Vapor Barrier questions

[Beercan321]

Hi Guys,
I'm having a 30x30 slab poured next week for a detached garage. The contractor wasn't going to install a vapor barrier. After doing some research and reading posts on this forum it seems that installing a vapor barrier provides a lot more benefits then not installing one.

The contractor has built hundreds of garages and had a great reputation but I still plan on keeping tabs on everything he is doing to make sure its done right.

He plans on putting the vapor barrier on a compacted gravel base then pouring the concrete on the barrier. Is this okay or should there be gravel or sand in between the vapor barrier and the slab? I've seen concrete poured directly on barriers before. It seems that is the most common method. I assume that is okay, correct? I read an online article about how not putting sand or gravel between the barrier and concrete will cause major cracking issues down the road. I have no idea if the article is accurate or not.

I know a vapor barrier seams need to be overlapped and taped. How much overlap should there be? Is there a specific type of tape that should be used? Should the barrier be taped to the footer walls?

Thanks for the opinions/advice!

 

[dave67fd]

If you contractor has a good rep and has been in buisness for sometime he should know the correct procedures. He may offer the barrier as a additional service as mine did. It's just something they can charge you more for.

 

[ConCretin]

Vapor barriers aren't 'necessary' for unheated detached structures but since they can't be retrofitted if your use of the building changes, it's a good idea to install one.

The vapor barriers can be installed directly on the sub-base. They used to recommend a sand blotter layer over the barrier but that practice has fallen out of favor for a number of reasons. You just need to use care so as not to damage it. For example, if you are using mesh, put it on supports rather than try to pull it up during placement.

The width of the lap doesn't need to be more than 6 inches or so if it is taped properly. You can buy a special tape for this purpose at a construction supply store though any tape that won't degrade over time will work fine. No need to tape it to the side walls.