Insulate before decking is put on?

suburbancowboy · May 18, 2011, 09:47:14 AM

I was finally able to get to the mountain lot this past weekend. Still a lot of snow up there. So I am going to start the build this weekend. my question for you all is about when to put the insulation in. My floor joists will be spaced 16" apart. I was planning on using the pink R30 insulation. It would be very easy to install it now before the floor decking is put on but you couldn't glue the decking to the floor joist. Not a big deal as I was going to use staples anyway to attach it. My other thought was to put in the insulation after the build. This would make plumbing easier and defer the cost till later. But would be a pain to put in later as I would have to crawl around in the 2 foot crawl space.

What do you all think and have experienced?

Redoverfarm · May 18, 2011, 10:00:37 AM

Bad idea IMO. Unless you are really fast framing and drying it in the insulation will get wet from rain and compress to be of little value. Yes it is a pain to crawl to install later but you will have to be under there anyway for the plumbing and the insulation will hamper you in that respect. If you are head strong on going this route you could install rigid foam but then there is another problem. If enough water penetrates your subfloor and has no avenue for escape you could be letting yourself into mold accumulation. Probably not that big of a threat but it is possible. The expense of rigid vs. batt would probably not be a good trade off just to accomplish a little labor later.

While we are on the subject of framing and floor joist might I suggest to spend a little more for Advantex or Shurfloor which will not delaminate when exposed to the elements like regular 3/4 T&G subfloor.

Well that is my $.02 worth

MountainDon · May 18, 2011, 12:33:30 PM

Absolutely no way I would put the insulation in first. There have been at least three cases I'm aware of where owner-builders did that and then has water get in. No way, no how, is my opinion. Rather than crawl around under my cabin to put the insulation in under there I hired the neighbors kid. He and a friend also nailed up the sheathing under the joists.

suburbancowboy · May 18, 2011, 01:46:54 PM

Thanks for the replies. You've convinced me to wait.

pmichelsen · May 19, 2011, 11:59:44 AM

Quote from: MountainDon on May 18, 2011, 12:33:30 PM
Rather than crawl around under my cabin to put the insulation in under there I hired the neighbors kid. He and a friend also nailed up the sheathing under the joists.

That was going to be my recommendation; I remember having numerous jobs like this when I was young.

May 19, 2011, 09:36:01 PM

I put the fiberglass insulation in before sheathing the floor, and it worked fine. Since it was a one-room, 16' x 20' cabin, I kept the floor carefully covered with a large tarp. Had no problems with wet insulation. Much, much easier than installing from below later.

MountainDon · May 19, 2011, 10:20:08 PM

I can't find the topic, but I thought there was a recent one from someone who had insulated and then screwed down the sub flooring sheets and covered it all with a tarp. After an interval it was discovered the tarp had leaked, water had gotten into the fiberglass batts in some places. Some was salvageable and some was not.

I originally thought of installing the insulation first as well to make it easier. Most of the advice I received was against doing that. So I decided to leave the insulation until the end. A day or so after the sub floor was completed it rained. It rained pretty much everyday until the walls were all framed and sheathed and continued until the roof was sheathed and tar papered. I was very happy I had not installed the insulation beforehand.

But to each their own. Some of us are luckier gamblers than others.

Also, I have seldom met a tarp that did not allow some water through someplace when laid flat. Maybe my luck in that department is on a par with the rainfall.

Possibly if "tented" the water will run away before leaking through, but then the wind issues may be increased.

Native_NM · May 19, 2011, 11:02:57 PM

If you put it in from the top, doesn't that place the paper between the joists and the deck? Is there any structural strength lost if you can't glue and screw the deck?

dug · May 20, 2011, 06:39:04 AM

QuoteIf you put it in from the top, doesn't that place the paper between the joists and the deck? Is there any structural strength lost if you can't glue and screw the deck?

The paper can be stapled to the side of the joist, leaving the top of the wood exposred.

QuoteRather than crawl around under my cabin to put the insulation in under there I hired the neighbors kid. He and a friend also nailed up the sheathing under the joists.

A good idea but make sure you supply those kids with some decent dust masks. Fiberglass insulation is nasty stuff under any circumstances, doubly so if working with it overhead in a tight space!

May 20, 2011, 08:01:07 AM

Does the side the paper is on matter from a vapor barrier standpoint? In wall insulation, the kraft paper acts as a vapor barrier and is on the heated side. I wonder what the ruled are for floors.

rwanders · May 20, 2011, 11:42:51 AM

I have always used unfaced fiberglas batts for floor insulation---then you will want to add a small mesh metal hardware cloth under FG to keep the critters out. You don't want to completely seal underfloor--lest it allow any future slow leaks to pool up in the insulation and grow mold and/or rot your floor joists.