Tiny House wall envelope condensation worries
Hi,
I have been reading on this forum for the last 2 weeks and found it very helpful that is why I want to ask a question to this knowledgable crowd.
My problem is this:
I am will start building a tiny house on wheels next week. the trailer has been delivered already and i am finishing the rough construction plans. the build will take place in perth, western australia which has a climate similar to your zone 3 or mediterranean, in other words. i might also move the house to a slight cooler area from here later on but it will hardly ever see frost.
i am limited to basic materials just because perth is fairly remote and anything special is very expensive and i am also limited for weight because the house is on a trailer.
my initial wall envelope from outside to inside was supposed to be this:
– western red cedar weatherboards
– air gap (rain screen)
– proctor wrap, which is similar to tyvek
– pine 7mm or 9mm plywood (a little less than 1/2 inch)
– sheep’s wool insulation in a 2×4 pine frame
– inside wall made of 3mm plywood (for airtightness, but vapour permeability) covered with tongue and groove pine boards / in some parts 7mm plywood covered with decorative wallpaper
after having read a lot about vapour retarders and so on i am now pretty worried about future condensation problems inside the wall on the sheathing, especially in a colder winter or a little further south of perth (the further south, the colder). i tried u-wert.net and WUFI for wall simulations and i can see a potential for problems. especially because the wetter winter here, still has quite warm daytime temperatures which could promote rotting.
could i put the sheathing on the inside ? maybe just the house wrap ? that is what some builders seem to do in australia.
could i use something less vapour retardant on the outside ? i also don’t have many options or space for xps foam on the outside. is there a structurally clever solution that does not require “fancy” materials ?
i would also like to avoid using PE foil, as it seems like overkill for this climate. would simply painting the inside walls before covering them solve the problem ? what paint would you use ?
what is a tried and tested lightweight timber frame wall envelope that doesn’t require drywall?
thanks so much for helping me out !
i am a cabinet maker by trade, not an architect, which i am now more aware of than ever before 🙂 . my blog address for the house build is : http://www.timberandteal.com
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Replies
i forgot to mention that i love robert riversong's idea of a 5:1 ratio of permeability from outside:inside wall. what is the best way of achieving this in my setup with fairly standard materials?
thanks again everyone !
Andreas,
Stop worrying. Your exterior layers (1/2 inch plywood, plastic housewrap, a vented rainscreen gap, and wood lap siding) are all vapor-permeable enough to dry fairly rapidly, especially with a vented rainscreen gap. You're fine -- especially in your very mild climate.
Andreas,
My experience with Tiny Houses is that most of the moisture related problems they experience come from the occupants generating high levels of indoor humidity, and that incorporating a good passive or active ventilation strategy is the key to their longevity.
I agree with Malcolm, larger energy efficient houses have humidity issues (there are GBA articles on it) so no surprise tiny homes are in the same boat.
I wonder if there are very tiny HRVs out there
thanks for the reassurance. i might still try to make the inside a little more impervious by putting at least a layer of indoor paint on it, or a primer under the wallpaper. would Intel DB+ or Intel Plus work in my environment or is that overthinking it again ?
there actually are small HRV units. we were looking at the german made Zehnder Comfospot 50, which seems easy to install and not too large for our house.
One more quick question. My subfloor is completely enclosed from the bottom with galvanised sheets of metal. is it a good idea to put vents, or i guess you could call them vapour duct ports, in it to enable it to dry out or will that just introduce more moisture coming into the vents from the ground up ?
the floor setup from bottom to top is this :
- galvanised sheet metal
- 2x6 joists with sheep wool insulation
- plywood subfloor
- underlay ? maybe felt.
- floating tongue and groove engineered flooring boards
some people put a vapour barrier in between insulation and subfloor. i am reluctant to use more barriers than necessary, especially because i would create a space between metal and barrier that is completely enclosed.
So the question again: would the floor dry up towards the inside or should i put in insect and rodent screened vents into the metal floor sheets?
Thanks so much to everyone for helping out !
Andreas,
Q. "Some people put a vapour barrier in between insulation and subfloor. ... Would the floor dry up towards the inside?"
A. Your floor assembly won't get wet. The main reason is that your plywood subfloor is a fairly effective vapor retarder -- enough of a retarder to slow outward vapor diffusion during the winter, but not enough to stop all inward drying.
I agree with Alan who agrees with Malcolm. Small volume houses and apartments are not challenged by exterior moisture so much as by occupant expiration, cooking, sink and shower, etc. humidity. Moisture accumulates faster than a 15 cfm fresh air rule per occupant can accommodate without making the occupants cold (occupants being in the fresh air stream in small spaces). I am facing this cool air stream with a 1100 ft sq main floor and passive vent and exhaust only ventilation where the passive make-up air is not diffusing and warming fast enough for my wife to be comfortable in certain areas of the house (hot water heating).
The sheep's wool is hydroscopic so it will absorb moisture but it will be slow to penetrate the paint and walls to provide any useful moisture management in such a small space at any useful rate.
Robert Riversong is a good person to consult with. If you like his stuff, why not contact him with this design issue. In small houses he favors the use of passive Airlets (see Aldes, and Panasonic), hydrscopic insulations (cellulose), and exhaust only ventilation.
In your case, a low cost low energy, humidity sensor-driven ERV, exhaust-only fan (kitchen or bath) or dehumidifier might be needed.
http://vents-us.com/images/cat/704_291_cat_file.pdf
They have a solar powered unit I believe.
Look around, there are now some small efficient ERV-HRV's You have to keep the occupants out of the fresh air stream. One way to do this is to locate opposite of the convention. Put vents low, behind buffers and use some creative diffusers to mix/warm the incoming air. Also, a passive air stack with a cool air trap might also help to release warm moist air vertically and rely on under-door infiltration instead of passive vents.
This looks like a lot of fun and a DIY project:
http://www.ventive.co.uk/products/ventive-splus/