x97). More importantly for the builders, MDF is substantilly more expensive. I think the latter is probably the biggest reason it doesn't get used for underlayment.


Its great for carpeted applications adds sound absorption. Just don't ever let it get wet. 

We mostly use Advantech now unless we are doing a repair and need to match up.


----------



## Gough (Nov 24, 2010)

straight_lines said:


> Its great for carpeted applications adds sound absorption. Just don't ever let it get wet.
> 
> We mostly use Advantech now unless we are doing a repair and need to match up.


I cringed when some friends' GC put down two layers of PBU under hardwood in a kitchen remodel.


----------



## VOC breath (Sep 4, 2011)

Thanks all for the insights. If my floor isn't MDF, it is very low-budget particle board. Very small fibers - or, as Gough said, "wood flour." I ran screws through it 12-15" apart across the entire second floor because it was originally nailed down 35 years ago and I've got nail pops all over the place. Polyurethane is overkill, but the time and materials are so minimal, I'm going to do it.


----------

