We are seeing a lot of floors where the SURFACE DPM(epoxy) applied to the subfloor with NO DPM UNDER the subfloor has failed. ( a subfloor with rising damp) Surface DPM's should only be applied to floors with residual building moister. NOT to a subfloor with rising damp. yes i know plenty of manufactures say you can, but truth being known the DPM does not fail. The subfloor fails. So keeping that in mind then the product technically has not failed. So to give you a idea of why. When a subfloor has residual building moister (moister trapped from mixing the concrete) you are applying a surface DPM to hold back how much moister escapes over a set time period. That being to lower the amount of moister leaving the subfloor, so 'for example' 75% rh ( RH = moister relative to humidity in the air around us) If we did not apply a surface DPM then the moister leaving the subfloor for example may be 90% RH which is way to high for floor coverings and in tern will cause them to fail. Are you all still following me So back to the point, if a subfloor has rising damp (no DPM UNDERNEATH or failed DPM ) then applying a SURFACE DPM will most likely end up with failer. Its a 50/50 chance you take. Hence don't do it ! The reason why it fails is due to pressure build up. Water will try and find its own level. If the ground level around the subfloor is higher then you will be faced with Hydrostatic Pressure: Force caused by water under pressure.(to keep it simple) What this means is the water outside will try and level out. It will try and force its way through the subfloor and either push the surface DPM off the subloor or even push the subfloor up until something fails. To give you a idea of how strong this pressure is please view the pictures. You will note that these are actually swimming pools. (they sufferer from the exact same issue, just more obvious of what causes it ) The water has collected underneath and actually pushed the swimming pool out the ground! Study the pics and i hope you will get a idea of how strong the forces are. Hence you cant expect 1mm of epoxy stuck to the top of a concrete subfloor to withstand these pressures.
In theory they wont be effected by Hygroscopic pressure. BUT if you was to install a Surface DPM and then next door neighborhood plants raised flower beds etc you are now effected.
