I’ve just completed a chair that will require a slip seat, and I’ve only been moderately happy with the few I’ve made in the past. I’d like to do it right, but after reviewing several FWW articles on this subject, it seems that there are a lot of different approaches including the use of either plywood or webbing as a support for a foam cushion, with batting between the foam and the top cover.

My question of those having experience in this, what specification of 1″ thick foam should be used in a desk chair? The seat will be 1/2″ baltic birch plywood, rather than webbing. I’ve looked at the “high density” foam available at the local fabric store chain, and it has a very low density (1.6 lb) and an ILD of only 36. It doesn’t seem as though it would offer much of a cushion at all. One FWW article stated that the author uses no less than 61 ILD, especially for thinner pieces like this. 

I’ll probably have to order it online. What specification should I be looking for? The DIY source for upholstery foam that I saw online has about 12-15 different specifications available. 

Also, are there any preferred online sources? 

Thanks.