Raised floor insulation question.

deck99 · February 19, 2009, 01:44:10 AM

Is it true that if I box in the bottoms of my floor joist that I need to add some type of ventilation? I planned to box in the bottom of the floor joists with pressure treated ply. I'll then add the insulation from the top of the joists leaving a small space between the ply and the insulation.

I've read posts where they say you need some type of air flow in your floor joists. If this is true wouldn't it work to drill a few holes with a hole saw in the bottom ply and insert a few small louvered screen covered vents for breathabilty? Or does this let too much cold air in and defeat the purpose of insulation?

Thanks,

deck

DirtyLittleSecret · February 19, 2009, 07:21:32 PM

I've been wondering what the preferred way of sealing this up was as well. Last thing I want are creatures tearing out my floor insulation or having mold bloom due to over sealing.

February 19, 2009, 10:26:57 PM

I enclosed the underside of my cabin floor with 3/8" CDX plywood. First I installed approx 8 inch wide strips of 1/4 " hardware cloth down each side adjacent to each beam. I bent that hardware cloth in a L-shape and nailed one leg to the beam and the other to the bottom of the joists.

Then 4x8 ft panels of 3/8" CDX plywood were nailed in place. The edges of the panels at the beams were spaced about 3 inches from the beam, nailed over the hardware cloth. The panels were ripped as necessary on the other side to also leave an approx 3" space.

Quote from: deck99 on February 19, 2009, 01:44:10 AM
...to box in the bottom of the floor joists with pressure treated ply. I'll then add the insulation from the top of the joists....

There is a potential disaster waiting to happen with this approach. If you get rain before the cabin is dried in you might end up having water seep into the insulated bays. I had thought of doing this very same thing as I thought it would be easier to install the insulation and the underside sheathing in this manner.

In the end I was quite pleased that I did not install the insulation first; it rained every day for a couple weeks starting the day the joists were installed. As for it being more difficult to install the insulation and bottom sheathing, I found it was easier than I originally thought it would be... I hired a couple hungry local mountain guys to do it. All I did was crawl around underneath twice to check their work and pull $250 out of my wallet.

deck99 · February 19, 2009, 11:38:22 PM

MountainDon,
Assuming all stays dry do you see any other issues with my approach?

deck

MountainDon · February 19, 2009, 11:44:59 PM

No, not really too much. Little things like having to cut here or there to install plumbing or something. It is a gamble; maybe a better bet in some locales than others, or at some seasons. It is your call; there are a number of builders on this forum whose luck with rain pretty much matched mine.

February 20, 2009, 11:58:34 AM

Okay... I need an opinion here too, as this has me scratching my head.

I was planning on covering the entire bottom of my floor joists with 1/4" shop cloth to keep the critters out. No panels for covering underneath, just the shop cloth. I had hoped to do this as I was laying the joists down, as I am planning on putting the beams down first, then shop cloth on top of the beams as I put down joists.

I had planned on then insulating from the top, and decking it. I am concerned a bit about the rain, but I'm guessing that if I don't sheet the bottom that it ought to drain pretty well. It is going to rain on me 95% of the days up there in the afternoon. From time of laying the deck to having a roof with a tarp over it, I expect 3 weeks. (yeah right

)

Intent on this cabin being a three season shelter. Maybe a winter weekend once in awhile. I had even thought about skipping the insulation underfoot.

Comments?

-f-

MountainDon · February 20, 2009, 01:02:54 PM

I have had a limited experience with damp fiberglass. Some areas developed black, mold like appearing.

This was in a PT wooden box in the ground containing some water control valves. Not quite the same as in an above ground situation.

So my answer would be depends; depends on how wet it gets and how long it stays that way. Fiberglass itself does not absorb water but the is one heck of a lot of surface area in a batt.

peteh2833 · February 20, 2009, 04:49:09 PM

I think I'm going to spend the extra money and buy the DIY Foam insulation kit and crawl under there and spray foam it. Its alittle more money but I think it will be worth it. Pete

MountainDon · February 20, 2009, 05:10:00 PM

Wear disposable clothing.

It's sticky icky stuff.

February 20, 2009, 07:27:33 PM

Do you have any suggestions on what kind of foam to use??? I will wear a protective cloths !!!!!!!!!!

