Cape insulation at rafters


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Old 11-13-13, 09:35 PM
K
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Cape insulation at rafters

I need some insulation advise. I've got a 15 yr. old Cape Cod style home. It has the typical knee walls, transitions, and overhead attic. The front has two dormers and the back has none... it is a single continuous knee wall attic.

I have been in the back knee wall attic and have taken temporary steps over the years to help mitigate the problem but it really needs to be re-done from scratch.

The back attic and transition are insulated on the roof rafters. I have continuous eave vents and a ridge vent. I have pink foam baffles continuous from eaves to ridge vent. These things:
http://ehowdiy.com/images/installing...mage470-02.jpg

Laid on top of those baffles(between rafters) are some R19 unfaced fiberglass insulation. Finally a 3 - 5 mil poly is stapled on the outside of the rafters. Couple problems, the poly was never stapled well enough so its dropping, the insulation is falling, and the poly's seams were never sealed.

I also don't like the pink baffles as they don't block the top plate of the wall below or prevent wind-wash from eaves into the insulation! I can't afford to spray foam and I'd rather keep a cold roof. I know I can never get a decent R49 recommended for my area on the rafters but I feel like the wind wash is my biggest energy problem right now.

I would like to take it all down to the rafters and add 1" blocks to the sides of rafters and nail the pink fan-fold to it (sealing it all) making my own wind proof baffles. I would than add back the unfaced R19 batts and finally seal the outside of the rafters with 2" tongue and grove foam boards. This should get me somehwere around a R30.

Fan-fold:
[url=http://www.homedepot.com/p/Owens-Corning-Foamular-1-4-in-x-4-ft-x-50-ft-Fanfold-10UM/100320301#.UoReLxoQZ7o[url]
**sorry this url tag won't save properly but will work if you copy/paste into browser!

Tongue & Groove
http://www.homedepot.com/p/Owens-Cor...The Home Depot

So... my problem is will both foams (even the 1/4" fan fold) act as a vapor barrier? I only want the fan fold to act as a wind barrier. I don't want to have the batt insulation in between 2 sealed vapor barriers.

Am I better off using
 

Last edited by Kris Michaud; 11-13-13 at 10:21 PM.
  #2  
Old 11-14-13, 03:57 AM
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Why not use Tyvek? No vapor barrier and will block the wind.
 
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Old 11-14-13, 08:43 PM
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Thanks for the input. Maybe I'm not understanding you completely or your not understanding my post completely.

I thought of it but since tyvek is more of a fabric type material it would be (in my opinion) more difficult to replace my baffles with it in between the rafters... I'd be afraid the batt insulation would just collapse it and block the eave to ridge ventilation. That is why I was considering some sort of rigid material. Maybe cardboard (but if it frosts over it might just turn to a mess)?

Now if the tyvek were to be installed on the outside of the rafters I would agree... I'm trying to prevent the wind from pulling the hot air slowed/trapped into the insulation out the ridge vent.
 
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Old 11-14-13, 09:31 PM
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As a side note, I want to eventually add built-ins into the knee walls so I want to make sure I've got a decent wind barrier at the rafters before I introduce new holes into the the walls.

Maybe I'm over complicating this and should just use drywall to build my new venting cavity? I'm just afraid drywall exposed directly to the outside air would be a breeding ground for mold!
 
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Old 11-15-13, 11:31 AM
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Old 11-15-13, 10:53 PM
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Gary, those are some great links! I think I need to re-read them a few times to fully digest the information!

Actually the first link literally had me thinking of going the unvented route (if I hadn't just paid to extend my ridge vent during my metal roof install).

Page 72 (last page) seems to be where I'm going with this retro.

My retrol insulation on the front was based on an article called "Taming the beast" which was specific to capes. On the front I moved the insulation to the knee walls and removed the baffles. I used 2" foam boards to prevent wind wash against the knee walls AND used rigid foam/spray foam to seal the key junctions (under the floors and at eaves). However, I am considering moving the thermal/vapor barrier back to the rafters... but first I would like to deal with the back knee wall attic; which still has insulation at the rafters. I find that side of the house is very drafty compared to the front. If I can get a game plan together for the back attic than I can apply it to the front attics and dormers!

Thanks again for the info.
 
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Old 11-17-13, 09:16 PM
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You're welcome. That is why we are here. Glad you have it figured out. Here are a couple more showing the air infiltration areas other than the wiring/plumbing holes through the top plates below, sometimes air comes all the way from the basement (be sure to air seal there or crawl space also); Energy-Efficient Building Design

Stack effect; http://www.wag-aic.org/1999/WAG_99_baker.pdf

Gary
 
 

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