Insulating crawl space

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Old 09-24-15, 01:32 PM
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Insulating crawl space

Hi all. Not sure if this is the right location for this thread but thought I'd give it a try. I found out last winter that the room right off of the back porch is not insulated. The walls are from when it was closed in, just not the floor. That floor got so cold in the winter that shows left sitting by the door would still have snow on them 2 hours later. I've read different things online like spray insulation but I'm sort of on a budget. My question is would it be a problem to get regular faced r30 and put between the joists underneath? I can get to them with about 4 foot of clearance between the joists and the ground. The space is vented as you can see in the pictures. I also hear about putting a vapor barrier. Would insulation with a built in vapor barrier be sufficient? Isn't that what the facing is? The space to insulate would be from about the electrical box to the back porch. Any input would be great!Name:  image.jpg
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Old 09-24-15, 01:56 PM
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What's the floor of the crawl space? If dirt, you would want a continuous vapor barrier over that, taped up about a foot onto the walls.

Insulation in the ceiling of the crawlspace is indicated if you intend to leave it vented to the outside like this.
 
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Old 09-24-15, 02:20 PM
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Hi Td,
There are two approaches, insulate the ceiling and ventilate below, or insulate the surrounding crawlspace walls and not the ceiling. In the second option you allow that space to be indirectly heated to actually add heat. I like heating it as you get warm floors.

Some of the determining factors are, what is down there? If water pipes and heat ducts, then it becomes a potential issue of insulating them.

Regardless of which approach you choose, air leakage is a major factor in making that space cold. Where the wood framing rests on the foundation is a common area where cold air pours in. of course you already have the vents, but if you insulate the walls you would close those vents.

Floors over a cold space are always going to be hard to warm up as the cooler air in that room falls to the floor pushing the warm air up.

And as ss said, a vapor barrier over that dirt floor.

Here's a link on crawlspaces:
New Light in Crawlspaces | Building Science Corporation

Bud
 
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Old 09-24-15, 04:56 PM
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So if I want to add simple batt insulation in between the joists, all I would have to do is install a vapor barrier directly on the dirt floor below? Then I can just install the insulation facing side toward the floor and be good to go? This seems to be the cheapest option for me as well as the easiest to install by myself. Can someone please explain the effect of a sheet of plastic covering the ground as opposed to stapling it to the bottom of the joists after insulating?
 
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Old 09-25-15, 08:08 AM
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The difference is in where you're stopping the moisture - before or after it's in the crawl space, basically.
 
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Old 09-25-15, 08:53 AM
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Up here in Alaska the vapor barrier always goes on the ground because chances are that condensation will occur on non-permeable surfaces and when it does we want it away from wood framing and other things that could deteriorate. Some folks extend it up the sides of concrete or block foundation walls but always stop it short of the sill plate or other wood members to avoid water reaching them through capillary action.

We also avoid using faced batting insulation against the rim joist because it can trap water for long periods. XPS rigid insulation works well for this because it blocks most of the airflow without trapping water long term. Unfaced batting is also used and is much cheaper but either way all the large holes and gaps to the exterior get caulked and/or filled with spray foam prior to insulating.

As far as I know none of the new construction here has vented crawlspaces with insulated floors. Many homes were built that way and some are still OK but none very efficient. Many of them are being converted to conditioned crawlspaces to save energy, improve indoor air quality and avoid moisture rot.
 
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Old 09-25-15, 11:52 AM
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Yes, general thought in mostly heating areas is to close the crawl space to the outside and open it to the house air. The reverse is also true in that in southern climates it's much more likely the crawlspace is sealed from the house air and vented to the outside.
 
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