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Spray foam sealed attic 1.5 story house question

Zuhl | Posted in Plans Review on

My wife and I are planning to build a 1.5 story home. By this I mean that we will have a game room, office, and a planned future moive room upstairs in the “attic” space. The office will protrude out of the 1 story roofline as a former, but other than that, everything is contained within the 1 story 8/12 roofline. The rooms are centered and sized such that there are no hip walls, and all have an 8 ft beam.

My question is: if we spray foam insulate this house and use sealed attic techniques, does this eliminate the need to insulate the walls of the “second” floor rooms? The way I see it, they are all already contained within the fully sealed and insulated envelope of the house and therefore the walls of those finished attic rooms should be no different than the cieling of a 1 story home. And that, as I understand it, is left uninsulated if the rood decking is what is insulated in a sealed attic home. I suppose there may be some extra heating upstairs from stack effect but this is in any two story house.

My hope is that I can essentially skip the insulation process for the upstairs walls and save the costs. In this case, the insulation costs should be the same whether those rooms were up there or not.

Can anyone advise on whether this is a good idea or fundamentally flawed? Has anyonne done this? Thanks!

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  1. GBA Editor
    Martin Holladay | | #1

    Once you have insulated the thermal envelope of your house -- which in your case will evidently include the sloped roof assembly -- then any wall on the interior of this envelope is called a partition. Partition walls don't need to be insulated. You can make as many partition walls as you want, or as few as you want. These partitions don't affect the thermal envelope.

    Needless to say, you need to define the thermal envelope of your house, and make it continuous. You need to know where the envelope is, and make sure that the insulation is continuous and that the air barrier is continuous. That means that the second-floor windows in your home -- the one in the two-story foyer and the one in the upstairs office -- are part of the thermal envelope. The walls in which these windows are installed are part of the thermal envelope and need to be insulated.

  2. homedesign | | #2

    I live in a 1-1/2 story Texas Sprayfoam House ... and if I had a do-over I would build a One Story not-so-foamy house.
    In addition to the three "dormer walls" on the front and the three "dormer walls" on the will also need to Isolate the Garage and the rear Porch from the "conditioned attic" with airtight & insulated walls that extend to the underside of the roof deck.
    So..the "bonus rooms" are not exactly a free lunch.
    An 8/12 pitch will add 20 percent "topside" surface area and the dormer walls and separation walls will add even more surface area.
    Because sprayfoam is soooo expensive is very likely that the contractor will recommend only five inches of open cell foam....well below minimum code even for South Texas.

    If you want to build a not-so-smelly "green" High Performance/Low Energy Home ...
    I think a better approach would be to ditch the "Bonus Rooms" and build an "Airtight Attic floor insulated with Cellulose Galore"

  3. wjrobinson | | #3

    An aj low E insulation option: insulate all walls, partitions, ceilings, and floors.

    1-low cost, subs availability, easy
    2-improve energy savings when shutting down rooms.
    3-sound abatement is nice added benefit.

    Many of us live in homes far larger than needed day to day. I am thinking a home like my own could decrease HVAC energy costs in half. May be trying this out soon.

  4. Zuhl | | #4

    Thank you for all the responses. This is helpful. My neighbor across the street did a sealed attic with closed cell in the walls and 11" of open cell on the rafters. They have a 3800 sq ft one story and their august electric bill was $160. That is what my current 1600 sq ft home is. However, I know they spent over $10k on the insulation. I really like the idea of the spray, but a problem I have with it, is that in Texas we have much smaller temperature differences inside to outside than up north, and our electricity is relatively cheap. So while I want a well insulated home, I am not interested in a 20 year return on investment (which is actually much longer when you consider that the money used is financed and iterest is acruing).

    I was also considering that maybe it would be a good idea to insulate the walls of the upper rooms as well for sound insulation as was mentioned. If I build them as partiion walls in a sealed attic, I wonder if sounds would transmit freely across the whole house since there is nothing in the walls or ceiling to stop it..

    Thanks again.

  5. Zuhl | | #5

    Another question: with a sealed attic, if you build attic walls to isolate the conditoned space from areas over porches or garages, what happens to those areas of the attic? Do they need to be conventionally vented since they are not insulated? Thanks!

  6. stuccofirst | | #6

    there is an acoustic drywall product that may suffice instead of insulating the interior walls. It deadens vibrations by having two different density layers.

  7. GBA Editor
    Martin Holladay | | #7

    Q. "With a sealed attic, if you build attic walls to isolate the conditoned space from areas over porches or garages, what happens to those areas of the attic? Do they need to be conventionally vented since they are not insulated?"

    A. Attics over porches or garages do not need to be vented. The only reason to vent them would be to satisfy the demands of a local building inspector. (The issue isn't worth arguing about.)

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