bizarre insulation / slab question.
Ok not sure how it came to this but I have a bizarre insulation situation emerging. Wondering (but doubting) if anyone has ever encountered something similar.
Basically we had a significant backfill issue below new basement slab-on-grade in a highly inaccessible area. Rather than cart tons of backfill in our contractor had the bright idea to have a suspended slab between old house foundation wall and new perimeter foundation, held on steel angles on top of which is welded a steel bed for concrete. I guess that this is a type of construction in commercial buildings.
What I didn’t realize until looking at it today is that by not backfilling there will be an air pocket that will be effectively encapsulated within the slab. That is to say something like the following:
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA slab
HSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSF h is old house S is steel F is new exterior foundation
H–––––––––––––––––––––––––––––––––––––––F H is house F is new ext foundation – is air
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX is grade with R-5 or R-10 and vapor barrier.
At this point my choices are the following:
1) just hope for the best
2) Ask contractor to stuff or spray as much possible insulation material into encapsulated air gap.
3) start over / do something different – obviously most expensive.
Help?? Does this make sense? I am sure it sounds bizarre
Thanks
TC
Replies
I'm confused
FYI all crawlspaces need access and might need to be vented.
Well it is only 4" deep. Solidly encased in concrete on all sides. Is it still a crawlspace?
you mean that the lowest spot on this is only going to be 4 inches of space? make life easy fill it with concrete.larryif a man speaks in the forest,and there's not a woman to hear him,is he still wrong?
Heck he could have filled the space with high density foam panels.
And the air gap is bad because...?
If the vapor barrier is directly on the ground and the air gap is above it, I don't see a problem (but I'm not a foundation or concrete expert).
My main concern would be that the concrete is strong enough not to crack (it's supported by steel, right?).
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Forrest
I too am somewhat confused. What is the physical dimensions of this air pocket? Why do you think an air pocket is a problem?
I'm guessing you are worried about heat loss through the exterior foundation wall under the steel?
Have them place some ridgid foam sheets to the inside of the foundation wall before the steel is in place. If it's really cold where you are, it wouldn't hurt to also insulate the exterior of the foundation or even use ICF's to rest your suspended slab on.
If you want radiant heat, you'll probably have to have the engineer's ok to run the tubing in the self supporting slab. We would ask and run the tubing if you wanted to use radiant or not, since eventually everyone wishes they would have. It's ok to install the tubing, having it inspected and then leave it capped.
Best of luck.
Beer was created so carpenters wouldn't rule the world.
OK let me try again. Here is a picture (not ascii....)
Any suggestions?
Alligator,
What your pdf illustration does not show is any vertical insulation on the inside or outside of your new perimeter foundation, which you have labeled "cinder block wall" in your sketch. The sloping "horizontal" insulation under the slab is shown with an airspace between the insulation and the slab; this insulation will be somewhat less effective than it would have been if it had been installed directly under the slab, because concrete block masonry is not airtight (although it can be made so). Vertical insulation on the new concrete block perimeter wall is essential.
The heat loss from the air space above the insulation and vp as you have it drawn won't be that significant.
As Martin pointed out the loss through the edge of the slab and wall will be the biggie.
If the slab and foundation wall are already done, the best you can do now is add exterior insulation to the foundation wall. Take it all the way down to the top of the footing (below frost line).
Dave
If you were using prestressed or poststressed concrete panels for your slab, I wouldn't worry.
If you had an engineer spec the rebar placement and concrete mix, I wouldn't worry.
If your span is under 4 to 6 feet, I wouldn't worry.
Other than that I'd worry.