1947 Cape Cod Attic Insulation Advice Needed

Discussion in 'HVAC Heating & Cooling' started by lordoftheflies, Jun 29, 2019.

  1. lordoftheflies

    lordoftheflies Member

    Joined:
    May 28, 2013
    Location:
    NY
    Yeah no active equipment in any of the crawl space.

    I started blocking off the joist bays with the 2" xps foam. Fun times as I had to rip up one of the floorboards - no access to get the spray foam can in there. I will have to remove two rows of floorboards on the other side.

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  2. lordoftheflies

    lordoftheflies Member

    Joined:
    May 28, 2013
    Location:
    NY
    Did the third knee wall crawl space today.

    I'm concerned about the missing top plate in the knee wall itself. On the other two I the faced insulation (faced the wrong way) kind of lent itself to making it easy to just spray foam where the foam board meets the knee wall. On this one there was only unfaced insulation that's thicker than r-19 that was way too thick and kept falling over and was held in place with cardboard and spare foam pieces (all done by my contractor buddy who obviously doesn't know what he is doing).

    So, I think my plan of action now is:

    1. Remove all the knee wall insulation
    2. Install blocking in each knee wall stud bay with 2x4s.
    3. Buy some R-15 rockwool for the 2x4 stud bays
    4. Buy some more foil faced foam board

    How does that sound @Dana?

    The only "active" thing in any of the knee walls are the pull chain light fixtures. Actually, on the one I was working on today the traditional incandescent bulb was touching the faced paper of the insulation and it was all black around the bulb. I can't believe I never noticed that before. I moved it lower (needless to say I know).

    On the other side of the rockwool is the bathtub.

    2019-08-13 15.15.39.jpg

    Below you can see how much the existing batts are sticking out of the 2x4 studs.

    2019-08-13 15.15.47.jpg
    Installed more 2" xps foam board in the joist bays to air seal.

    2019-08-13 17.30.25.jpg

    What a mess.

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    2019-08-13 17.30.37.jpg

    I can't tell what it is but it's quite a bit thicker than R-19 (you can see the R-19 that's faced and it doesn't stick out that much)

    2019-08-13 17.30.42.jpg
     
  3. lordoftheflies

    lordoftheflies Member

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    Location:
    NY
    Actually I only had to remove one row of flooring from the 1st knee wall space.

    Now all 3 spaces have the joist bays blocked off and foamed. I will have to remove 2 built in drawers in the remaining section of knee wall space in order to insulate.

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  4. lordoftheflies

    lordoftheflies Member

    Joined:
    May 28, 2013
    Location:
    NY
    Regarding 4.

    I was thinking maybe 2" xps foam board would be nice...but at $34 a sheet............compared to $12 for 1/2" sheetrock.....But then the foam board is R-10 vs R-0.5.

    The foil faced 3/4" foam board is $16.24 a sheet........

    Hm............
     
  5. Dana

    Dana In the trades

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    Jan 14, 2009
    Location:
    01609
    If most of the R-value is at the roof deck there isn't much benefit to adding foam board to an R13-R15 insulated kneewall. XPS is about the worst thing you could do for it, since it has the highest enviromental cost and highest $/R.

    The batts do need an air barrier on the attic space side, but that could be drywall. If you wanted to bump up the R value with continuous foam that's OK, but if you're not going to also install drywall over it don't use polystyrene (EPS or XPS), since it has worse characteristics from a fire.smoke hazard point of view. Foil faced polyiso (any thickness) is somewhat safer, but polyiso with a true fire rating is best. Since there is probably wiring in that kneewall (?) with at least some potential for starting a fire it's in a different category than using it on the rafters.

    With the batts that are too thick for the framing it's fine to leave them in place and just trim them almost-flush with the stud edges with a batt knife, leaving a half-inch to an inch of batt proud of the stud edges to guarantee a compression fit. I suspect they're R25s (8' nominal loft) designed to fit into 2x8 framing bays (7.25" deep), which when fully compressed into a 2x8 bay perform at about R24. If you leave fully an inch of compressible excess it'll be about R15 when compressed to 3.5" in a 2x4 bay.
     
  6. lordoftheflies

    lordoftheflies Member

    Joined:
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    Location:
    NY
    The roof deck has R-19 batts and the R-4.4 foil faced polyiso....so not quite enough for the climate zone.....So if I do what you say with the existing batts and 1/2 drywall that will leave 19 + 4.4 + 15 + .5 roughly = R-39.

    For fireproofness then wouldn't using rockwool be the ideal choice? I'd rather just rip out everything that's there only because it's install the wrong way anyways and I won't need a vapor barrier with the rockwool either. I think two bags of the thermal batt will be sufficient and it's easier to work with than fussing around with the batts.

    There is only one outlet on the kneewall of one of the spaces. The other two don't have any outlets and the only wiring is for the pull chain light fixture. Also, I do have AFCI breakers at the panel for all living space homeruns......

    What about adding top plates to the knee wall stud bays? Worthy of the effort and cost?

    Thanks Dana.
     
  7. Dana

    Dana In the trades

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    From a code compliance point of view you can't just add the kneewall R to the roof deck R- in fact only the roof deck R counts, even though an air tight insulated kneewall is still doing something.

    Yes, rock wool is considerably more fireproof than fiberglass.

    You don't need a vapor barrier with either fiberglass or rock wool, but both need an air barrier to perform fully to spec. That air barrier could be housewrap, drywall, foam board, masonite, MDF or any number of materials as long as it's sealed.

    Installing blocking to prevent air from moving through the kneewall attic is important for overall thermal and moisture performance. That blocking could be made of foam board, if you like. Even cut-up corrugated cardboard sealed in place would work- the idea is to prevent convection or wind currents from becoming a thermal bypass through the kneewall attic that is insulated at both the roof deck and kneewall. I'm not sure if fire codes would require a 2x4 blocking there, but using high denstity rock wool batts does a pretty good job as as a fireblock in stud cavities, even if not fully addressed by code.
     
  8. lordoftheflies

    lordoftheflies Member

    Joined:
    May 28, 2013
    Location:
    NY
    Well, I think solid blocking (foam board or wood) will be the way I'm going to go. Along with removing the batts and installing rockwool.

    Today I took out one of the built in drawers/dressers in the kneewall. Let me tell you, whoever built this should be shot. Nothing was sealed. The floor of the dresser was completely exposed to the attic...which is why the drawers always smelled terrible......because I found about 800lbs of mouse droppings in the space.

    The rockwool insulation was pitiful. In two of the rafter bays the insulation had just fallen down.

    What a disaster and I'm glad I decided to open it up...because just like everything else in this house, it was done horribly quick, cheap, and lame.

    Here it is with the drawers removed. Look at the floor. What is THAT?? There was some kind of paper faced foil.

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    No sealing done in between the drawers and the wall.

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    Look at the base of the dresser! WADUHECK?

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    This is what it looked like when I pulled the frame out.

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  9. lordoftheflies

    lordoftheflies Member

    Joined:
    May 28, 2013
    Location:
    NY
    Here I'm looking sideways towards the center of the house and there's a dormer on the other side of the wall.....The roof extends past the dormer and I won't be able to get in there to do anything.

    2019-08-15 17.30.28.jpg

    Just to the right of this picture is brick where the chimney goes up on the outside of the house.

    I AIN'T NO EXPERT BUT I DON'T THINK I'M SUPPOSED TO BE ABLE TO SEE THE BLUE SKY FROM INSIDE OF A SEALED WALL NOW AM I??

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    Oh and what have we got here? A nice buried junction box. What an absolute joke.

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    Almost all cleaned up. Will remove the rest of the poop infested insulation next time. Tacked down a piece of plywood. Not sure why they tacked down such a skinny piece of wood (they actually doubled it up on top of each other smh).

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  10. Dana

    Dana In the trades

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    Jan 14, 2009
    Location:
    01609
    Aluminized kraft facers on batt insulation was pretty common in the very late 1950s into the mid-'60s, but fell out of favor in cooler climates since they sometimes created moisture traps and mold conditions due to the extremely low vapor permeance of the thin foil. They are still sometimes used in the cooling dominated regions of the US, giving a low-E/radiant-barrier affect when facing an air gap (such as an open attic.)

    I'm also pretty sure you shouldn't be seeing blue sky through the wall. Perhaps you should only go in there at night or when it's cloudy. :)
     
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