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Old 11-05-2011, 12:26 PM   #1
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Insulating Between Joist on a Outside Wall


durign are home inspection the inspector told me that i should put some isulation up between the joists on the outside wall (Appro 1.2 sq ft sections). I bought Roxul R14 to put up there and all the other stuff, vapour barrier, black adhesive silicone, and staple gun. When i pick up the stuff the worker at Home Depot said not to seal off the the bottom the new vapour barrier i am about to install. But to me tht woudl allow air to get in a cause condenstaion.

So my question is do i seal the whole piece of vapour barrier, or do the leave the bottom section unsealed.

This guy had a "I am in Training" button net to his name.
Thanks for the help.

I have included a picture if you would like a visual aid.
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Old 11-05-2011, 07:53 PM   #2
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Insulating Between Joist on a Outside Wall


The rim joist area should be insulated. If you're using Roxul, cut it a bit oversized and friction fit it in the joist bay. No adhesive is required.
Posting where you live will help with the responses.

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Old 11-05-2011, 09:32 PM   #3
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Insulating Between Joist on a Outside Wall


when it comes to insulating the ribbon where the joist bays are open to insulate i use 2" rigid foam and use spray foam around the edges of hte rigid foam to create a seal
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Old 11-05-2011, 10:45 PM   #4
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Insulating Between Joist on a Outside Wall


Were I you, I'd read about vapor barriers before I buttoned up the place. There are good reasons to NOT have a true vapor barrier, but rather a vapor retarder. Buildingscience.com has excellent information about them, and don't just install them "because that is what everyone does". It depends on where you live, whether or not you have air conditioning, below/above grade, etc. Make sure you have to have one before buttoning up. Regardless, air sealing is of utmost importance.
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Old 11-06-2011, 08:57 AM   #5
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Insulating Between Joist on a Outside Wall


I used kraft faced r-13, due to I did not want a air tight basement. I did go around and seal any cracks with caulk and spray foam, and also covered our old windows with foil faced foamboard. Before you do anything, go through and seal any joints on the ductwork, and also cover hot water pipes with foam. Replacing the dryer vent helps, also placing a closing vent on your bathroom vent fans also helps. Also changing out that fiberglass on the basement walls with 2" XPS foam board would be better.

Building science has a lot of good info, and this subject has been beaten to death on here also.
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Old 11-06-2011, 10:27 AM   #6
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Insulating Between Joist on a Outside Wall


As said, the plastic faced f.g. batts can be a problem: http://www.buildingscience.com/docum...ent-insulation

Because the rim joist is cold on the inside face, condensation will form from the warm room air. Faced batt insulation or Roxul will not stop this air movement, actually making matters worse because you are blocking the warming of the rim with the insulation. http://www.buildingscience.com/docum...-at-rim-joist/

2” foam board (0.75 perm.) slows the moisture drive and stops the air leaks (edges canned foam) ----not the facing (1-5 perm.) on f.g. which changes its permeability rating with moisture (hygroscopic): http://www.buildingscience.com/docum...vapor-barriers

Faced fiberglass batt inside only if foamboard outside, Fig. #3: http://www.buildingscience.com/docum...ce-insulation/

“So my question is do i seal the whole piece of vapour barrier, or do the leave the bottom section unsealed.------- yes, seal the whole piece. You need to stop the inside air from reaching the rim joist--- page 10: http://www.buildingscience.com/docum...apor-retarders

This guy had a "I am in Training" button net to his name.” ----- training to direct you to the proper aisle, not give correct answers.

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Old 11-06-2011, 01:32 PM   #7
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Insulating Between Joist on a Outside Wall


gbr's last point is very key.. NEVER and i mean NEVER ask young people who work at big box stores advice on how to do something. the info the big box store employees is wrong almost 95% of the time.. the only guys to ask at these stores are the old timers who are retired from their trade. their bodies are worn out and they cant do the work anymore but they want to stay involved some how.. the young staff are simply repeating what theyve been told in training seminars and god only knows who gave the seminar

i dont know how many times ive been in a bigbox store to pick something up and another customer has asked an associate "what to use" or "how do i do this".. i hear the response and bust out laughing or just get a look of disgust on my face at what they tell the person.. its that bad honestly

as for my method of fitting rigid foam up into the space with the rim board, this is a standard practice for r-2000 homes in canada when spray foam insulation isnt used up in the rim board to give it a better seal
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Old 11-06-2011, 01:57 PM   #8
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Insulating Between Joist on a Outside Wall


I am in Calgary, Alberta, Canada.

So it sounds like i should put rigid foam against the wall then spray foam the edges. Then follow up with a batt of insulation and then seal off with a vapour barrier.

is that whats you think should be done.

Thanks
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Old 11-06-2011, 04:46 PM   #9
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Insulating Between Joist on a Outside Wall


if you use extruded foam and a bead of spray foam around the edge you have a vapour barrier right there. any fibreglass after the fact will help insulate that much more

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