What are some thoughts on under slab rigid foam insulation?
The plan is to lay 2" insulation under my slab, I will need 30 sheet @ $28/sheet thats $840.
I will have radiant heat in the slab. If I use 1" I could save about half and save even more if I use 3/4 or 1/2.
I have often seen it recommended to use 2" under the slab, but 4" around the perimeter. I guess it all depends on your climate. You only pay for insulation once, but you will be pouring energy into heating the building for as long as you own it. If you are using a wood fired boiler for heat, then perhaps energy cost is less of an issue.
I bet someone with more experience in the matter will come along with better advice, but that is my two cents!
As a matter of fact I will be using a wod fired boiler.
When you try to optimize the cost effectiveness of slab insulation it can be a very local calculation.
In my mild climate I use 2" (R-10) at the edges and for 12" to 18" inside the perimeter. The center of the slab can then be 1" foam. This is over a gravel or sand base.
Here are other factors:
• The milder the climate the less insulation needed.
• The lower the cost of fuel the less insulation needed for maximum cost effectiveness
• The wetter the soil the more insulation needed
John is absolutely right about climate being a factor. If you haven't seen the thread before, there is some good dialog and great pictures here:
http://countryplans.com/smf/index.php?topic=5386.0
In case the link doesn't work as I intend, the flooring insulation is covered on the first and second pages of the thread.