Tiny house on wheels with exterior XPS assembly
I’m in the process of building a tiny house on wheels in zone 6 in BC.
Just wanted to clarify our wall, roof and floor assemblies for this particular zone.
Walls from exterior to interior: Cladding (type TBD)//Furring strips//Tyvek housewrap (as we will be doing outie windows)// 2” of XPS foamboard// 5/8” plywood sheathing taped for air sealing// 2×4 frame 24oc// cavity filled with fiberglass batts// Class 3 vapor retarder or some type of smart retarder// interior T&G
I am most concerned about the conflicting information about the necessity of having any type of vapor retarder on the interior if my XPS will act as a semi impermeable vapor retarder at the thickness of 2” that we have decided. Do I need one at all? Or would it be safest to install a smart retarder of sorts?
As we are building a tiny house on wheels we are quite concerned about the amount of potential humidity in the space if we are building to dry inwards. Hypothetically with our setup with the exterior foam we shouldn’t have too much condensation problems in the first place, but should this be much of a consideration in our design if tiny houses are notorious for interior humidity problems to begin with?
Roof from exterior to interior: Tin roof// Furring strips to allow for air gap (our area gets quite a bit of snow)// 5” of XPS foamboard// ice and water shield// Plywood sheathing taped for air seal// 2×6 joists 24oc// cavity filled with fiberglass batts// possible vapor retarder or none at all? // interior ceiling(material TBD)
Floor from exterior to interior: Steel trailer joists 24oc// plywood sheathing 5/8”//4” of XPS// plywood sheathing 5/8”// flooring (material TBD)
Again any recommendations on the use of any vapor retarder in both the roof and floor? We are hoping to eliminate any risk of thermal bridging with the current floor set up made similar to a homemade SIP. As the tiny house flooring is elevated off the ground should we be looking at higher Rvalues?
Thanks in advance for the input! 🙂
Christie
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Replies
I don’t think you'll need any vapor retarder at all with that much rigid foam on the exterior. I’d recommend you use polyiso instead of XPS though, which will get you a little more R per inch. Be sure to tape the seams of the polyiso.
If you go with polyiso, that 4” in the floor will give you about R26 which meets code in all climate zones. Since you could potentially have wind blowing underneath the trailer, it wouldn’t hurt to add some more insulation if you have the room.
Bill
>"I’d recommend you use polyiso instead of XPS though, which will get you a little more R per inch."
+1
Not only is it higher R/inch, foil faced polyiso is about 30% lighter (per inch) than XPS, and it's a HELL of a lot greener, at about 1/10th the CO2e of XPS at any given R-value, comparable to fiberglass batt or mineral wool:
https://materialspalette.org/wp-content/uploads/2018/08/CSMP-Insulation_090919-01.png
All XPS sold in North America is blown with HFC blowing agents currently banned for that application in Kigali Amendment (to the Montreal Protocol) signatory countries. In Europe almost all XPS is blown with low-impact CO2, and has the same performance and CO2e footprint of EPS.
Thanks Bill and Dana for the responses!
I will look more into the polyiso... I was having some difficulties sourcing it where I am especially during current times! If i were to find the polyiso, it would be more vapor impermeable then the XPS... thoughts in that case about a class 3 vapour retarder on the interior? Or no vapour retarder at all for interior drying? Should i be thinking of a vapour retarder for the ceilings and floors as well in this case? Thanks again for the input, much appreciated!
2+ inches of XPS doesn’t really offer any significant drying capability so you’re unlikely to see any practical difference between XPS and foil faced polyiso in terms of vapor permeability. Fiberglass or kraft faced polyiso is more vapor open, but I don’t think you need that. The foil faced polyiso is easiest to tape.
Bill