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Garage foam board instalation

I will be installing 1" foam board on the interior side of the garage foundation. About 2ft will be under ground. The builder told me I will have 3 days to install once they remove the forms before they back fill. I have a few questions.

1) What can I use to install the foam board with this green of concrete so it wont fall off?

2) Should I put spray foam on the seams before I push them together or should use a different method for sealing?

3) What should I use to protect the foam board that will be visable once the floor is poured. Is there some kind of coating i can spread on it?

Thanks for the help

Asked by jeff schlickbernd
Posted Mar 23, 2011 4:44 PM ET


8 Answers

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I had 2" XPS on the inside of the garage fdn, down to the footing. It was held on at the bottom by backfill, over which the slab was poured for a more permanent lateral force. At the top there was a 2x6 horizontally over the top surface of the fdn and hanging out over the foam board; A groove was cut into that 2x6. The exposed surface was covered with Duroc cement board (score and snap), held at the bottom by the slab and captured at the top by the groove in that horizontal 2x6.

Answered by Dick Russell
Posted Mar 23, 2011 7:31 PM ET


Do you have any pictures? Did you attached the 2x6 to the stud wall? What held the Duroc in place till the concrete was poored?

Answered by jeff schlickbernd
Posted Mar 23, 2011 9:17 PM ET


Jeff: Where are you located? 1" of foam may be pretty minimal, and not worth the trouble. Any reason to not use 2" or more? Why inside vs outside? Just curious. Often it is on the outside. If your slab is in contact w/ the foundation wall, and the foam below it, you'll have a path for the heat to slide right out. Is the slab isolated? Have you searched here and buildingscience.com for details/information on this?

Answered by John Klingel
Posted Mar 24, 2011 1:36 AM ET


Jeff, I don't have pics of that detail. A ramset was used to fire the 2x6 onto the top of the foundation, what was left after the 2x6 sill was mounted toward the outside. The foam was fastened to the concrete with ramset and wide washers, before backfill. The Duroc was fastened with adhesive to the foam and supported vertically by pieces of wood until the adhesive set. The top edge of the Duroc is captured by that groove under the 2x6. Ultimately, the Duroc is held by the slab.

Our garage is attached to the house, overlapping about 21 feet of the foundation and extending about 15 feet past the corner. With 2" of foam outside the house foundation, we just continued that in the same plane, which put the foam on the inside of the garage foundation. There is no foam under the garage slab, as it is not heated. The house has 4" under it, as it is heated space. The idea is to allow some ground heat, however minimal, come up to moderate the temperature inside the garage. The walls are 2x6 and insulated, as is the ceiling.

Answered by Dick Russell
Posted Mar 24, 2011 9:32 AM ET


I'm located in Nebraska. It will only be heated to 50 degrees and 65 degrees when im working on somehting once a week or so. The biggest reason for 1" or 2" is cost. I can get the 1" for $11.50 and the 2" for $25.50. Yes the foam board will act as a thermal break.. My goal is to provied a thermal break, prevent the frost from getting under the slab and to reduce the heating cost. I have been on buidldingscience.com and this approach is on there. Not the #1 approach but it does work because of the thermal break. Not sure how to thermal break the garage to door area. Any ideas? The reason for inside is time, money and looks. I have a very small window to accomplish this and would have to hire it done if i do exterior. Any advise is welcome. thanks

Answered by jeff schlickbernd
Posted Mar 24, 2011 1:47 PM ET


Not to your point, Jeff, but if you decide to go with 2" -- a smart move if you can afford it -- you can actually get it for $23 by using 2 layers. this would also have the advantage of allowing you to stagger the sheets.

Answered by 5C8rvfuWev
Posted Mar 24, 2011 4:35 PM ET


2 Inch would be better

inside works well if you use it to isolate the slab as well

They make foam adhesive and also foam safe adhesive. It only has to hold until the fill is in. Aint goin nowhere once the backfill is in

Again consider the 2 inches especially the 2 layers with staggered seams I do not think you will regret it

Answered by Keith Gustafson
Posted Mar 24, 2011 5:30 PM ET


Jeff - For Information on design analysis for use of XPS for Frost Protected Foundations

a) Fine Home Building magazine p 76- 81 February / March 1997

b) see http://oikos.com/esb/43/foundations.html#anchor300937 has 5 pages on Frost Protected Shallow Foundation & Council of American Building Officials

c) similar on http://www.toolbase.org/Design-Consruction-guides/Foundations/fpsf-case-...

d) best of all a download of Revised Builder's Guide to Frost Protected Shallow Foundations September 2004 by NAHB Research Center - some 34 pages on how to design for your location
--- Marvin

Answered by Marvin Oosterbaan
Posted Apr 11, 2011 11:15 AM ET

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