Monday, July 16, 2007
I spent some time the other day looking for "cost per square foot" calculations to have a guess as to what it might cost to build a structure yourself. Starting from the bottom, my assumptions were that a professional would have to pour a slab foundation, but that you'd do most of the site prep work on your own. So for a 6" thick, 3000 square foot slab it would cost around $4200 for the concrete (professionally trucked in and poured) and around $12,500 for the labor. One could see immediate cost savings if there were ways to eliminate the labor costs, but I figure most folks lack the ability to successfully pour, screed and float 3000 square feet of monolithic slab themselves. I'm sure one could pour sections, but still...for the square footage of floor space needed, you'd have to mix a LOT of bags of concrete by hand. If you had a lot of friends that you could rope in for a slab-pouring party, that could be a lot of fun too and save some cash, but I guess that depends on how much food and beer they consume!
On to the walls. I had a little difficulty here. Calculating wall costs using materials like AAC, ICF, and cement block using the information on the internet isn't very easy. Most places want you to call or submit your plan for an estimate, and considering the structural numbers I'm using for guesstimates is completely arbitrary at this point, I have nothing to submit. Many places don't list cost per unit, cost per foot or cost per cubic yard for their materials anywhere on the site; they want you to call or talk to a local distributor who also wants the same info - how big is your structure? How many doors/windows?
I wasn't able to come up with what I thought might be a reliable estimate for ICF construction. I was quite interested in it and thought it might be the way to go, but after seeing that the builder will need to purchase the block forms (these forms come in a limited number of shapes) that are the outline of the structure, and that the form must be complete in order to bring in a ready mix truck every time you need to fill the form, it doesn't look so attractive. Not to mention that the ICF form blocks vary widely in cost, and all seem to have different methods of assembly and unique hardware that would take some learning for a DIY builder. Not to say this still wouldn't be an option, it all depends on your build timeline.
Then there's AAC block. After looking at several sites, this material looks to be a good possibility for the DIY builder to use with good results. No specialized foam block forms. Cuts with regular hand tools (no special stone saws like you'd need for concrete block), so you could make curved doors or window spaces fairly easily. Builds like cement blocks. Don't need to hire professionals and a truck to pour cement in your walls. Lighter weight block. Very good insulation qualities, though not quite as good as ICF. (8" wall, AAC=R-22, ICF=R-22-46, climate dependent) You could assemble your structure one block at a time, as you have time. Cost for plain 12" thick AAC block seems to be around $197 per cubic yard of block. Plain cement block (CMU) is around $190.
There's another option as well; sort of an in-between - insulated concrete block. Couldn't find a whole lot of information about it. It's similar to regular concrete block except that instead of having empty, hollow cores in the block, the structure of the block is altered to incorporate an insulating foam insert, or the block itself is foam filled. While this does provide better insulation than standard block, it still has "thermal bridges" (where heat or cold can travel through a wall due to the exterior surface being connected by material directly to the interior surface) that reduce the efficiency of the structure.
Edit: I found a site here that has to do with educating consumers and builders about ICF construction. Good information, somewhat hard to wrap your mind around at first, but an eye opener. I also discovered that not all AAC blocks are created equal; one site mentioned the difficulty an owner-builder had with her AAC block- they weren't milled true! She couldn't get her walls perfectly straight, and upon calling the manufacturer to find out why, discovered the fine print: block size "nominal".
*Pictures: The colorful ones are ICF blocks, the string of blocks are AAC, and then there's the regular CMU or "cinder" block.