Installing flooring on top of decking

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T

TMackey

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Hi all,

I bought a fixer upper in the Santa Cruz mountains and am asking for your advice regarding the following situation:

The previous owner built an extension to the existing house but instead of doing it properly, she just first extended her outside deck (2x6 cedar boards on piers and posts) and built the wall frames directly on the deck. She didn't put down any flooring but just left the deck as is. There's also no insulation or any kind of moisture barrier under the deck.

I want to turn the room into a work space so I'd like to know how to put insulation and flooring on the decking without it rotting in a few years. The crawl space is so small that I can't get under the deck.

Thanks for your help.

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I would nort worry so much about insulating the floor, most heat loss is ceiling and walls.
To raise the floor with insulation and plywood would work but you would have to raise the door, that would depend on the structure above it. There maybe a 2x? on the flat that could be removed but as it was a deck you maybe right up against a beam or worse right up against the roof.
 
Thanks, Nealtw.

If I skip the insulation but put down subflooring, what kind of moisture barrier should I use, if any?

And do you think that the moisture rising up from underneath the house will rot the decking if there's subflooring sitting on the it and obstructing normal flow of air?
 
You deck likely treated and has good ventilation, A sup floor is consider a vapour barrier. What is the height difference between deck and house floor. If you have room, you might look into foam under sub floor. I think they do that in basements.
 
Ok good,

so it sounds like I don't need a separate vapor barrier then.

The height difference between the deck (which is also the pantry floor) and the kitchen floor is 3/4" (the height of sub flooring in the kitchen). But the front door leads directly to the pantry, which means I have to cut s strip off the bottom of the door once I install the flooring in the pantry, in order to be able to open the door.

True, I could put some of that hard foam under the subflooring. My only concern about that is that the floor could end up being squeaky.
 
But the foam would make your floor higher than the kitchen. With a decking below, i would not worry about squeaks as you could put lots of screws. But I would still like to hear from someone that put foam there.

You mentioned moisture before, You should have some kind of drainage around the unit. And two people could work a sheet of 6 mil poly under there..
I have dug a trench around one of these thing, I sloped the dirt under the edge of the building and then ran the poly under it so it went down the slope into the trench and filled the trench with gravel. But you also need a place for water to go. That way any water that gets in around the edge of the build runs out and away.
 
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