(n) pad inside existing residential S.O.G.

[radiocontrolhead]

Hi All,

I need to introduce some interior pad foundations inside an existing residential remodel. What are some best practices with respect to breaking the vapor barrier?

I do not know yet if there is a vapor barrier but assuming there is, the installation of a new pad will break the barrier. Could this be sealed up during the new foundation pour say in two phases? First pour being the thickness of the new foundation. then phase 2 to include 2" of sand under new vapor barrier under the new s.o.g.?

Or is breaking the vapor barrier for such a small area (say 3x3 feet)not considered much of an issue?

Thanks all for your feedback and expertise

 

[PropertyGuy67]

Correct me if I'm wrong, but you want breach the slab and existing vapor barrier to set new footings? They way I do it can be summed in one word: carefully. Also, I don't pour footings on top of the existing barrier. Make your 3x3 opening and remove the rubble, taking great care to not shred the existing barrier. Once exposed, try to slice it corner to corner, and pull the 4 triangle flaps back and secure. Excavate your hole. Cut and attach new barrier material to the existing flaps (trimming as needed) to create square flaps instead of triangles. Right before the footing pour, tape up the interior corners of the barriers (think of it as recreating a paper bag from a bag that had its four vertical edges sliced open). Pour your footing up to the underside of the existing slab. Set your post (if it is a post). Re-cut the barrier and snug it up to the sides of the new post, and re-tape it all. Pour the topping slab after the footing sets.

 

[radiocontrolhead]

Thank you PG67,

Would you have any resources you could direct me to look into about this topic? This has been a gray area to me for some time and want to make sure i have my bases covered

 

[radiocontrolhead]

anyone else have experience with this or have go to details/procedures to reseal the vapor barrier?

 

[phamENG]

The only other option I can think of is to cut a larger than needed hole in the slab (maybe by a foot) without damaging the barrier. Then cut the barrier to the size of the new footing, excavate and pour the footing, and install your column. Once the dead load is applied, backfill to the bottom of the barrier. Install new barrier material lapping the edges of the existing and taping properly. Pour back you slab and you're done.

If you damage it and can't lap it from above, you'd have to undermine the slab slightly to access it and do something from underneath. Not sure how you'd accomplish that...so try not to mess it up!

 

[canwesteng]

Lap the vapor barrier? Just tape it. I would drill a pilot hole to see how deep the slab, then sawcut a little less than this and jack hammer away. That should leave the vapor barrier in good enough shape to tuck tape it to new vapor barrier on top of your footing.

 

[oldestguy]

Why a vapor barrier anyhow? Is there a high water table there and moisture in the room a problem? Most so called vapor barriers are damaged during installation. If you truly want a water-proof vapor barrier, ordinary plastic won't do it. Remember moisture vapor goes from warm to cold zones all other things equal. If a real concern the guides above suggest about the best that you can do and don't be expecting any great results.

 

[HouseBoy]

Agree with OG on all points.
For virtually all residential floor slabs, a vapor barrier is not going to stop water if the water table is the problem.
Dig a test hole and see where water level is. I suppose it might be a seasonal condition but if there is no history of water intrusion, I don't think it would be adversely affected by such a move. An under slab sump would probably be more effective IF something is needed.

Generally, I simply require excavation below the sub-base by several inches to reach firm and undisturbed soil and the new footing can be flush with the surrounding slab. Often, that results in a thick footing that might not even require reinforcing. 3 ft square might but that rebar would be incidental.

I don't recall ever requiring attention to the vapor barrier for a new footing in a residential basement renovation.

 

[r13]

Is the pad to be provided in the ground floor, or the basement? If not an obstruction to living space, have you investigated the possibility to add pad on the SOG?

Vapor barrier is to keep damp out of the floor, it has nothing to do with ground water. If necessary, the suppliers website shall have information on how to repair/seal it.