Oak, relative humidity and Underfloor heating

As an independent flooring consultant, I regularly get asked to inspect and report on oak wooden floors that have cracked, warped, lifted, or delaminated.

Inevitably the floor failure has something to do with moisture.

• Usually, due the moisture content being too high
• Less frequent but becoming more common, due to the moisture content becoming too low.

This blog focuses on the latter “ to low”

The dimension of a piece of wood, even if engineered does have a relationship with the amount water that is held within it.

The moisture content goes up                    = The wood gets bigger

The moisture content goes down              = The wood gets smaller

The wood will absorb moisture from above and from below.

For now, I going to assume that you have prepared the floor properly and that there is no risk of moisture ingress from below and have concentrated on from above “ Relative humidity” (this is the water that is carried in the air) and  underfloor heating.

I will make one thing clear, engineered boards are not a magic way of eliminating risk from fitting oak floors in areas with under floor heating. An engineered board is usually constructed of a thin layer of solid oak ( Lamella) adhered to  plywood.

This piece of solid oak will try to behave exactly the same as a piece that is not adhered to plywood.

This oak layer will get wetter when exposed to high levels of humidity and will try to expand (remember I have assumed that you have prepared the sub floor correctly so that there is no risk of moisture from below).

This same piece of oak will also shrink when it dries out with lower levels of humidity.

You may ask yourself what is all this to do with underfloor heating ?

The underfloor heating also dries the oak out, to an incredibly low moisture content level. This may result in movement “shrinkage” resulting in gaps or  splits. Combine the underfloor heating with a low level of relative humidity and the oak will shrink further often resulting in greater splits, cracks, warping.

Turning the underfloor heating on full from being off may also result in your floor splitting, gapping, warping, or delaminating. Can you imagine your cold self jumping into a hot bath . No, you lower yourself in gently so that you gradually get used to the hotter temperature.

The same  should apply to the oak floor, you do not want to shock it.

Instead measure the ambient temperature of the room and set the thermostat one degree Celsius higher and then increase this another one degree per day until the desired room temperature is achieved.

I have not mentioned this so far, but it is very important. “ The floor temperature”.

Most manufacturers insist that the floor temperature at the point it is glued to your correctly prepared subfloor is not above 27°C.

Why  you ask yourself ?

Remember what an engineered oak floor is made from ? an  oak lamella (top layer) adhered to plywood, the key word here is adhered. The whole lot is glued together  with glue ! not surprisingly if the glue gets too hot it will melt, soften, and result in a raft of nasty symptoms such as delamination, wrapping, and splitting.

I can not count the number of times that the homeowner has stated that they never have the thermostat set at 27°C. “ it would  be far too hot”

I then have to explain that the floor temperature is different to the room temperature and that in  order to achieve a comfortable  21°C room temperature, that the floor temperature needs to be higher but this should be regulated.

There is another gremlin lurking in the background “Thermal block”  as a flooring consultant I sometimes get asked to look at floors that have failed and the manufacturer is being blamed for the failure as the homeowner has followed all of the rules, so there can only be one culprit faulty flooring.

Wrong ! Beanbags, low level furniture, rugs, packed cardboard boxes sitting on top of under floor heating will allow the heat to build up in the floor allowing it to get too hot.

Like me under my double duvet but with my head sticking out, my head is cool, and my body is roasting and all of a sudden, I wake up in a sweat.

It is the same with the floor 90% of the floor is running at a nice 25 degrees on the surface as the excess heat energy is rightly lost into the room, but under the bean bag , it just builds up and up and up it is not unusual to record 36- 39 degrees where thermal block has been allowed to occur. The result being delamination, warping and lifting.

So here it is in a nutshell

Control the relative humidity, ideally between 40 and 60% RH

Do not turn the underfloor heating on full when it is new or after it has been turned off.  Remember measure the ambient room temperature and then turn it up 1 degree per day until you reach your comfy 21 degrees.

Measure the floor surface temperature and never let it exceed 27 degrees at the glue point.

(or whatever your manufacturer stipulates)

Do not sit items on the floor that will allow the heat energy escape into the room.

Please remember to check the manufactures instructions as they may vary from the above.