Redoverfarm · February 20, 2009, 07:48:34 PM

Frank it is almost impossible to put the insulation in at the decking stage. That is unless you have a 8 man crew who could also erect the walls, rafters and sheet to get dried in on the same day or even 2-3 day window w/o rain. You could run the sheeting around the exterior to set the walls on and leave the interior open until you got the roof on but still would be limited in access to the exterior portions. If you are like others with some exceptions there is usually 1-2 weeks from the deck stage to the roof stage if you are lucky. Once the insulation gets wet it will take a long time for it to dry out and depending on the humididty and crawlspace ventilation several months.

The water will compress the fibers and you will loose what ever R Value you had to something less. It seems that fiberglass does not have the means to spring back to is original shape but rather stick together when wet.

Maybe I am off base but not by far.

peteh2833 · February 20, 2009, 08:02:23 PM

Thats why I waited to do mine. I'm going to try and get it done this summer.

Don_P · February 20, 2009, 08:27:50 PM

I've done a few with ply under the bottom side with no ventilation and haven't been bitten yet. I view it like a wall or a garage ceiling with living above, which we don't ventilate. That doesn't make me any more right than the folks promoting ventilating it. LSU and the FPL have wired up several post Katrina pier and beam foundationed houses with humidity/moisture and temp loggers and have incorporated several insulation scenarios, the report is due out in a few months, something to keep a lookout for.

If you have adequate space under there it isn't too bad, I've installed full sheets with only 1' of room (that was pushing the fun factor

) It goes best in weather where you can fully suit up.

I did manage to shoot a can of sprayfoam up the sleeve of my T shirt while unloading a truck one day. We were in a hurry, kept working and forgot about it... for awhile. I ain't never gonna get a bikini wax

NM_Shooter · February 20, 2009, 09:03:51 PM

Quote from: Redoverfarm on February 20, 2009, 07:48:34 PM
Frank it is almost impossible to put the insulation in at the decking stage.

Well heck.

Maybe I'll use 2" solid foam and call it good. Or maybe skip the insulation all together

February 20, 2009, 10:32:50 PM

You're going to be high enough off the ground that you could always add it later.

NM_Shooter · February 20, 2009, 11:36:19 PM

Except for one corner.... I'm only about 16" above grade there. It's not just the insulation, I'm worried about getting the shop cloth in there too. Maybe I'll start putting it together and take a hard look at it.

-f-

MountainDon · February 21, 2009, 01:17:44 AM

If only one corner is that close it wouldn't be too bad getting under there to work.

Of course that's easy for me to say... the guy that didn't even do his own floor insulation and sheathing.

But my guys would have no problem doing it. They would want mileage to do the job though.

Windpower · February 21, 2009, 09:52:57 AM

I have been mulling this over too

I plan to build the floor inside of the pole barn so rain/water is not an issue

but

there is no way I will be able to crawl under there to insulate it from the bottom ( did that once under an 80 year old porch over dirt and who knows what else and I had all of about 1 foot to the bottom of the joists -- I thought the itching would never stop and I blew black snot out for days -- ok I should have worn a mask but it was 95 dgrees and humid and the ex wife was bitching and well you get the idea)

so anyway that is right out

I like the hardware cloth idea to keep the critters out -- there are going to be zillions of mice until we get em trapped out -- not to mention the rat snakes (seen a few over 6 feet )

The idea I have been thinking about is to build the floor much like a side wall

frame it (say a 8 X 10 box , insulate it, hardware cloth it and put 3/8 cdx over it) then with the help of a couple friends turn it over and place it on the beams

I think I could use a block and tackle arangement to the roof trusses to lift them in place too

MountainDon · February 21, 2009, 11:05:19 AM

Keep in mind that "code" states there should be 12" minimum between the ground and the lower edge of the support beams and a minimum of 18" from the ground to the bottom of the joists.

I also believe it states you can go less, but then need to use PT wood.

I would not want to try to wrestle a boxed in section of floor into place, but that may be just me.

You will never trap out all the mice.

Windpower · February 21, 2009, 04:06:58 PM

"You will never trap out all the mice."

It's a goal

February 21, 2009, 08:47:18 PM

G/L

If you are successful I have some land in the Jemez....

Redoverfarm · February 21, 2009, 09:14:37 PM

By killing off the snakes you will have more mice. Killing prey & predator. The snakes will leave once they run out of mice.