Hello:

My single-story 30 x 42 house is built over bedrock on an 8 inch poured concrete crawl-space foundation. The floor of the crawl-space in uneven *** exposed *** bedrock, with headroom ranging from 3 feet on one 42 foot side up to 5 feet on the other 42 foot side. The footings, foundation walls and rim joist have been spray foamed to 3 inches. There are 2 x 10 floor joists supporting the OSB floor above – there is no insulation between the joists.

The crawl-space is a 'conditioned' space in that it is heated with a 240 Volt 4800 Watt Construction Heater and is included in the whole-house heat-recovery ventilation system. My fireplace has a 'forced-air' attachment that also heats the crawl-space.

The exposed bedrock acts like a 1,260 sq ft giant heat-sink!

To make things even more challenging, water seeps in to the crawl-space through some of the cracks in the bedrock, all most exclusively in two well defined areas where the crawl-space height is 5 feet – unfortunately, at opposite ends of the building. I have installed a sump pump to deal with the worst spot and will jack hammer another sump to deal with the other area. Water will always come in so, I will just 'manage' it and not waste my time in trying to 'eliminate' it. I run 2 household dehumidifiers down there.

Assuming that I can 'manage' the water, no matter what I do, I was thinking of these possible changes:

1. Insulate the floor joists with R-22 Roxul and maintain the crawl-space at 'some temperature' above freezing so that my water pipes won't burst – cost about $1,200.

2. Spray foam the bedrock with 2 inches of foam and heat the crawlspace to whatever temperature the house is heated to – cost about $2,400.

Any thoughts or suggestions would be appreciated.

Rob.

1. Install an impermeable membrane* on the ledge, and fastened to the rim joist or foundation walls so the moisture does not get into the crawl space. (*I'd suggest a heavy - 10 mil reinforced poly) Make certain it is lapped correctly so water does not get trapped.
2. Spray 2" of CCSF on top of the vapor barrier.

The poly will keep the crawl space dry, and the insulation will keep you from losing all that heat into the rock.

Yes the ledge is absorbing heat, but it's also being carried away by water which is probably colder then the ledge.

What do you intend to gain by using expensive CCF spray foam vs something like gravel + plastic + layered EPS?

I specified ccsf instead of sheet foam since the surface is not level or, presumably, flat. If he can flatten it with gravel, then by all means use EPS. There are situations where ccsf is the best material for the job. I'm thinking of a home I built where there is exposed ledge in the basement but it is too steep for washed stone to stay. The methods I suggested would work in that home but the owners, despite lots of problems, have resisted doing anything.

Posted By jonr on 30 Mar 2014 08:10 PM
What do you intend to gain by using expensive CCF spray foam vs something like gravel + plastic + layered EPS?

The bedrock's surface is very uneven making the use of any 'sheet' material very difficult. I have, in less steep areas, filled smaller cracks & undulations, with washed gravel to 'flatten' the floor area.

Best pictures I have are here.

Rob.

Posted By Bob I on 30 Mar 2014 02:44 PM
1. Install an impermeable membrane* on the ledge, and fastened to the rim joist or foundation walls so the moisture does not get into the crawl space. (*I'd suggest a heavy - 10 mil reinforced poly) Make certain it is lapped correctly so water does not get trapped.
2. Spray 2" of CCSF on top of the vapor barrier.

The poly will keep the crawl space dry, and the insulation will keep you from losing all that heat into the rock.

Yes the ledge is absorbing heat, but it's also being carried away by water which is probably colder then the ledge.

Bob - What would you do for a thermal barrier?

the ccsf (closed cell spray foam) would be the insulation. You could of course do more than 2". Would be of some value to use a thermal gun to check the temperature of the ledge and the water, although it's getting late to do that. - that would help in determining how much foam you need. If the rock is 10o you probably want more insulation than if it's 50o. Of course you can always add more.
Or are you asking about fire protection? If so, the only good option I know of would be intumescent paint.

I recognize this is not an easy job in a tight space, but what are the other options? Probably will require a mask with an outside air supply.

•••Or are you asking about fire protection? If so, the only good option I know of would be intumescent paint. •••

Bob- to the best of my knowledge, Canadian jurisdictions do not allow intumescent paints as thermal barriers on foamed plastics.

aha! Concrete works great as a fire barrier, but more work & hard to do now. Perhaps a heavy layer of sand? Options? the ledge has to be insulated somehow. What about Roxul?

Bob:

I sort of thought (and hoped and prayed!) that the ccsf, over a couple of inches of crushed stone - where possible, *** would BE the vapor barrier. ***

Is that wrong?

Thanks,

Rob.

not exactly. It is a vapor barrier, but not a water barrier. I was thinking of "bulk" water which may or may not stay under the foam; water that follows the ledge into the crawl space could go thru voids in the foam and end up on the surface. Maybe it won't, but it's a PIA to find that out afterwards, so the poly is insurance.

I repaired my own basement last year; it was wet; water followed ledge into & thru the basement. I did what I recommended to you; heavy poly up the walls (where it came in) and across the floor, then 2" of foam, then concrete. Basement has been DRY for a year now, so I know that works.

Thanks Bob. :)

There is a large area covered with crushed stone and because of that, it is flat. I see, from one of your replies, that I could use EPS in that area - the rest will require spray foam.

The whole floor will get the poly.

Question: What is the correct way to handle the seams in the poly?

Thanks, yet once again, for your help.

Rob.

Edit: What would be the thickness of the thinnest amount of concrete that I could place over the spay foam - this part will only have 'traffic' when I need to do some plumbing or electrical.  I'll put thicker concrete in the flat area but nothing much will happen there either due to the limited head-room - about 55 to 60 inches.

Will the foam adhere to the poly ok or does it need a layer of gravel over it. I have wanted to put some roofing foam over my crawl space floor on top of poly so I can roll around in my crawl space on a creeper. It would also keep the cooler ground isolated in the winter.

Rob: I know that 2" of concrete will work; unsure about smaller amounts. If it's too thin it will crack easily and be a PIA. Just tape the poly seams. Dow Weathermate tape, or Tyvek tape works OK.
Kogashuko: spray foam sticks to pretty much everything it touches. Not sure what foam is strong enuf to stand up to wheels, but you can experiment.

I am pretty sure the roofing foam will stand up to it. Was afraid of it coming off of the plastic. But come to think of it the closed cell sticks to the plastic drop cloths I use with fury.

Would love to get concrete like the OP has the option of doing but as I have said it has a house already on top of it.

Posted By kogashuko on 06 Apr 2014 02:49 PM

Would love to get concrete like the OP has the option of doing but as I have said it has a house already on top of it.

kogashuko:

The pictures were taken [before] I built the house - the house is now built. I was thinking of using a stucco sprayer to cover the spray foam.

The house looks like this , as of this past fall.

Bob:

Did you use spray foam or rigid foam sheets?

I ask this because I once saw spray foam on plastic and it seemed to buckle the plastic into waves as it cured.

Thanks,

Rob.

Rob - I think you need to make sure you have drainage medium over all the rock so that if water pops up in different places later, it can find its way to the sumps.

Consider what will happen if the pumps shut down and the crawlspace partially floods. Will it float the foam?

Can you terrace parts of the crawlspace to level them off (like rice paddies ;-) and then use sheet foam?

If you use 2" concrete, you might want to add poly fibres for reinforcement.