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Insulation and vapor barrier between floors?

Fatcat175 | Posted in Energy Efficiency and Durability on

I have built a two story cape cod in zone 6a. Normally I would not consider insulation between floors but, I only have enough cash to finish one floor and plan to live in the house this way for a few years. The heating system is radiant heat, 2 zones upstairs and two downstairs. It is the Onix 3/8 radiant hose stapled tight under the Advantech floor with a 3″ gap then the foil faced bubble insulation.

The upstairs will not be insulated until I am ready to finish that level. I was planning to use Roxul 5.5″ r23 between floors for sound deadening and insulation, should I have a vapor barrior too? Or would this cause a problem as the bubble insulation is already a vapor barrior, it’s stapled every inch three inches below the floor. I did all the walls with the Roxul and did use a vapor barrior on the inside as the code says. I’m planning on using standard 5/8″ Sheetrock on everything. The floor is 12″ TJI 16″ OC. I would insulate the upstairs and not the floor but this would require me to do the electrical too.

So to sum it up the floor system is: Advantech, Onix 3/8 stapled underneath, 3″ air gap, foil face bubble insulation ( r 8 with this air gap ) Roxul r-23 5.5″ then 5/8 fire code Sheetrock. ( guessing no vapor barrior but want to be see before Sheetrock is on)

The eves have vents and will have proper vent to ridge cap eventually. Floor size approx 33′ x 44′ part of upstairs is cathedral other is under an attic. 12 x 12 pitch. Hope I included everything sorry for the long question.

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  1. Expert Member

    I have only dealt with this on projects that had unfinished basements, and most of the problems they had won't affect your situation. Our code wants the electrical panel to be within the heated envelope, wants all basements to have at least rudimentary electrical services - and that those services be mechanically protected against damage for the first four feet off the floor. Which means drywalling, which means insulation first... you see how the knock-on effects become complicated.
    In your case the only complication I can see is air sealing the second floor to in effect create a huge vented attic. Probably just continuing the subfloor over the stair opening and taping the joints, as well as sealing all penetrations would be sufficient.
    I'll be interested to see of other posters can think up anything we are missing. Perhaps the second floor has the same characteristics as a flat roof, and would be better served with the insulation on top of the subfloor?

  2. GBA Editor
    Martin Holladay | | #2

    In Climate Zone 6, you don't want to include interior polyethylene (although if you live in Canada, your local code inspector may urge you do install it). I'm not sure whether you did install interior polyethylene, but you might have (since your wrote "I did all the walls with the Roxul and did use a vapor barrier on the inside as the code says").

    For more information on the use of interior polyethylene, see these two articles:

    Vapor Retarders and Vapor Barriers

    Do I Need a Vapor Retarder?

    One problem with your plan is that your PEX tubing will be on the exterior side of your insulation, so the fluid in the PEX can freeze. To avoid this problem, you need to either (a) make sure that the PEX in your floor assembly is drained of all fluid, or (b) charge your hydronic system with a solution of water and antifreeze.

    I think that the decision to install bubble wrap in your floor assembly was a bad one, for two reasons: (a) bubble wrap is fairly useless as insulation, and (b) you now have a wrong-side vapor barrier that may cause problems. Until you begin heating the upstairs of your home, the foil-faced bubble wrap will be cold, and will be a condensing surface for moisture.

    Here is a link to an article with more information on bubble wrap: Stay Away from Foil-Faced Bubble Wrap.

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