Condensate/mold damage with radiant under a tub

Arthur Binder

I looked at a new house last week where the plumber installed a tub an a basement slab. He installed it to code and had a roughly 12" x 12" box out for the tub drain. The house is heated with radiant heat. I was called to look at it because the homeowners complained about a mildew smell and had a mold remediation company come out. They didn't find any leaks in the plumbing or walls around the tub but said that there should have been insulation and a vapor barrier over the drain box out and the cold air from the soil mixes with the warm radiant heat and caused condensation. I am not sure what to tell them but that doesn't seem like a likely source for the water. The house has perimeter drain tile that drains by gravity out the back of the house (built on the side of a hill) and it is located in Decatur, IL for anyone contemplating the climate.

Gordy

The box out could indeed alone be the cause of the mold/mildew smell. Is this a seasonal issue, or year around?

Certain soil types can hold moisture well thinking clay soil types.

Was the slab insulated for the radiant heat?.

I think I would try containing the box out for the plumbing in the slab with spray foam, and a layer of plastic sheeting.

If there is concrete exposed tape a 1'x1' square of plastic sheeting down to the concrete sealing it around its whole perimeter with tape. See after a couple of days if moisture collects on the plastic sheeting.

There may be drain tile, but was it done correctly. Inside perimeter, and outside?

I really can't see the radiant being the sole cause of the issue. Miles of it in basement slabs. With, and with out insulating the slab.

Arthur Binder

Not sure if seasonal or not, was just noticed recently. The basement slab is insulated. Perimeter drain tile is inside foundation only.

RobG

Have you shut off the water feeder to the radiant system to monitor the pressure to see if there is an under slab leak?

Arthur Binder

Yes, there is no leak, the system is 30% glycol based and with feed valve off pressure has held

RobG

That's a new one to me. Most homes these days have at least a rough in for a basement bath and have no problems. I did work in one home with mold issues that was built on top of a spring. The solution for that home was to install a French drain following the sewer and then branching off into a retention pond. I would look for leaks in the plumbing and heating system. They can be devils to find. Since you have already tested the heating the next step is to pressure test the water.

Gordan

The notion that moisture in cool damp air would condense when it hits something WARMER than itself is... strange.

Gordy

Funny how radiant heat is always the scape goat when there is an issue no one can wrap their minds around. The radiant is not on, but only in the heating season. So if this is a year around problem then look else where.

I tend to think there is wet earth around the rough in opening, and the smell is more wet dirt than mold. Drain tile done right is one thing inside, and outside the footing is best.