Floor insulation question

[Chalco]

Hi, I'm building a workshop separate from the house. It's going up on pilings. The inspector who approved the pilings said that I should get R30 for underneath the floor. So I got some. It's 10 inches in depth which is more than the joists. Is this ok? Should I just staple the insulation to the joists and use chickenwire to squish it up and staple the chicken wire to the joists? Lastly on the Certainteed website they said to put plastic on the ground to reduce moisture. Is this what I should do?

Any advice at all is greatly appreciated!

[Longhorn]

1. The R-value of the insulation is based on the full thickness.

Reducing the thickness by squishing it to fit would reduce the R-value, in which case, you might as well buy insulation with a lower R-value that will fit into the joist space. In our area (Central Texas,) R-19 is recommended for crawl space.

Other than finding some way to rig your floor framing to fit the insulation such as replacing the joists with thicker ones or adding on to the existing joists or loosely draping nylon mesh under the R-20 insulation so that it doesn't compact the insulation, my husband thinks you're better off using a lower R-value. Most heat loss is through the attic. The greatest benefit from crawlspace insulation is air infiltration control as opposed to heat loss control.

In answer to your question about putting down plastic, my husband says, yes, but even more important is adequate ventilation through the underpinnings. A lot of this depends on where you live.

That said, he remembered an article in the August 2004 edition of The Journal of Light Construction about how to control moisture in the crawlspace by sealing it. This method also eliminates the need for insulating the floor because you seal the ground and insulate the crawlspace walls instead. Here's a link to the article: http://www.jlconline.com/cgi-bin/jlconline.storefront/4217601f000e845a27187f0000010535/Product/View/0408fixi You have to pay $2.95 to read the whole thing if you're not a member but I have the article here if you'd like me to copy it and fax or mail it to you.

I hope this helps! :hi:

[Chalco]

2. Thank you so much

Now for another question. I don't have a crawl space per se but a one story building built on pilers (spelling?) . Right now it is open all the way around underneath the floor. So according to what you said I should insulate with R19 and the put up some kind of barrier...like what?

[Chalco]

3. Just got the article, thanks

[Berserker]

4. The only problem with using a lower R-value

is the fact you have to go by the building code. And it will be inspected. In Minnesota we get temperatures of 20 below zero and colder and this type of addition needs to be insulated as per code. We have built this type of addition you are talking about. We installed 1/2 inch treated plywood on the underside of the joists and then installed the insulation then the sub-floor. I agree that using floor joists in the place of dimensional lumber would be a better way to go. The span dictates the depth of the joist. And local code dictates the R-Value. Also yes it is true that you should lay a sheet of poly on the ground.

[Berserker]

5. Chalco

When you drop the R-Value and build that addition, or insulate with a lower R-Value the building inspector will make you redo the project. If you have a building permit (and you should) you will also have and inspection.

[Chalco]

6. Thanks

The inspector's coming Tuesday am to check out the joists, so at that point I can just ask what I need to do. He's the one that suggested R30 to begin with and I already bought it. So I'll see what he says about the situation.

The work so far looks good. Pilings are excellent and firmly placed in cement.

We'll see what happens Tuesday.

[Berserker]

7. Let us know how it goes Chalco

[Chalco]

9. Passed inspection. While he was here I asked again about

the insulation. He agreed that R19 would be better given that the joists were 7 inches, heat goes up and I'd have to extend the joists in order to fit the R30 in. So the plan is to put R19 between the floor joists then put treated plywood underthat, then put a vapor barrier on the ground and lattice around the sides.

I'm saving the R30 I bought for the floor and putting that in the ceiling.

Thanks for all your help!

[Kolesar]

8. If you squish 10" of insulation into a 7.5" space...

...I don't think it will cost you a whole lot of insulating capability. That is only compressing it 25%. You will still have about R-20 there. The reason I say that is because Owens Corning makes a "dense" 3.5 inch wall insulation that gives you R-15, where as most wall insulation is only R-13. I did a lot of reading about insulation with my complicated house projects.

Here is a url about floor insulation
http://owenscorning.com/around/insulation/project/floors.asp

Here is an insulation-stuffing mention from the "walls" page:
http://owenscorning.com/around/insulation/project/exteriorwalls.asp
"2x6 stud walls allow for more insulation because of their extra depth. Depending on local codes or your desired R-value for the wall, you may use 5 1/2" thick R-21. R-19 fiber glass batt insulation (6 1/4" thick) can also be used. Compressing it into a 2x6 cavity will lower the R-value to R-18. "

So if you compress 6.25" insulation to 5.5 inches (about 12%), you lose ~5% of the R-value.