# Laying rockwool directly on top of slab on grade. Problems?



## Windows on Wash (Aug 30, 2011)

What is the moisture content in the slab currently? Where is the home? What is the grading like?


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## jppie1 (Feb 6, 2015)

The drainage is good. I'm not sure what you mean by grading, but it is a 4 inch slab built on a vapor barrier on about 4 inches of gravel. Moisture level low (according to plastic taped over slab test loop) 
House is located in Osaka Japan, mild winters, it rarely gets below freezing and summers are hot (90s) and humid


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## Windows on Wash (Aug 30, 2011)

Grading is referring to with the exterior grading away from the home is. This is to eliminate the possibility of moisture infiltration as a result of poor drainage.

I'm not sure that any type of batt insulation is a good idea in this scenario. The general default recommendation is to go with a rigid foam given it's lessened moisture sensitivity.


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## jppie1 (Feb 6, 2015)

Cheers! I will have a look into xps. Do you know if xps best laid directly on the slab, leaving a gap between the insulation and sub floor or between sleepers leaving a gap between the insulation and slab?


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## Windows on Wash (Aug 30, 2011)

I'm not 100% certain on that. If it holds true to most of the other applications in which you're trying to keep the floor surface above warm, typically you do leave and airspace towards the warm side.


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## Gary in WA (Mar 11, 2009)

The water from flower beds abutting the slab edge can wick inside unless protected by a moisture barrier at the slab edge; Fig./Photo 6; http://www.buildingscience.com/docu...nvestigating-and-diagnosing-moisture-problems

The gap should be at the top for warm toes, allowing the heat from the room to condition the cavity, same as a wall; http://www.buildingscience.com/documents/insights/bsi-064-bobby-darin-thermal-performance

Pp. 3; http://www2.dupont.com/Tyvek_Weathe...rior Insulation Building Science Bulletin.pdf

XPS is over-kill, IMO, the R-Wool you have is fine (with a vapor barrier under slab), install it with joints tight and stagger seams both ways; http://web.ornl.gov/sci/roofs+walls/insulation/fact sheets/slab insulation technology.pdf
Seal the plywood joints with house wrap tape or similar for an air barrier to stop room moisture from reaching a cooler slab. You will get little thermal bridging through the wood sleepers but your temps are mild. If worried, use EPS foam strips under them, or even fanfold- if available there. http://www.buildingscience.com/documents/insights/bsi-059-slab-happy

Gary
PS. Welcome to the forum!


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## jppie1 (Feb 6, 2015)

Gary. Thank for the reply! It is super helpful! I wish flower beds were a problem, I'm quite literally surrounded by houses. Will spend some time reading through the links. Cheers!


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## Gary in WA (Mar 11, 2009)

You're welcome! The links are like eating a restaurant meal vs fast food..... lol. I heard housing is tight over there. 

Gary


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