Will this exterior wall system work for my tiny house?
I live in South Dakota in zone 6a (Sioux Falls). In a few months time I will be constructing a tiny house on wheels using 18 gauge structural studs. Starting in the wall cavity will be sprayed closed cell foam. On the exterior sides of the studs (and any other exposed metal areas- the steel trailer) I plan on using strips of 1 inch thick xps rigid foam with a compressive strength of 60 lbs/square inch. Also I will use steel strapping all around the outside to give it some shear strength and prevent racking. Then over the foam I will use 1/2 inch plywood to give the spray foam something to attach to and install some house wrap to prevent the sheathing from getting wet, but allowing drying to the outside. Then I will use 1/2 inch strips of vertical plywood to have an air space behind the cladding and give me something to attach the cladding to (which will be some type of tongue and groove wood boards). This will also be how I finish the roof except I may use felt paper (unless the house wrap would work) and then a standing seam roof.
I am not sure about the interior walls and ceiling because I’m not sure if I will need some sort of fire barrier from the spray foam. On the floor I would use strips of foam again to cover any metal of the trailer and then use a 3/4 plywood for the subfloor. The trailer cross members will be spray foam as well with strips of foam and 1/2 inch plywood on the underside of the cross members, and finally maybe some galvanized metal to protect the underside.
Sorry for such a long post and it turning into the roof and floor as well but just trying to be thorough.
Any thoughts or comments would be greatly appreciated. Thanks for reading.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Damon,
Steel studs don't perform very well from a thermal perspective. They are thermal bridges. Wood studs would be much better.
In most cases, installing spray foam between steel studs is a waste of spray foam. (The thermal bridging through the studs defeats the foam.) If you insist on steel studs, all of the insulation should be on the exterior side of your plywood sheathing. The easiest way to insulate a steel-stud wall is with one or more continuous layers of rigid foam on the exterior side of the wall sheathing.
Thank you for your response Martin.
You say in most cases the spray foam would be a waste. What would be a case where it wasn't a waste? It would be nice to use the spray foam to seal up everything nice and tight. I would of course use some type of air exchange system in the tiny house.
I like the steel studs because of the straightness, strength and the ease of working with it (I've been doing commercial steel framing for over 8 years now). It is also just a little lighter than wood, though I've been toying with the idea of using structural 20 gauge to really see a difference in weight, but I don't know if that would be a pain to weld.
And also, would a strip of 2 inch thick foam (R10) work to cover all of the metal exposures, and then put the sheathing over that and spray foam the cavities for an air and water barrier? Or would I have to sheath first and then install continuous layer(s) of rigid foam? If I do it the second way, wouldn't the plywood sheathing be trapped and unable to dry properly if it had a little moisture in it at all?
Damon,
Q. "You say in most cases the spray foam would be a waste. What would be a case where it wasn't a waste?"
A. I suppose that you could install a continuous layer of spray foam on the exterior side of your sheathing. If you do that, at least you won't have the thermal bridging. But using rigid foam is easier and cheaper than using spray foam.
Q. "Would a strip of 2 inch thick foam (R10) work to cover all of the metal exposures, and then put the sheathing over that and spray foam the cavities for an air and water barrier?"
A. Putting rigid foam strips between the metal studs and the wall sheathing would help from a thermal perspective, but it would reduce the strength of the walls. Ideally, wall sheathing should be fastened directly to the studs -- that's how you get the strongest wall and the best bracing.
Installing spray foam between your studs does reduce air leakage. However, this type of spray foam installation does not create a "water barrier." The water-resistive barrier (WRB) is usually installed between the siding and the sheathing. Here is a link to an article with more information on WRBs: All About Water-Resistive Barriers.
Sorry, I meant to say vapor barrier instead of water barrier. I would use a tyvek drain wrap over the sheathing. And as far as the shear strength of the wall, would it be just as good to run sheathing on the inside and maybe even some 1 and 1/2 inch cold rolled channel clipped to every stud?
Also, would I be able to get some sort of thermal break from the trailer if it could be sprayed with material like they use to line truck beds?
Damon,
Q. "As far as the shear strength of the wall, would it be just as good to run sheathing on the inside?"
A. Yes, that could work.
Q. "Maybe even some 1 and 1/2 inch cold rolled channel clipped to every stud?"
A. I don't see how adding cold-rolled channel would help brace the wall. When in doubt, I suppose you could consult an engineer -- but that seems complicated compared to the easier approach (using wood studs and conventional plywood or OSB sheathing).
Q. "Would I be able to get some sort of thermal break from the trailer if it could be sprayed with material like they use to line truck beds?"
A. I imagine that the R-value of a sprayed-on truck bed liner would be quite low, so I can't imagine that it would provide much of a thermal break.
How big of a waste is closed-cell foam between steel studs?
The thermal bridging of #18 steel studs is more than twice that of milled 2x framing lumber. The comparative performance breaks down this way:
With R13 batts the "whole-wall-R" after calculating the thermal bridging a milled lumber 2x4 wall comes in at about R9.5-R10, which includes addition R-value assumptions for thestructural wood siding, wallboard, and both interior & exterior air films.
If it's steel studs, with R13 batts it comes in a hair shy of R7. Same with 0.5lb open cell foam.
With R15 batts or 0.7lb open cell foam it's just about exactly R7.
With 3.5" of closed cell foam (R22-R23 center cavity~ ) the performance only rises to about R7.5.
That has to be a VERY expensive way to gain another R0.5!
That's truly a waste of expensive foam.
If you're going to use steel studs, make ALL of the high R/inch insulation as continuous unbroken sheathing. (Cheap 16" x 3.5" thick fire & sound abatement batts between the studs may still be worthwhile for both the additional R6-7-ish performance)
To Dana; so is that figure of the closed cell foam taking into account the 2" rigid foam strips applied directly over all the studs too? Wouldn't the 2" rigid foam be a good thermal barrier against the studs to help even the playing field?
I would like to just use continuous foam on the outside of the sheathing, but in the tiny house, every inch counts and my wife is worried it will shrink it up too much to have at least 4 inches of continuous foam.
We are now looking into the possibility of SIPs which would be a great improvement over traditional stick framing and lighten up the load a bit as well.