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Old 01-15-2008, 06:22 PM   #1
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Insulation headache - Do I need a vapor barrier?


I think I am going to insulate the basement joists over my enclosed porch. The basement area under the porch does not get very warm and I have a lot of cold air intrusion up into the porch through the floor.

- Do I need a vapor barrier? My basement is pretty damp (moisture problems to be remedied at a later date). If so, would it go 'up' against the porch flooring? If I use a backed fiberglass batting do I have to cover it with drywall? or only if the kraft paper would face out?

I am dizzy trying to figure this out.

LFW

PS - Joists are 6 inches deep - I want to get at least R-25


Last edited by Leah Frances; 01-15-2008 at 06:33 PM. Reason: addenda
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Old 01-15-2008, 06:57 PM   #2
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Insulation headache - Do I need a vapor barrier?


Use unfaced insulation and install the vapor (poly) barrier on the side that is "exposed" (over the underside of the joists).
DO NOT use insulation with craft-faced and the poly barrier or you will create a "chamber" that can trap moisture and breed mold.

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Old 01-15-2008, 07:04 PM   #3
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Insulation headache - Do I need a vapor barrier?


I would solve the air infiltration problem before adding any fiberglass. Fiberglass does nothing to keep out the air..it just filters it. Use a few cans of sprayfoam and foamboards if needed to stop the air first. Then do as Atlantic says.
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Old 01-15-2008, 07:13 PM   #4
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Insulation headache - Do I need a vapor barrier?


I thought I couldn't use foam board unless it was covered by drywall. Because of the fire issue. Would the foam board act to create a 2nd barrier?

This part of my house is circa 1880s. There are actually two floors to the porch. The first is severely canted away from the house; and is covered by the second, a level pine floor spaced up to make it level (the thinnest part is 5 or so inches- I think it might be circa 1940s).

To get the R-25 will I have to add to the joists? They are only 6 inches deep.
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Old 01-15-2008, 07:21 PM   #5
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Insulation headache - Do I need a vapor barrier?


Regardless if you use foamboard or fiberglass you will have to cover it if its in a living space. What is your basement like? Is there a dirt floor or are the walls that bad? Is this actually a crawlspace? If so I would insulate the walls. My point was just that solve the air issues first and then you can add any type of insulation later. Most people think that fiberglass insulation will solve air intrusion problems but it isnt true it barely even slows it down. They even use it in some furnace filters after all.
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Old 01-15-2008, 08:12 PM   #6
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Insulation headache - Do I need a vapor barrier?


The short version of the basement is that the room under the porch is not living space.

The long version: The area under the porch consists of two parts (like the floor). One portion @ 24% is over a crawl space, appears properly insulated. The other 75% is over basement - that is five feet below grade.

The house is brick and mortar. Most (but not all) of the basement walls are plastered, most of the floor is concrete; some of it is herringboned brick over sand (gorgeous!); there is @ 8 square feet of the area directly under the porch that is sand (excess moisture in the basement seems to drain into this area). The 'room' under the porch is adjacent (connected by a doorway) to the room where my oil-fired boiler lives (hence the desire to stop the drafts as much as possible). It is used for storage.

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