ᐅ Timber Frame Construction – Moisture Issues from Airtight Layers?

Hello everyone,

I hope this is the right section for my concern.

Here is the situation: At the end of October, we started having a two-story extension built on top of our garage by a professional company. The extension was constructed using a timber frame method.

We were advised to apply a bitumen layer on the flat roof of the garage due to the risk of rising moisture (when a wet car is parked inside). We followed this recommendation.

The extension was then built on the garage / the timber was laid. Insulation material was placed in the gaps between the structural timber. A membrane (airtight and waterproof) was installed on top, followed by OSB boards.

By chance (we needed an opening to the garage for the heating system installations), we discovered that water was actually standing in this cavity (between the garage roof and the ground floor ceiling)! It can be ruled out that the water entered from the outside.

We were told that the issue is caused by the airtight and waterproof sealing on both sides. The residual moisture in the wood can no longer escape.

We have now removed the OSB boards and the membrane, and there is indeed a significant amount of water standing in the ground floor ceiling space.

My question: Is it possible that that much moisture can still come from kiln-dried structural solid wood? Or does this suggest that "wet" wood was used (a defect)?

And is it sufficient, after initial drying/ventilation, to reinstall the OSB boards with ventilation holes (and this time using a vapor-permeable membrane)? I am concerned that this will not provide enough ventilation—especially since a vinyl floor is planned as the final finish on the ground floor ceiling.

Thank you very much in advance for your answers!

KVH would never release that much moisture, even if it were wetter. What makes you rule out that the moisture is coming from the outside? And who exactly came up with the idea to seal the insulation airtight and watertight on both top and bottom? What do you actually expect from the vapor barrier? Are the structures heated? Is the garage kept unheated? There is too little information for too many questions.

The extension is not heated yet and is still unfinished. The vapor barrier was originally planned before the idea of the bitumen layer came up and was not removed when we decided on the waterproofing for the garage roof. The garage is not heated. The heating only turns on in the garage when there is frost.

We had a building physicist consult with us. They also believed that the moisture is coming from the wood. It rained several times during the construction phase as well...

The plan now is to install the heating in the extension (hopefully next week), turn it up high, and open the windows. How long do you think it will need to dry out before the floor can be installed again?

A rough estimate:
KVH (solid structural timber) should have a moisture content below 18%. If a generous 2% of that moisture were dried out and all the water collected on the garage roof, with 2 cubic meters of wood used, that would be 40 liters (10.6 gallons).
Now, I can’t quite picture the amount of water you described, but it could easily add up to a bucket full.

That would roughly add up, then.

The question now is, when did the wood release its last remaining moisture? We want to close the floor on the ground floor and install a top floor layer. Should I be worried that in a few weeks, water will have seeped out again? The water can't go anywhere there, and it basically cannot evaporate or only minimally through the floor structure. Does that mean the space between the sealed garage ceiling and the ground floor slab will always remain damp? What effect might that have on the structural timber in that space?

We can’t be the only ones facing this kind of issue. Surely no one builds their timber construction directly on the soil, but rather on a concrete foundation.

I’m wondering where our mistake in reasoning is...

Could you please take some photos of the situation?
I also find it unlikely that KVH is leaking liters of water. If it can’t be rain, then condensation is the only possibility.