Hi everyone... I'm new here and love this thread. There is no question that there is a lot of confusion over the evaluation of moisture in concrete slabs. I especially love the people that put their electronic meter on a slab - observe the red light flashing and the beeping sound coming from this device - and then pronounce the slab to be "too wet". Sheesh!
Meters are Guides... NOT Gods!
So - I have a question for CC... Based on your message dated 03/18, you have a somewhat similar ERH reading in the slab - yet the MVER are on two ends of the 'acceptability spectrum'. When using the Rapid RH device, what is considered to be 'acceptable'?
Hi Ken, glad you came on board! There are a lot of great questions here, and we all learn something with each post. I'll second your comments that meters are not gods, and add that interpreting the meter's information is 90% of the battle.
As for acceptable RH levels, I guess I could go two ways with this. The safest is to say: Follow the manufacturer's recommendations. If the manufacturer does not specify an acceptable RH level, call their technical department and ask! I have done this several times (back before RH became more a common parameter in the US) and was always pleased to be able to have a good discussion with the manufacturer's technical team and come away with a warranty based on RH levels.
The second way you can look at RH is to go with your own knowledge and experience. I would have no problem putting any floor covering over a slab that is at 75% RH, and almost all manufacturers will agree with that. Yet I have been approving test installations on 80% RH slabs for many years and have not had a failure on those. Yet. I should say 'yet' because sometimes failures take time to manifest. It is some consolation to me that there are several flooring manufacturers that specify 75% RH and will write a warranty extending to 80% RH on a case by case basis. I believe this is because we have found that if the concrete is well prepared and the installation of the floor is done well, 80% RH will be fine. (Disclaimer: Always check with the manufacturer before exceeding the written RH or MVER rates to receive their approval, or the manufacturer's warranty could be voided.)
I have had the good fortune of working with very large companies that are willing to take the risk of stretching the boundaries of installation requirements. In that context I have had moisture sensitive sheet goods installed on slabs with up to 96% RH without failure. But that installation was done by gluing a homogenous sheet vinyl directly to concrete using an epoxy adhesive. No patching compounds were allowed. So far it's been 6 years and is still failure free!
So to get back to the question, 75% RH is almost universally accepted for moisture sensitive flooring, and 80% RH will be acceptable most of the time, even if it takes some leg work to get approval. Above 83% - 85% the concrete is still actively hydrating and you are walking on thin ice if you don't install a breathable floor covering.
Concrete Answers for Flooring Problems
Your last sentence really has my attention.
Thanks CC. I will have to give this some more thought before I make any more comments.
Here's why I say that:
Years ago we had more asbestos in flooring materials, and our adhesives were much more forgiving. Under-slab vapor retarders were not as common and concrete moisture was seldom an issue. If there was a vapor retarder installed, it was probably 6 mil poly, and those poly retarders degrade over time. There was also a time when sand blotter layers were the norm, and we now know that blotter layer is a great conduit for water movement under the slab.
I have been finding the majority of slabs I test that are over 10 years old have very high moisture levels.
Concrete Answers for Flooring Problems