I will be building a 3/4 underground basement. The uphill wall will have 8" ICF blocks to support the backfill load. Should I consider a 6" block on the other three sides to save money since 8" is not required? If so is there a good way to reduce the size or do I regress to some basic formwork to make the transition?

Why not do 6" all the way around?

The 8” is required but only on the uphill side to support the load from the backfill. It is a retaining wall. I am trying to cut the cost of the non-retaining walls that only support a vertical load and therefore do not need to be 8”.

If it is only 3/4 backfilled on the uphill side you do not need 8" there, even with heavy clay backfill. And if you backfilled that area with gravel next to the house that would lessen the pressure exerted from the backfill and the wall requirements.

It is a pain to change sizes on a corner. I would do the whole thing in 6".

If you are limited by engineering (I don't like to say that!) and have to have that core thickness and want to transition to a thinner wall section, it can be done. I have transitioned from the Durisol 12" ICF (outside form width) down to 10", 8" and 6" (outside form width) without any problems or extra effort. However, there is more work when you transition from their 10" down to 8" or 6" if using their off the shelf corners. I typically custom cut an end block to create a return corner down to the thinner wall section instead of using a corner block. You can do the same with most ICF's, but as Brad said, it takes a little more effort - you end up making a custom corner to achieve what you want.

Thanks all. I have not used ICFs since 1979 and am trying to figure our which block to use. This forum has been helpful but the bad dreams about bellied out walls and blow-outs are returning. ICFs now look much better than anything I remember.

I will look into Quad Lok. My engineer here in Carmel, CA is pushing for the typical CA overkill. The project is Maine with stable soil so I am going to try for 6” and be careful with the backfill. Any recommendations for structural and civil engineers around Bethal, ME?

Does CA use the IRC (International Residential Code)? If so, I am sure the 2006 IRC covers your situation, with 6".

Mark Purvis at MK Construction at 207-478-8428 has done some good ICF work in Maine and can recommend an engineer. I saw one of his Durisol installations and I am sure that he had an engineer working on it.

The Lite-Form ICF installation manual that you can download online shows transition corners that are available to change wall thickness at a corner. An 8" to 4" wall transition is shown in the manual in diagram 27-1, but it also states "Transition ties for other combinations are available." If interested, you might give them a call.

Call the ladies at New Form 207-469 2711. They are excellent.