Saturday, October 11, 2014

What if: Jack Frost not welcome here!

One of my complaints about the van is how quickly the interior heats up on hot days and cools down on cold,days, though, because of radiant energy, the heating is more drastic than the cooling.

True, having a structure that isn’t essentially a steel box is a good start towards resolving this, but a little insulation never hurt anything either.

Planning for 2” thick walls, and allowing for a ¼” skin on either side of the wall, leaves 1.5” for structure and insulation.  

I could carry that even further and plan for thicker walls with more insulation, but since the windows, poorly insulated single pane RV style, will comprise about 35% of the exposed sidewall area, and the structure, also a relatively poor insulator, another 8% to 12% there is a point of diminishing returns on adding insulation.

I can compensate somewhat for the windows by having insulated inserts/curtains on hand to put over them on those cold, cold nights or as protection against the raging sun, (I use Reflectix metallic bubble material in the van and, judging by how hot the sunny side gets, it has proven to work very well.) but there's not much I can do about the thermal-bridging created by the structure except use wood rather than a metal of some sort because the former, though a relatively poor insulator is one heck of a lot better than steel or aluminum. Which works out really well for me since I'm definitely a wood man. No, not that kind! I'm talking about wood that grows on trees!!!

In addition I'm looking at using a roll-on bed liner paint, which now comes in a whole variety of colors, over the entire shell as the exterior finish. I've been a long-time follower of Glen Morrissette's blog and about a year ago he built out a Volkswagen van to be his full-time home and used this type of paint on the exterior of the whole van.

Since he has barely mentioned the finish in the year since I would guess it's holding up pretty well. (And he isn't shy about abusing it by driving down very narrow wooded tracks either!) I don't know if this finish has any desirable thermal conductance properties but it can't be any worst than the usual Filon (That fiberglass sheeting you see on the exterior of some RV's today.) and certainly better than the aluminum skin you see on most the other RV's. I'll also bet it's a lot easier to renew or repair dings and scratches than the other skin options as well.

For an extra bonus, a painted surface creates a mono-coat skin, unlike the other two exterior options mentioned, which have seams at the corners that require calk and trim. To carry that attribute even farther, by rounding over the corners slightly, say with a 3/8" round-over bit, and finishing them off with a lightweight fiberglass tape laid in resin, I can eliminate troublesome sharp corners (Paint likes to migrate away from sharp edges as it cures) and add some extra sealing and strength to the vulnerable joints.

But that's more about construction and this is supposed to be about insulation. So, either high density foam blocking or sprayed in foam insulation will be my choices.

I’m a fan of high density, closed cell spray foam for a few reasons. It has a very good R factor per inch, is very good at sealing up air leaks and also acts as a vapor barrier. And, when you build the wall structure and apply one of the skins then fill the voids with spray foam, the foam bonds very tightly to everything it touches, seals any possible air leaks, and when cured adds greatly to the structural integrity of the component.

Being closed cell, even if water leaks in past the outer skin somewhere, it has a very hard time penetrating the closed cell foam so is less likely to saturate hidden areas then lurk and grow nasties.

But then, no matter how skilled you are at spraying the foam in, you have to cut and shave it flush with the open side of the structure before you can apply the second skin. And spray foam is expensive stuff, costing about $1 per board ft.  A 10’ square area 1.5” thick would require about 15 bd. ft. of foam or $15 plus over-spray and waste.

This is a photo of the open cell foam sprayed in to insulate the steel barn some 9 or 10 years ago. You can see the rough surface left behind even by professional installers. (The closed cell foam I sprayed in myself to insulate the well-house is completely concealed so I can't get a photo of it but looks about the same except it's blue.)

Block foam is less expensive than spray, coming in at about $6.80 for the same 10 square feet. But that doesn’t account for the waste of cutting 4’x8’ sheets to fit the bays in your walls. And you want the fit to be tight so air movement is minimized, so careful cutting and fitting is required. And to get something similar to the added structural properties of spray foam, you want to glue the block foam in place with an evenly spread coat of a good adhesive, all of which adds to the cost and labor. But then again block foam can be bought in 1.5” thick sheets so once it’s installed the exposed surface of your bays are already nice and even when it comes time to secure the second skin.

One of my previous, block-foam insulated, rigs under construction.

Having learned from experience that getting block foam properly fit and fastened requires a great deal of care and patience, and even then will still require a little extra sealing around the edges, I’m leaning towards the spray foam, which I've also used in the past but not in a rig before.

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