Anhydrite drying times

[Perfect Tiling]

Hello all.... quick question. I have a customer who's built a new home. I've tiled the walls and have the floors still to do. They're 50mm anhydrite incorporating piped underfloor heating. The floor was poured in December and heating has been on for a few weeks now. However, I put down my hygrohood and the reading will go down to 79% then the next week its at 96%. I even bought another one and put them side by side but they both read within 2% of each other. When lifting the hood there is no dark stain underneath to suggest the floor still retains water....so do I tile or not???? I have a decoupling mat ready to go down but I know that once I seal the floor and put it down with porcelain tiles on top I'll be sealing in any moisture and causing possible problems....whats the best option....thanks.

 

[Dumbo]

Are putting hoods in place and leaving them or do you keep taking them up. What are you sealing them to the floor with .

 

[Just Rizzle]

what adhesive are you proposing to use to lay the matt

 

[Ajax123]

At 96% the fact that there is no dark stain suggests there is something wrong with the reading. Are You leaving the boxes down too long shoukd be nax 72 hours. Is the heating off for the test. Should be switch off 48 hours before testing. The screed temperature will also affect the result. What are you sticking the boxes down with. It should be a fully sealed gasket of water free tape. In good drying conditions and with heating on it should take a matter of a couple of weeks to dry so I suspect the result rather than the screed is wrong.

 

[Perfect Tiling]

Many thanks for the replys guys..... I'll try to answer them. The goods are stuck down with the tape that they came with so it it the correct waterproof tape. The heating is off for the test.....but I didn't realise that they should only be down for up to 72 hours max..... they've been down for a couple of weeks. The adhesive I normsnor use is BAL SPF.....after 2 costs of APD primer. That's what I've done in the past an never a problem.....yet. No-one up here does a gypsum based adhesive.

 

[Ajax123]

I suspect you are reading interstitial condensation within the chamber on the hygrboxes which will change wildly over the course of the day and will start to interfere with the results after about 72 hours.

Lift them and make sure they are dried (leave them off for 24 hours in a warm dry place) then put them down again as you have already done. If they go over 80% within the first 4 hours your screed is not dry. If they are below 80 after the first 4 hours then read the result after 24 hours. Then read it again after 48 hours. If the 2 are reasonably consistent and below 75% at 20C then you are good to go.

Does no one stock Norcros or tilemaster up your way? Nothing wrong with BAL it's just not flexible enough to cope with a slightly damp screed where as gypsum addy is.

 

[Perfect Tiling]

Hi Ajax and thanks for the info. I took the hood off and left it as you said and put it back down on friday. switched it on on sat morning and it read 81%. I'll see what it says today. The floor must be dry....the heating has been on and the weather up here has been dry and warm for weeks...plus the screed has been down for months.

 

[Ajax123]

Ok. If it's a new hygromerer it will be calibrated. This means it will b mirecaccuratd than one that's been thrown around the back of z van as it were. That means you need to be at 80 or below to consider it suitable. The temperatures currently mean that screeds ardcquite warm which will bump months reading. My suspicion is tgat you are right and its dry. However to be safe use gypsum based adhesive and its good to go anyway. Your alternative is schluter ditra or durabase.