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Community and Q&A

Air sealing plank attic floor

baxt1412 | Posted in Energy Efficiency and Durability on

I have a house in climate zone 5 built in 1918.  The attic is approx 728 sq ft with a single peak around 8 ft and tapers down to all edges.  It is roughly a square with a staircase and a single dormer jutting out front.  There is approx 6 inches of >30 year old loose fill cellulose with a plank floor on top of it.  I know the ultimate best thing to do would be to rip up the plank floor, vacuum out all of the old insulation and air seal but we aren’t planning on being in this house more than a couple of years and it’s hard for me to give up this amount of prep and time with my current situation.  

I can seal around my bathroom vent fan, chimney, and plumbing stack with the appropriate materials but my next step planned was just to blow in additional cellulose on top of the plank floor to achieve a total of r60.  I’m estimating about R20 with the old cellulose.  

However, would putting down a house wrap and taping the seams be advantageous in achieving a better air barrier?  Would I run into issues with having the plaster ceilings below, old cellulose, new housewrap, and then new cellulose?  That doesn’t seem to be too much work – certainly less than tearing up all of the plank floor and having to dispose of all of the wood in addition to renting the insulation vacuum and disposing of all of that too.  

Or do I just go forward with super beefing up the existing insulation with additional blown in cellulose and call it a day?  I’m going to have a roofer/siding company come out and check out ventilation in the attic – The plank floor extends all the way out and there are no baffles so I’m guessing the attic intake ventilation is just through being old and leaky! The soffits do have the perforations but I’m guessing this was just because that’s the soffit material that the siding company had – either that or they’re useless right now anyway as they aren’t ventilating into the attic well with the cellulose and plank floor in the way.  

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  1. Expert Member
    DCcontrarian | | #1

    What are you doing with the staircase? That's much more important.

    1. baxt1412 | | #2

      It is a full staircase with about 5 steps, a landing with subsequent 90 degree turn and another 5 steps, a full sized access door (ie a regular door). It is a solid oak door, weather stripped, with a door sweep, and closes tightly without give. I could consider adding rigid foam to the back side but then that would also decrease the access on getting things up into the attic should the need ever arise. I didn't include in the post as I didn't think it was as pertinent, but I plan on making mini knee walls about 18" tall just to retain a very small area for storage should the need arise in the future for temporary storage - ie redoing a bedroom and needing to store dressers etc up there while doing that.

      I would be much more apt to take off the treads of the stairs than the whole attic floor as that is a much more doable project in a shorter amount of time. I could either fill in the staircase with cellulose or figure something else out. You bring up a good point though, that's a lot of volume that I could insulate in.

      1. Expert Member
        DCcontrarian | | #3

        With air sealing parallel paths are really important, it's like waterproofing only half of the bottom of a boat.

        1. baxt1412 | | #4

          I guess part of my question is just is it a huge waste to blow in a bunch of extra cellulose if it isn't perfectly air sealed? I've heard plaster ceilings themselves are a good air seal but of course I have top plates. No smoke detectors or can lights and just one plumbing stack and chimney and one bathroom fan so not a ton of other penetrations.

          1. Expert Member
            BILL WICHERS | | #5

            Ideally you want to air seal prior to insulating, but the insulation will work regardless -- but with reduced performance if you have a lot of air leaks. Blown cellulose is the least affected by this though, and the best at limiting air leakage if you can't do a proper job of air sealing first.

            DC is right though: while I wouldn't say it's completely useless to insulate and air seal the other areas, you DO need to address that staircase. A good exterior door usually comes with good weatherstripping these days, but you can retrofit old doors too. What you want to do is to make sure the entirety of the attic floor is sealed off from the living spaces below. How exactly you do that depends on what the air leaks are, and where they are located.


  2. Expert Member
    DCcontrarian | | #6

    It's hard to know for sure without seeing it, but one way is to build a knee wall the height of the insulation around the stairwell and then cap it with a hatch made from foam.

    Blown insulation is generally believed to have good air sealing properties, but there's no way to know for sure without testing it.

    1. baxt1412 | | #7

      I get it. I can snap some photos if it would be helpful. The problem with sealing the attic stairwell is the stairwell is two tiered - the first is against an exterior wall with no ledge on the exterior wall side and then it turns interior and goes toward the inside/central part of the attic. I suppose I could make a partial wall just inside that exterior wall to have a “top plate” and then make a hatch on top of that but, again, it kinda seems like more work than it is worth likely if the door has overall a good seal already. I truly do appreciate all your time and comments - I’m thinking I’m going to just take advantage of the tax incentives and blow a ton of cellulose in and call it a day. I’m sure I’ll see a noticeable improvement in comfort and I’m okay if it doesn’t affect energy bills as much but I bet it helps a bit

      1. Expert Member
        DCcontrarian | | #8

        The issue with the stairwell is that it has a lot of surface area -- the walls and the steps themselves.

        A basic principle of building science is that your building envelope has to be continuous, you should be able to draw it on the plans without lifting your pencil. This wasn't understood when your house was built, a lot of people don't understand it now. So the challenge with the stairwell is, where is the edge of the building envelope? If it's the staircase itself then you have to make sure that the stairs, walls, and bottom door are all sealed an insulated. If you can make it at the top of the stairs, parallel with the floor, it's a much smaller and more regular area that you have to deal with.

        I do see that having an exterior wall on the staircase complicates things, if the building envelope is at the top of the stairs that wall becomes part of the envelope too and has to be insulated and sealed.

      2. Expert Member
        BILL WICHERS | | #9

        To add to what DC just said, remember to feel empowered here: YOU can decide where the building envelope is in many cases. Try to define it in the easiest place to build it, such as a flat wall with a door in it instead of the entire perimeter of the staircase. Take some time to look at the structure, then decide where to define that barrier. You want to look for two main things: try to make the envelope as simple as possible geometrically, with a flat wall being best (easiest). Next, look to see how the proposed location will tie in with the rest of the home’s building envelope, again looking to make those transition points as simple as possible. The more complex the area you need to air seal, the harder it will be to do a good job sealing it.


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