# building of Room under Deck



## Frankietoo (Oct 4, 2015)

I have a raised concrete deck off of my house, and am building a room below it on a 5" concrete slab. The deck above is concrete on 5/8 sheathing and supported by 2x6 double planks on 16" centers. I want to put 2x6 installation in the ceiling , but do not know if I should have an air space or use a vapor barrier


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## keymaster (Jul 28, 2015)

Frankietoo said:


> I have a raised concrete deck off of my house, and am building a room below it on a 5" concrete slab. The deck above is concrete on 5/8 sheathing and supported by 2x6 double planks on 16" centers. I want to put 2x6 installation in the ceiling , but do not know if I should have an air space or use a vapor barrier


"Concrete deck" on 5/8" sheathing with 2x6 (sistered?) at 16"?? Is this decking such as a composite (e.g. Trex), or poured material. Structure doesn't seem adequate for the latter. 

Photos would help, and more details.


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## Frankietoo (Oct 4, 2015)

*Room under deck*

The deck was already here when I bought the house recently, but I had it checked out by the city and house inspectors and was told it was well built. So I don't know if it was some type of special concrete, but as the person was in the business I am told, he probably used the proper material. I am just not sure of the proper way of putting installation underneath for the room I want to build


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## oh'mike (Sep 18, 2009)

Because you do not have ventilation to keep fiberglass dry, I am guessing that foam,either sprayed of sheets would be the best answer.

I suggest we wait for an insulation pro to spot this thread--you have an unusual situation---


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## oh'mike (Sep 18, 2009)

Moved to 'Insulation' for you----------


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## Frankietoo (Oct 4, 2015)

I am wondering if instead of using 2x6 installation bat in my ceiling, I use 2x4 bats, would that give enough air flow as one side is open to the outside where I will put soffits. Not being much of a construction person, it sure would be nice to here from someone who knows


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## gregzoll (Dec 25, 2006)

What you have is what we call a "Cold Room". Made to keep vegetables and other goods from rotting over Winter. Also the rooms are used as shelters during bad weather such as a Tornado or Wind Storm.


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## oh'mike (Sep 18, 2009)

For air to flow, there must be an entrance and an exit---

You are describing an unvented space---fiberglass is not a good choice.

I do hope one of the insulation pros looks in on this thread.


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## Gary in WA (Mar 11, 2009)

Welcome to the forum!

Is the concrete slab above- waterproofed? Does it leak at all?

Gary


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## Frankietoo (Oct 4, 2015)

Yes it is completely waterproof, has been there for a number of years and there is no sign of any moisture


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## Windows on Wash (Aug 30, 2011)

Rigid foam, if you are taking the DIY route, would work here. 

As Gary pointed out, any moisture that does enter the concrete will have no way to dry out to either side if it does get in there. 

A humidity porous insulation would allow for condensation on the cold side of the concrete so an airtight and moisture impervious insulation is preferable here.


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## Frankietoo (Oct 4, 2015)

I have uploaded some pictures so you will have a better idea of what I am trying to do


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