Best Insulation Under 4-season

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Old 08-26-19, 11:09 AM
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Question Best Insulation Under 4-season

Hey, I have a 4-season room, and underneath is just 2x10 joists exposed to the outdoors. What's the best stuff to use for this? Just fiberglass, or that hard foam stuff? Spray foam? Just want to use the best thing I can.

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Old 08-26-19, 01:50 PM
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Spray foam will generally get you the best R value for the space.
That being said, its not cheap, and what ever is in the space being sprayed, is there for good.
 
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Old 08-26-19, 03:33 PM
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Are the floor joists accessible from above? Or do you have to do all this work from below?
 
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Old 08-27-19, 06:50 AM
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It will be done from below.
 
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Old 08-27-19, 09:11 AM
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If you update your profile to include at least your state it would help come up with a more accurate answer.

Styrofoam sheets cut to fit the space would be both moisture and vermin resistant.
You also can get fasteners to hold it in place unlike fiberglass which would need solid sheeting to protect and keep it in place.
 
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Old 08-27-19, 01:27 PM
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Not sure what happened, I've always had MA in my profile, not sure why it isn't showing. It used to show up before the forum look changed.

Anyways, I'm thinking of just doing R30 fiberglass... that will cost about $250, as opposed to the hard foam board which would require 3 layers to get R30, totaling about $500. I'm getting a quote for the foam as well.
 
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Old 08-27-19, 02:32 PM
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As GregH said you will need something solid to protect the fiberglass. Raccoons ripped out most of the unprotected fiberglass under my 4 season room so I had it replaced with spray foam. I only had about 14 inches clearance to work in so I was not able to install solid sheet.
 
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Old 08-28-19, 06:21 AM
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But even with closed-cell spray foam, you need many inches to get to a specific R value don't you?
 
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Old 08-28-19, 06:32 AM
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I had 4 inches installed. I don't remember the R value (you could probably look it up.) I think it was the equivalent of the 8 inches of fiberglass that was removed. I'm not sure that R 30 is necessary in a floor. My floor is heated so I can't tell the difference.
 
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Old 08-30-19, 07:55 PM
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Insulation.

Is there any chance the insulation will get wet? If not, R19 fiberglass will be the least expensive and easiest to install. Next in line would be Styrofoam then poly iso insulation boards for cost and installation. The hard part of the install is cutting all the pieces to fit into the joist bays. The bays are almost never perfectly parallel and when you cut the boards they should fit snug into the bay. If they are too wide and you force them, they will break. If you cut them too small there are areas that are not insulated. Spray foam installed correctly will provide the greatest insulating value but, it is usually not a home owner job. It costs the most.
 
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Old 08-31-19, 12:27 PM
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I just purchased a 300 board ft closed cell foam kit from the box store. I was filling in a space that I had formed out on my steel building. First half went like the videos the second half not so much, big globs that expanded at differing rates. I believe it was lack of pressure, definitely warm enough tanks and surface.

You say BEST, which is going to be closed cell foam, IMO. Closed cell foam needs nothing to protect it and is semi rodent proof. R value to suit your area but surely no more than 3 inches probably 2". Bonus all the leaks and some of the squeaks will vanish when the foam is applied.
Lived on a farm house in Iowa and had fiberglass under the floors. Once a year I had to crawl under the floor and redu the insulation due to critters, especially mice. Mice turds above you,,,,,,,,,, yuck!
They do make poly sheets like Tyvek , used here as a guide. That could be put over the fiberglass, joists for a finished product and critter barrier.
Total cost of both compared to foam? Foam is more expensive but more attractive less trips below the floor
The bigger building supplies have polyurethane sheets in various thicknesses. You can cut it with a circular saw or table saw for fit to the joists. You may have large amount of waste due to your spacing and lengths. Caulk the edges for air tight.
 
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