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Insulate between first and second floors?

Tim_in_Delaware | Posted in General Questions on

I’m finishing a fairly large (2200 SF internal, 3000 SF external walls)  two-story addition to my vacation home on the Chesapeake Bay (climate zone 4A).  It’s a much larger home than we need, but it was a foreclosure, beautiful lot, couldn’t pass it up, etc.  The shell of the addition was already completed by the previous owners – fully framed, and finished on the exterior.   The home basically comprises 3 areas:
– Original (finished) two-story 2000 SF portion where my wife and I will live when it’s just the two of us.
– Addition 1200 SF main floor (unfinished – contains powder room, dining room, great room) where we will gather when we have family, friends for the day.  Probably will use it 20-30 days a year
– Addition 1000 SF second floor (unfinished) w/3 bedrooms and a full bath.  Will use it when we have more than two visitors spending the night (probably 5-10 days a year, mostly in warm weather).  The plumbing will be arranged so that the water can be blown out of the bathroom plumbing lines (each floor independently) in the wintertime.  This portion will be insulated with fiberglass – R-39 in the ceiling, R-21 in the walls.

Given the limited use of the addition, we want to minimize heating/cooling costs (propane furnace and central A/C for main floor, and thinking a separate all-electric system for second floor).  We plan to install an exterior door at the base of the stairs going to the second floor to isolate the top floor.  However, we plan two sets of 60″ french doors between the original home and the main floor of the addition.  We’re debating between exterior doors (certainly more energy-efficient, but kinda strange to have exterior thresholds dividing your house?), vs interior doors (are there doors without thresholds that will minimize air leakage?).

Second, we’re debating whether to insulate the ceiling of the main floor of the addition (i.e., between the main floor and the second floor), as well as the 2×4 walls between the addition and the original house (both floors)  I’m assuming we would just use fiberglass.  Is this a good use of several thousand dollars?  If so, I assume we should use kraft-faced, treating the unconditioned spaces as if they were outside areas?

Any comments regarding the doors and the insulation question would be most appreciated.

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  1. Expert Member
    BILL WICHERS | | #1

    For your “external doors indoors”, what kind of floors are using? If you’re using hardwood, a transition piece under the door and a rubber sweep would probably make an unobtrusive seal. The hardwood floor transition pieces have beveled sides and ride up on top of the flooring. This would serve to give a slightly raised spot in the floor for the door sweep to seal against, but wouldn’t be a trip hazard and wouldn’t be a full exterior-style threshold.

    Between floors you don’t normally need faced insulation except as an installation aid. In your case the other part of the house, if in “unconditioned mode” might count as outdoor air. I’m not sure on this, but I’m sure others on here will comment. If you’re planning on fully shutting down the unused portions of the house and letting them freeze, you may have some unexpected problems with interior trim details due to expansion/contraction. It may be better to “minimally condition” these unused spaces and only heat to maybe 45-50 degrees or so.

    Regardless of whether or not you need a vapor barrier, you need to insulate between all the zones. That includes walls and floors. You need insulation between any two areas that will be kept at different temperatures for any period of time. You may want to consider mineral wool over fiberglass if you also want to maximize sound attentuation between zones. It’s a good time to think about sound transmission between zones if that’s of concern to you since there are other details beyond just insulation to think about.

    You will want a fully zoned HVAC system with different zones for each space. You’re essentially building condos so each needs to be seperate from a utility standpoint (heating/cooling/water) so that you can shut down the unused spaces without affecting the occupied ones. Make sure all your water lines are pitched back to your utility room or wherever you will be setting up to blow out lines. In my cottage we do this every year, and it’s important to ensure there are no low spots in water lines where water will pool and not be blown out. This is very important if you don’t want to worry about freezing.

  2. Trevor_Lambert | | #2

    Are you planning on just shutting off the heat to the second floor, and allow it to settle to whatever temperature the weather and insulation dictates? If so, minimizing heat loss from the first floor might be worthwhile. You could probably determine this through mathematical modeling. On the other hand, if you have any target temperature at all that you're going to maintain, I'm not sure there's any point; heat you lose through the ceiling will be heat you may have otherwise had to put up there anyway.

    For the french doors, I would be looking at making some custom interior doors, with insulation equivalent (or better) to exterior doors, but without the structural/security enhancements that you don't need. Maybe a simple sweep on the door bottom that would greatly reduce air leakage.

  3. user-2642926 | | #3

    Backing up Zephyr7's sweep thought, here's some nice product that I used in my house (admittedly, for the door into the attached garage.) They make several products and are easy to install if you have access to a router.

  4. GBA Editor
    Martin Holladay | | #4

    I'll reinforce advice you've heard:
    1. Installing insulation to separate the sections of your home that will sometimes be heated from the sections that will sometimes be unheated makes sense.

    2. I have installed "a transition piece under the door and a rubber sweep" -- also described as "a hardwood floor transition piece that has beveled sides and ride up on top of the flooring" -- under an interior door and I agree with Zephyr that such a transition piece is unobtrusive.

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