Floor Box - Day 18

(2.5 hrs)

Now things are happening! Today I busted out the Roxul rock wool insulation and started installing it in the Tyvek-lined bays of the floor. I thought it would take a longer time than it did; the rock wool was quite quick and easy to work with.

As a reminder, rock wool is made principally from slag--waste rock--from the steel industry. It is melted down and blown or spun into tiny filaments, then pressed into insulating batts (i.e., soft, wooly, but cohesive rectangles of material, like felt). Fiberglass is the only other common type of insulation that comes in batts, and it has a terrible reputation for being nasty, noxious stuff that will irritate your skin, lungs, and every mucous membrane it touches. Rock wool is pretty lovely, in comparison, so I don't think I strictly needed to wear long sleeves, gloves, and a mask... but it was a little dusty, and I was feeling sensitive, so I did.

(Aside: the Roxul company also makes a rigid board-type insulation from the same spun rock material. I know it to be used on the exterior of one tiny house, but I've never seen it applied anywhere else.)

Rock wool batts

Each batt of rock wool is 48" long by 5.5" deep by about 15" wide. It also comes in a 23" wide version. Since my joists are all varying distances apart--not a regular 16"-on-center or 24"-on-center schedule--I decided to just go with the narrower batts. I figured it would make more sense to fill in gaps than to trim down virtually every batt.

I'm not sure it ended up making any difference at all, since I quickly found that cutting the batts lengthwise to fit--whether a narrower bay or a gap where the 15" was too narrow--was a mess.  

Cutting Roxul with a small hand saw

Lesson 25: Roxul cuts much better crosswise than lengthwise, at least for narrow cuts. I ended up just cutting each batt into sections as long as the bay was wide, and using as many sections as it took to fill each ~37" long bay. I used a small hand saw to cut the batts. I understand a bread knife is also suitable for the job. Since rock wool is non-toxic--it doesn't need added chemicals to fireproof it, since rock naturally doesn't burn--I could very well have used a bread knife, washed it, and returned it to the knife block. The saw worked perfectly well, too.

After installing (most) of the insulation, I went around with a can of Great Stuff and filled any gaps with foam. I also foamed the last section between the trailer and floor box, in the back.

I bought four packages of Roxul and will use only two, plus most of the third. I hope Lowe's takes returns on special-order items! Or maybe I'll keep the last package and find uses for the extra insulation. (I've been bouncing around the idea of insulated exterior window shutters...)

Location of future shower drain... sigh

I didn't totally finish installing the insulation, because two of the bays in the back left corner need the grey water drain lines to be roughed in first. ("Rough in" means to install the pieces that won't be accessible later in the build.) The plan is to cut holes in the subfloor and undercarriage and fit the pipes through, then insulate around the pipes. Unfortunately, the shower drain pipe will emerge from the shower exactly where a joist is, so we'll need to cut a bit out of that joist to accommodate the drain and a 90º elbow immediately underneath. We'll use an elbow so we can route the line away from the joist as soon as possible. We can't cut all the way straight through the joist because 1) that would entirely, rather than just mostly, compromise the structural integrity of the joist; and 2) there's a trailer rib directly under it, and we definitely don't want to cut through that.

However, Dad and I did get around to installing the first piece of 3/4" plywood subfloor! Woohoo!

First piece of plywood subfloor, and it feels great!

The subfloor plywood extends all the way to the edges of the trailer, except at the wheel wells. We (okay... Dad) cut a rectangular notch out to fit around the wheel well. It worked out so that the notches are cut from the end of the sheet of plywood, rather than the middle, which makes it easier. (Thank you, Dad, for being so good with the Skilsaw! It came out perfectly!)

As you can see in the picture to the right, the notch also circumvents the narrow ledge inside the wheel well. It didn't make sense to have plywood against metal there (or even plywood with just Sill Seal between it and the metal); it would be an invitation to condensation and rot. Instead, the insulation with which I'll eventually fill the wheel box will go right up to the metal there.

Sill seal under edge of subfloor

We did use Sill Seal between the trailer's width-extending flange and the plywood, however. I hope that it will provide a bit of a thermal break as well as hinder any air/moisture leakage into the interior of the floor.

One thing that is not advantageous about the extended-width trailer design is that there is a portion of the floor that is not insulated, where those width extensions are. There, there is only plywood and Sill Seal between the inside of the house and the great outdoors. That portion of the floor will be mostly covered by the insulated walls, but the walls are thinner than the width extensions, so there will be some heat loss around the bottoms of the walls. And whenever warm temperatures meets cold, there is a tendency for water to condense out of the air and become liquid on whatever surface is there... that surface being the subfloor. Boo. I suppose I could put insulation up under the width extensions... hmm, now there's an idea! Too bad Great Stuff doesn't stick to metal very well!

Anyway, for the record: just like with the undercarriage plywood, we glued and screwed the subfloor plywood to the joists, using a 1/4" bead of Liquid Nails, 1.5" deck screws, and a 6" (on edges) and 8" (in the field) fastener schedule for the screws.

Also for the record, boy, I'm noticing that I refer to a lot of materials by their brand name (e.g. water resistive barrier/house wrap = Tyvek, construction adhesive = Liquid Nails, spray foam insulation = Great Stuff, sill plate gasket = Sill Seal). I swear it's just an easier shorthand; I am definitely not getting rewarded for promoting the brands! And as such will feel free to bitch and moan about specific products if the need arises, as well, muah ha ha!

For example: ugh, stupid Lowe's brand house wrap tape! Shredded like crazy!

There, see?