GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

How do I thicken slab with 2″ of foam underneath?

beichenlaub | Posted in Green Building Techniques on

I am building a house on a slab (with in-floor heat). There will be 2″ of blueboard underneath the entire slab. I need to thicken the concrete slab16″x8″x10 feet in the center. How do I do this and maintain the continuity of the insulation? Pour the pads first? Cut holes in the blueboard? Help, I start next week!

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    You have three choices:
    1. Increase the thickness of the entire slab so that the slab can support the bearing loads required. You should consult an engineer if you go this route. Most "raft slab" foundations do it this way.

    2. Install extra insulation in the areas where the slab is thinner.

    3. Rake the crushed stone to conform to the shape of the thicker concrete, and install 2 inches of foam on a slope where necessary. (Later edit: see Armando Cobo's comment: he advises using pins to hold the foam in place.)

  2. beichenlaub | | #2

    What if I made boxes the shape of the thickened slab (with the top open). lay out the full sheets of blueboard and mark where the boxes belong, cut out the layer of blueboard and mark where the boxes should drop down, pull up the layer of blueboard and bury the boxes, lay the blueboard back down around the boxes. If you pour the thickened boxes first. it should work. Correct?

  3. Expert Member
    ARMANDO COBO | | #3

    We always do the following detail with no problems ever... (you need to add appropriate rebar)

  4. GBA Editor
    Martin Holladay | | #4

    I guess you choose option #3 -- I'm glad that it works (as long as you use pins).

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |