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Old 01-22-2012, 01:08 PM   #1
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Insulating the attic in my Cape cod


have a cape cod and my attic space needs some insulation (there really isn't any) and I'm considering blowing some insulation up there but there's a caveat.

I 'drew' a picture w/ microsoft paint to help depict my dilemma.. in the 'attic' there is rolled insulation (the red line) on the attic side of the upstairs 'knee' walls. on the floor of the attic they installed 'sub flooring' to use as additional storage space. under the subflooring there is nothing but air, then the drywall from the lower level living space (see the 'box' on the top right side of my picture).

the triangular portion of my attic needs some insulation. there is a ridge vent for the roof (above the upstairs living space) and there are soffit vents where the lower level walls meet the roof line behind the gutters.

What i was considering doing was putting a hole in the attic subfloor near the soffit vent (to allow air to escape while filling the gap) and a hole near the upstairs knee wall (to use as the entry point for the cellulose).. my question is, the joists are only 2x8 so the max thickness of insulation that I can blow in there is 8".. will this be sufficient thickness to be effective ?
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Old 01-22-2012, 02:31 PM   #2
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Insulating the attic in my Cape cod


Not really.

8" is not ideal nor code compliant unless it is 8" of a much higher R-Value than fiberglass.

I would cut back a section of the sub-floor to allow access to the joist cavity that extends under the finished floor. Install foam blockers and seal them to prevent cold air from entering that interstitial joist cavity.

If you use the space for storage, feel free to leave the rest of the sub-floor and make sure you have 8" of insulation in there.

If you cut a large enough hole in sub-floor and there are a bunch of fasteners sticking up through the drywall ceiling, you could slide 4" of foam in there and finish it off with a 3.5" fiberglass batt. This would give you about and R-35+ in that cavity which would be a huge improvement in the insulation layer.

Make sure you seal up that outside top plate as well while you are doing the rest of the sealing work.
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Old 01-22-2012, 05:28 PM   #3
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Insulating the attic in my Cape cod


what if I blow insulation into the cavity, then unroll a higher R value on top of the subflooring that's in there ? I have no intentions of using this space for storage.. I simply want to keep the heat in the house that I'm paying for..

Further, the attic space above the upstairs living space is not insulated (that I'm aware of).. I do not have a trap door to get myself up there.. could I blow a bunch up there from the sides ? the knee walls do not meet the roof decking.. there is ample space to get the insulation blower up there and blow a ton of cellulose up there, obviously I would want to get the baffles on the underside of the decking to keep the air flowing from the eaves to the ridge vent, but I think I can get enough cellulose to blow up there ... maybe not quite the 15" that's required by code, but I believe I can get 'some' or 'quite a bit' up there which would be better than the none that I believe I have now..

thoughts on blowing into the joist cavity (to fill the space) then roll some high R value on top of the subflooring ?

blowing above the finished space from each side ?

Thanks for the input.. it is much appreciated.
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Old 01-22-2012, 05:39 PM   #4
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Insulating the attic in my Cape cod


That is fine too.

If you are going to blow insulation, just blow insulation over top of the board as well. The only thing you need to make sure of is that you don't block the soffits so that you obstruct soffit incoming air.

If you blow insulation up in that attic, you may obstruct the ridge venting and create ventilation issues.

Cut a small inspection hole and see what is up there. If there isn't any insulation, you can add it as long as you don't block the top side of the chute into the attic.
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Old 01-22-2012, 05:51 PM   #5
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Insulating the attic in my Cape cod


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That is fine too.

If you are going to blow insulation, just blow insulation over top of the board as well. The only thing you need to make sure of is that you don't block the soffits so that you obstruct soffit incoming air.

If you blow insulation up in that attic, you may obstruct the ridge venting and create ventilation issues.

Cut a small inspection hole and see what is up there. If there isn't any insulation, you can add it as long as you don't block the top side of the chute into the attic.
I would definitely get baffles for the soffit vents so they're not blocked, then drill holes in the subflooring near the eaves (for air to escape as the cavity is filled) then a hole near the bottom of the knee walls and blow towards the soffits to fill the cavities...

Should I blow insulation under the upstairs living space or close it off and use expansion foam to seal each section ?
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Old 01-22-2012, 06:08 PM   #6
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Insulating the attic in my Cape cod


Close it off and seal it.

You don't need to drill holes in the board to vent that space. Blow insulation over top of it to a level that is deep enough. If you do that, the wood wont get to dew point and the insulation underneath of the board is not air tight so it will allow for some convection.
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Old 01-22-2012, 06:17 PM   #7
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Insulating the attic in my Cape cod


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You don't need to drill holes in the board to vent that space.
I wasn't really going to drill holes in the board for ventilation, I was thinking when cellulose is blown into a wall cavity from the outside (retrofit) they drill a hole at the bottom (for access) and at the top of the cavity for air to escape as the cavity fills w/ insulation. I guess I was thinking about the cavities that I have as 'sideways wall cavities' in that I should drill a hole near the eaves (of the cavity) for air to escape as the cavity fills with insulation and use a drilled hole near the knee wall for access to fill the cavity.

You're suggesting just make an access hole (or remove a board or two for access) and fill each cavity w/ no hole near the eaves for air to escape as the cavity fills ?

(obviously I'm all for doing less work but wanted to make sure I could get each cavity as full as I could LOL)
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Old 01-22-2012, 07:24 PM   #8
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Insulating the attic in my Cape cod


Fill the space via the access that you will need to cut in the subfloor to proper install the blockers in the joist bays.
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Old 01-22-2012, 07:30 PM   #9
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Insulating the attic in my Cape cod


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Fill the space via the access that you will need to cut in the subfloor to proper install the blockers in the joist bays.
got it..

Thanks a bunch for all the advice..

Sure appreciate it.
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Old 01-22-2012, 10:29 PM   #10
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Insulating the attic in my Cape cod


If not using the attic for storage, I'd remove the sheathing as the cellulose below it has nowhere to evaporate the moisture that would be in the air. Without an airspace on top side of the cellulose, it could mold on the bottom side of the sheathing. Cellulose wicks moisture to a dryer area of cellulose to dry by evaporation, helped along by the stack effect, the way I understand it, anyway. Add foamboard (with/out suitable ignition barrier- if required) or housewrap to the attic side of kneewall: http://oikos.com/esb/51/sideattics.html

Gary
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Old 01-23-2012, 05:37 AM   #11
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Insulating the attic in my Cape cod


Gary's option of removing the board is ideal.

If you are cutting it back to install the blockers, you can just rip another chunk out of the middle for that matter.

If you are blowing cellulose over the top, it should not make a huge difference and especially given the small amount of square footage and if you seal up the top plate (exterior).
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Old 01-23-2012, 06:27 PM   #12
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Insulating the attic in my Cape cod


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Gary's option of removing the board is ideal.

If you are cutting it back to install the blockers, you can just rip another chunk out of the middle for that matter.

If you are blowing cellulose over the top, it should not make a huge difference and especially given the small amount of square footage and if you seal up the top plate (exterior).
you're suggesting blowing on top of the sheathing as opposed to under AND above ?

I could try and rip it all out but that's alot of work LOL.. (also might do the tyvek on attic side of knee walls.. didn't give that much thought)

Thanks for all the input.
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Old 01-23-2012, 06:31 PM   #13
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Insulating the attic in my Cape cod


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If not using the attic for storage, I'd remove the sheathing as the cellulose below it has nowhere to evaporate the moisture that would be in the air. Without an airspace on top side of the cellulose, it could mold on the bottom side of the sheathing. Cellulose wicks moisture to a dryer area of cellulose to dry by evaporation, helped along by the stack effect, the way I understand it, anyway. Add foamboard (with/out suitable ignition barrier- if required) or housewrap to the attic side of kneewall: http://oikos.com/esb/51/sideattics.html

Gary
the construction of my second story resembles the second picture on the page that you have linked to.

I will double check how well it is all sealed together next time I crawl out there.

Thanks for the link
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Old 01-23-2012, 10:01 PM   #14
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Insulating the attic in my Cape cod


That picture shows a subfloor with f.g. below/above, which is fine, just like a wall on its side. Cellulose needs the air space above the top layer to evaporate moisture out from wicking. Plywood/OSb is a vapor retarder (Class 2, 0.75 perms) which you want on the wall to dry to the inside, but in an attic, it can't because it is the "first condensing surface": http://www.buildingscience.com/docum...bout-diffusion

If it didn't have the vapor drive (stack effect) behind it, I'd say go for it.
I'd use a skilsaw, blade depth set just bit more than sheathing, cut alongside each joist to remove in sections, leaving the two inch wide pieces glued on the joists.

Gary
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Old 01-24-2012, 07:21 AM   #15
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Insulating the attic in my Cape cod


Cut back the sheathing to put in the blockers and seal of the exterior top plates. Make sure you seal any other penetrations in the drywall ceiling that the board is over top.

Once that is done, blow in insulation under the board and on top of it.

You should not have enough diffused moisture to create any issues for the plywood and blown in fiberglass will allow for convective air movement anyway.

Having the board inside the insulation will keep its surface temperature far enough away from dew-point that you should not have any condensation issues on the board itself.
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