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Community and Q&A

Open vs. Closed Crawlspace and Attic

marygordo | Posted in Energy Efficiency and Durability on

* Building a new home, zone 4 east of Seattle about 30 miles
* planning to make it very air tight, well insulated
* NO equipment of any kind in the attic
* NO equipment of any kind in the crawlspace
* open or closed (aka encapsulated, conditioned) crawl space?
* open or closed (aka encapsulated, conditioned) attic?

Thanks for any, all replies

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  1. Andrew_C | | #1

    The crawlspace should be within the conditioned envelope, built like a short basement.

    Vented attics are generally cheaper and more robust. If you’re not in a fire zone (think burning embers), I would vent the attic.

    There are a lot of articles here on these topics, use the search function. Read the articles first, before the Q&As.


    1. marygordo | | #2

      Appreciate the reply. Most everything I could find before asking the question is information quite dated.

      1. Expert Member
        Akos | | #3

        +1 on Andrew's suggestions.

        A sealed and insulated crawlspace is cheaper to insulate than the whole floor of the house since it is a much smaller area. It is also much more energy efficient and gives you much warmer floors. About the only drawback is that if you are in a radon area, you need to add in sub-slab venting, which is pretty simple if done upfront.

        The other benefit of the a sealed insulated crawlspace is that it is the same temperature as the house, which means it is a great spot for both ducting and mechanical equipment.

        So you want a sealed crawl and feel free to run ducts in there if convenient.

        Simplest attic is a vented attic built with raised heel trusses with lots of insulation on the attic floor.

        1. marygordo | | #4

          Thanks Akos !

  2. Expert Member
    BILL WICHERS | | #5

    I would check how much more it would be to excavate your crawl space into a full basement. Basements are sooo much easier to work in, and they give you plenty of space for mechanicals (HVAC, etc.), and extra storage. The extra cost to excavate a full basement is often not as much more as you might think, as long as you don't have any site specific "wierdness" (really high water table, really shallow depth to bedrock, etc.).

    If you go for a full basement, you insulate the walls similarly to a crawlspace. Subslab insulation is a good idea too. If you go with a crawlspace, absolutely build it sealed and insulate the walls, not the floor of the house above.

    Vented attics are the way to go unless you're in a fire zone as previously mentioned. I would build with a vented attic, do a good job of air sealing the attic floor, then insulate with blown cellulose up to at least code minimum R value for your climate zone. I would use a full ridge vent, and a proper one -- not the "shingles over foam" kind. I like to use PVC trim board soffit material that has integrated vent slots, but there are other options out there for soffit venting. Try to have a little more intake (soffit) vents compared to outlet (ridge) vents, so that you don't depressurize your attic.


    1. Expert Member
      MALCOLM TAYLOR | | #6


      I hate basements.

      There is a big jump in what our code requires once you move from a crawlspace. There is effectively no such thing as an unfurnished basement anymore. It has full electrical and insulation - and those have to be protected up to the five foot level, meaning you might as well drywall the whole thing. Once you are done, having spent much the same as on the above ground rooms, you are left with poor living spaces, with little light or access to outdoors, that are prone to damp and flooding - which are more likely due to the depth. I'd never include one unless there was no other option.

      1. Expert Member
        BILL WICHERS | | #7

        Well that's disappointing! The biggest issue I'd see then is finding somewhere else to fit mechanicals. Attics are obviously not a good option there (and maintenance on things in attics is miserable). Do you include a mechanical room in your new builds instead, or do you squeeze things into crawl spaces (which are only slightly better than attics for maintenance)? I suppose since I started out as an installer, I think about maintenance issues more than some :-)

        BTW, if we don't have basements, think of the hardship producers in the future will have when trying to find a suitable scary location for their horror movies... ;-)


        1. Expert Member
          MALCOLM TAYLOR | | #9


          Part of my preference comes from living in an area where we typically only have to excavate down 18" for a slab, and the chances of encountering rock if we go much farther are pretty high.

          To me the most adaptable house is one with a trussed roof and conditioned crawlspace. The majority of the houses I design have cathedral roofs with a slab foundation. On those I usually include a flat ceiling over some of the service rooms - either the laundry or bathrooms - and use the area above for mechanicals.

      2. Expert Member
        Akos | | #11

        I think this basement/no basement falls into similar category as strapping out the ceiling, certain areas it is the only way.

        In my area, having no basement is practically unheard of. Even most cottages up north have it, if the bedrock is high, the whole house is lifted to build a "basement" that is almost at ground level.

        At least in the city, I think it comes from the fact that basements are not part of gross floor area, so it is the easiest way to add square footage without getting a zoning variance.

        I can say that I would definitely miss my basement if I ever had to give it up.

  3. plumb_bob | | #8

    I also have a strong dislike for basements. Joe Lstibruk says "they either have leaked, or will leak", or something to that effect. A good 4' crawl will offer fair access for mechanical and maintenance, and can be constructed so that the odd wetting should not be too hard to clean up.
    We should be building structures that can last for 100s of years, I think it is fair to expect enough crazy weather and events within that time frame that many basements will flood.

    1. marygordo | | #10

      Zephyr7, Malcolm Taylor, plumb_bob
      The above comments, suggestions were greatly appreciated. Thanks for taking the time to respond. Each comment gave me more to think about.

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