Can I Use MDF As Underlayment? Critical Considerations

Getting the right underlay for your home or work environment is vital, as it affects the smoothness of the finish and the life expectancy of your new floor. Modern density fiber (MDF) is an excellent choice for your underlayment since it’s cost-effective and easy to work with. 

You can use MDF as underlayment but it is best used in areas not prone to moisture. MDF sheets of 18-22mm are a high-density material that provides a consistent finish and resistance. Set MDF underlay over an uneven floor to create a smooth surface but be sure to use a water-resistant membrane or moisture-resistant MDF if moisture is likely. 

In this article, we’ll examine why MDF proves a popular underlay option and ways to enhance its performance. Read on to discover the tricks to get an underlay that will last for years.

Is MDF The Right Underlayment For Your Project?

Before you decide whether MDF is for you, it pays to understand the function of underlay and isolate the purpose you need it to fulfill.

Underlay sits between the structural subfloor floor and the decorative surface fitting, like carpet, vinyl, or tiles. A hard underlay like MDF gives a smooth, consistent surface since it covers any imperfections present in the subfloor.

Its high density also makes it resistant to wear. Underlay also adds comfort and reduces noise. Lastly, it limits damage to the flooring material by disbursing the effects of repeated impact from foot-traffic.

MDF is an obvious choice since its qualities fit many functions required of underlay, but is it right for the task at hand? 

Pros and Cons of MDF as an Underlay Material

Pros	Cons
High Density	Warps when wet
Affordability	Swells when wet
Easy to work and cut	Absorbs moisture
Smooth finish	Brittle
Capable of withstanding humidity more than wood material	Potentially toxic dust
Can be acclimated to the environment to reduce movement despite atmospheric changes

While MDF is excellent for cutting and finishing, it is brittle. Though it takes screws well, it is likely to break or split if the screws are too close to the edges. We’ll look at how best to fit MDF later in the article. 

Another potential downfall to MDF is that it’s prone to expansion or bowing when wet. However, its movement due to humidity is less than that of solid wood (source).

If your MDF underlay will be exposed to liquid spills or frequent damp conditions, opt for either a plywood underlay to sit on top of the MDF or use moisture-resistant MDF. 

Using a water-resistant membrane underneath your MDF is advisable, also, especially if the underlay is over a concrete subfloor. Similarly, coating MDF with a few coats of polyurethane varnish will improve its water and wear resistance. 

Despite a couple of potential flaws to MDF, you can easily overcome the shortcomings. And remember, there are many positive attributes to MDF and circumstances in which it delivers an affordable, practical choice. It’s now time to consider another important question.

Where Do You Plan to Apply MDF Underlay?

MDF can be placed and is set by many builders as an underlay in many situations. However, as we’ve discussed, you may want to consider an alternative if you’re seeking an underlay for a bathroom. 

If the MDF underlay sits upon upper-level floors over a timber subfloor, you’re looking at the right underlay material. MDF is also workable over a concrete floor with a framework of wooden struts supporting the MDF, especially underneath its joins.

Finally, when considering MDF underlay and you’ve identified its purpose and where it will be placed, you need to look at the flooring it will be underneath of.

Certain flooring types are not compatible with MDF, while others will benefit from the advantages it offers.

What Type of Material Is Your MDF Underneath?

Since MDF is a hard underlay with a smooth finish, you can place it underneath vinyl, LVT tiles, or carpet. It’s not uncommon to see treated MDF used as a flooring surface.

With a few coats of varnish, it provides a gleaming, warm underfoot experience. You could also use MDF as an underlay for a floating wood floor, but you’ll want a foam or rubber membrane for extra softening.

Managing Your MDF Underlay

Even though MDF is susceptible to moisture in the atmosphere, there are techniques you can apply to reduce its swell or shrinkage. These are detailed in greater depth on this site.

Essentially, it is best to acclimatize MDF to its new environment for a few days before cutting and setting. Once acclimated to the general humidity of a place, its likelihood of swelling or shrinkage reduces.

If your MDF underlay will go into a high humidity area, consider sealing with a polyurethane varnish. When placing MDF underlay on a ground subfloor, place a water-resistant membrane underneath to limit MDF’s absorption capability.

Another way around absorbency issues is to varnish the MDF, so it repels liquid. Alternatively, select MDF with a veneer, which would act both as underlay and flooring.

If you opt for MDF with a veneer finish, you’ll want to use a foam underlay to soften footfall and enhance comfort. 

How to Attach MDF Underlay to Subfloors and Flooring

You know to factor certain factors into how you work with MDF. You also understand that MDF remains a popular building material and underlay for many reasons, not its durability.

But now, you’re faced with attaching this absorbent material to subfloors and flooring.

Attaching Your MDF to Your Flooring

You have three options for pinning your MDF underlay to the subfloor or bonding the underlay to the flooring.

Adhesive

MDF is absorbent, which means it can reduce the effect of glue. For this reason, you need to select the right kind of bonding material. Thankfully, since MDF contains reconstituted wood fiber, you can use any adhesive that works on wood on MDF, too.

Carpenter’s glue is perfect for joining MDF to MDF, but this glue is not water-resistant. As a general rule, you’ll want to use polyurethane glue.

If sticking MDF to a different material, check the surface it’ll attach to and select an adhesive capable of bonding mixed surfaces.

For more in-depth information on adhesives that you can use to bond MDF with various materials, MDF Info is a good place to start.

Screws 

Screws are an effective way to connect MDF to subfloor structure, but you need to take precautions given the composition of MDF. Opt for a straight shank screw, first of all.

Next, clamp the MDF around the area where you plan to sink the screw. Clamping will compress the MDF, holding it in place and limiting its ability to split.

You must take one last step before placing the screw, and that is to drill a pilot hole. Doing so will preserve the MDF’s core and ensure the screw sinks in without damaging the material.

Staples

Staples offer hold, though not as strong a hold as screws. When stapling MDF, do so at least 12mm from the edges and 25mm from corners.

Apply the staples legs at an angle that falls between 15° to 30° to the surface plane; doing so will increase the withdrawal strength and limit delamination risk.

It is not a good idea to attempt to staple edges of MDF sheets to one another because this can lead to breakage. 

One last point to make about MDF’s composition is that you should take some precautions when working with this material. 

A Note of Caution

As much as we love working with MDF in construction, we know that this is one material we have to respect. Its dust is known to contain toxic chemicals. Therefore, when you’re cutting or sanding MDF, do so in a well-ventilated area, be sure to wear a dust mask, and follow safety guidelines.

Conclusion

MDF is a good option for an underlay. Be sure to assess the location it’ll be placed in, treat it, and take the appropriate measures to increase the longevity of use and reduce bowing or extraneous wear.

Attention to these details when placing your underlay will ensure your MDF underlay more than fulfill its purpose.