Hybrid insulating concrete form & plywood form

[carolyn]

Hello,

I am looking for a cross between the insulating concrete forms and the more common plywood form. I would like to use the insulating forms on the exterior, and plywood on the interior.

I like the idea of heavy insulation on the exterior of the foundation wall. I also like the idea of passive solar, and using the foundation as a heat sink - absorb the heat during the (warmer) day and release it during the (cooler) night. If I insulate the inside of the foundation walls, I lose the benefit of that heat storage source.

So, the way I see it, I can either use the standard plywood forms, and insulate after the concrete has set, or ideally, I can use the insulating forms only on the exterior, and save some labour.

Any comments?

Thanks, Carolyn

[marson]

from a construction point of view, you would be better off pouring a conventional concrete wall and insulating the outside with 4x8 rigid foam. you don't have to use plywood--look into renting concrete forms.

carolyn wrote:

[CWatters]

I thought the floor was more important than the walls. eg most of the sun coming in through the windows heats the floor rather than the walls.

[Italian]

I may be missing something here but if you have "heavy insulation " on the exterior how is the heat going to get to the concrete walls to sink in?

carolyn wrote:

[atufft]

I had a hard time understanding the situation exactly, but I've used foamboard as form board for concrete foundations, and can't see why using molds is an advantage. It certainly is not cheaper. It depends upon the height and amount of concrete poured behind the foamboard because it is not as strong. However, both foamboard and plywood (or molds) could be used, and later the plywood removed, without so much concrete on it, and used for subflooring or other construction purpose. The HomeDepot in our area sells a relatively cheap styrofoam material with a vapor barrier on both sides. I have put in two foundations with gravel and drain pipe already in the ground outside the foundation, using the foamboard as the form holding back the gravel. I used wood stakes at 1 foot intervals behind 3/4" thick foamboard to hold back the gravel and drainpipe long enough for the concrete to arrive. When the concrete pumper came and the filled the void, the liquid concrete mix held back the foamboard and gravel, and the stakes were easily removed and discarded. As a result, I have insulation/vapor barrier stuck to the footing of the foundation, and on the outside gravel and drainpipe. All of this was done in one step. In a raised foundation, OSB or plywood should be used to support the 3/4" styrofoam insulation/vapor barrier, or use 2" thick styrofoam and place the stakes on the outside. Hope this helps.

Italian wrote: