Insulating a cistern??

littlebitfarm · #1 10/05/09, 08:46 AM

When I built my house, I had an 8,000 gallon cistern poured level with the ground. It served as the back patio. A few years ago I enclosed it with a sun room and tiled the concrete surface. That floor is like ice all year round. There is no heat gain because it is all sucked up by the floor and the water. Obviously the only solution is insulation.

I can't do the underside surface of the concrete. The opening is only about 2' square and I would have to drain it before I could work in there and then would have no water. So I must do the top tiled surface. The complication, at least in my mind, is on occasion the cistern overflows. So whatever I do must not hold water or let mold grow.

My thought is to put a raised floor in there. It is a step down from the house, so there is room to do it. Will just have to create a step down through the sliding door to the outside. First thing I would do is apply a water proof membrane on the tile surface, which should cut the humidity in that room a lot.

Two of the outside walls are concrete block, backed by soil. Third wall is 2 X 6 construction with the sliding door in it. Fourth wall is the rim joist to the house. Can I use something to form floor joists (metal studs?) above the tile surface and then have it filled in with the spray foam insulation? That should fill all the spaces and not allow any standing water or mold, right? Then what could I use as a floor surface?

Thanks,

Kathie

rambler · #2 10/05/09, 02:56 PM

Metal studs are for flimsy interior walls. You would need substantial I or C beams to hold flooring. I think you will end up with wood when you check prices.

Overflowing the cistern is a bad thing, now that this is living space & enclosed. I think you need to address that issue. You need to put an overflow drain in the cistern, if you need to run drain tile to get to a lower elevation, you need to put a hole in a cistern wall & run tile away from it. You need to eliminate the problem, and this might be chaper than all the waterproofing & raising the floor. Concrete folks can drill a 5 inch hole in a poured wall pretty easily a couple inches below the top.

Once the overflow issue is taken care of, then putting down those plastic/wooden flooring pieces designed for a damp basement should work well. They don't give them away, but cheaper than building a whole floor, and would only raise your floor a couple inches. Likely they offer enough insulation, but if not you can add an inch of ridgid foam or some such below. Of course follow the manufaturer's instructions on that.

I think you can do this a lot cheaper & easier by addressing the problem, rather than trying to out-engineer it. I'm sure you'll get other good options from others.

--->Paul

Harry Chickpea · #3 10/05/09, 05:44 PM

The amount of insulation needed shouldn't be too much. How about a couple layers of the closed cell foam sheets used under laminate, then cover that with the plastic boards now used for decks? They are dimensionally stable and don't twist like real deck boards. Mold can be addressed with bleach, and if you just tack those floorboards at the ends, you can go back in and remove them as needed or even add crossboards and more foam without damaging them if the surface is still too cold.

SolarGary · #4 10/05/09, 09:08 PM

Hi,
I agree that keeping the cistern from overflowing would be good thing to do and would simplify the insulation problem.

I think that you could just use rigid foam board insulation (the pink or blue extruded stuff) right over the current floor, and then put the new floor right over that. As long as the new floor has enough strength to spread things like high heel loads over a small area, the foam board should have enough bearing strength to do the job fine.

If you could solve the cistern overflow problem, just carpet over the current floor would insulate a bit.

Gary

littlebitfarm · #5 10/06/09, 08:11 AM

The cistern overflow isn't a constant problem. It's maybe once a year when I forget to disconnect the downspouts. Because the house and cistern are built into the side of a hill I only have access to one side of the cistern. I think rather than digging a hole big enough to wrestle a big drill in I can modify one of the pvc lines and add a drain there.

There are 2 inlets to the cistern, both 4" lines. One is buried under about 4' of dirt and I don't really care to tackle that. The other one has an elbow maybe only a foot under ground. I can pull that elbow and put in a "T". Then plumb an overflow off that.

I would still want everything waterproof. Won't know until next spring if the overflow is enough. It will certainly reduce the chances of overflowing but I'm sure a gully washer could still overflow it.

Thanks for the input!

Kathie

rambler · #6 10/06/09, 11:30 AM

Quote:

Originally Posted by littlebitfarm

The other one has an elbow maybe only a foot under ground. I can pull that elbow and put in a "T". Then plumb an overflow off that.

That could work. You'd want the T angled up a tad (only a tad!), then angle the fdrain line down from there. So the water coming in doesn't go through the T until you want. And the drain tile slope away so groundwater doesn't come in backwards, and so the overflow gets going away right away.

I would look at those basement flooring panels, they have dimples on the bottom to allow moisture to move around; a waterproof plastic type coating on the bottom; and OSB type top to be warmer & provide a surface to put your flooring on.

There is a bit of a pic of this type of subfloor & some ideas here:
http://basementflooringguide.com/

--->Paul

Mallow · #7 10/06/09, 11:34 AM

I'd say fix the overflow and use a water barrier along with treated 2x4's and blue foam in----ation sheets.

fishhead · #8 10/06/09, 02:28 PM

I'm going to insulate a cold floor in my house by laying foam board down and covering it with CDX and then finally linoleum. A contractor friend said he did that and has moved freezers and other heavy things over his floor without any change in the floor. He did say to use long nails.

littlebitfarm · #9 10/06/09, 08:52 PM

I would really love to stick a hot tub out there but I have no idea how much weight that floor can support.

Kathie

artificer · #10 10/08/09, 08:45 AM

The easiest solution is to use the extruded polystyrene foam. (pink and blue stuff) The pink stuff from Owens Corning also comes in a higher strength formula for putting under concrete slabs. Its called Foamular 250, vs. the normal 150. The number coresponds to 25psi vs. 15psi compression strength. I have a pallet of 600 here that was used on a roofs walkway. Probably overkill for what you need, as long as you use a decent underlayment. 3/4 marine plywood would be great, if you can afford the extra cost. If you use OSB or plywood, at least get Exterior grade.

I agree with everyone else on solving the drain problem first.

If your floor is made up of the concrete cistern top, foam, then sub-floor, it will hold more than you think. Using Foamular 150 with 15psi strength, each square foot will support 2160lbs ifs its spread out. You have to be careful with point loads, however. A 90 woman in stiletto heals can put over 1000psi of pressure on the floor if her weight is on one heal while walking. If your hot-tub spreads the load out over the floor, it will be fine.

The second part of what will the floor support question is how is the cistern top made? How thick? Steel reinforced?

Michael

littlebitfarm · #11 10/08/09, 04:37 PM

Quote:

Originally Posted by artificer

The second part of what will the floor support question is how is the cistern top made? How thick? Steel reinforced?

Michael

The cistern has 8" sidewalls and the top is 6". I think there is a metal mesh in there but I don't think there is rebar.

Kathie

artificer · #12 10/08/09, 04:47 PM

What are the dimensions of the floor/cistern top?

Do you have access to the cistern? Could you put supports in there?

What is the drop from the main level to the cistern floor?

Michael

littlebitfarm · #13 10/08/09, 05:27 PM

Quote:

Originally Posted by artificer

What are the dimensions of the floor/cistern top?

Do you have access to the cistern? Could you put supports in there?

What is the drop from the main level to the cistern floor?

Michael

Top is roughly 10' X 20'. I do have access to the cistern, roughly 2' X 2' opening. I suppose I could but something in for support when the water level is lower. Not sure what I would use. Sure don't want treated wood down there. Also wouldn't want that is oily in there either. Drop down is about 10".

Kathie