Building envelope for zone 8, but living in zone 6
skibow
| Posted in General Questions on
This is an odd question since the house will be moveable, but what climate should I build for? I know I’m not going warmer than zone 6, and might go as cold as potentially zone 8.
Is it an issue to choose a building envelope combination for zone 7/8 and then keep the house in zone 6?
Say there was an unforeseen circumstance that forced me into zone 4 or 3?
Thoughts on how to be versatile when choosing a combination of insulation, vapor retarders, etc?
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Replies
skibow,
Yes it is an odd question. You say a movable house which suggests a "tiny house", a format that I have problems with. You would do well to search the Q&A for similar builds. Many problems arise from the need to anchor onto a metal trailer frame. Thermal management of the floor layers and controlling condensation is one problem area I recall. I suspect that water and waste management schemes are also a trouble point.
As to the thermal question. Unless you are planning on having very limited windows in a small square footage movable building, the windows will dominate your losses. Trying to reduce heat losses by padding out the walls and roof insulation will be difficult. Maybe Litezone ultra insulated windows could be a useful pursuit if you end up in a CZ8 location. And then you still have to deal with doors that typically are quite inadequate thermally.
If you do build with multiple CZ's in mind, exterior insulation offers the best path IMHO. Sheathing and framing will be protected against condensation problems for most zones. I feel that even in CZ3-4 it will perform safely. If you moved to New Orleans I would have to re-think that position.
You do not mention your energy choice for heating. What do you plan for a CZ8 siting? Even in CZ6 it will be tricky to blend an ACHP with other forms to bridge the coldest bits. Basically, if you design for CZ8 you will only be better off in lower HDD environs.
I am hoping to figure out a way to use continuous external insulation (mineral wool, I guess) on the floor, walls, and roof. I will definitely have windows and I do expect that to be the main loss - still planning to keep a reasonable wall to window ratio. I'll look into Litezone - sounds expensive though.
I plan to use wood heating as my primary source and probably a diesel heater as my backup/supplement.
Can you clarify what ACHP and HDD stand for?
skibow,
oops -typo ASHP air source heat pump. HDD heating degree days. The window area affects all homes, just potentially a bit more in tiny homes. The ones I see being built in my area are as much as half window and door to wall area. As has been discussed many times in GBA, better windows are the fastest path to controlling heat losses. Increasing the R value of wall or roof insulation is less useful. The diminishing return of increased R values across a wall for a given delta T is easily calculated, same for showing how one can halve the losses through a window with relatively modest window upgrades from code minimums.
Generally speaking, buildings in areas with high HDD values will benefit more from good walls and windows than buildings in low HDD areas. The climate zone categories are a bit slippery. My own CZ6 local HDD values are in the 7800 range. The coldest night temps of -15F only occur a few times a year. Mostly I average around 0F for a month at most. A lot more time is spent at 35-40F which adds up the HDD profile but really doesn't warrant extreme window or wall R values. Still, I have to design to the lowest temps.
The ultimate temps you are asking about in a CZ8 could include -40F for extended periods. The code standard window will be a sheet of ice inside, a really good R-7 window will still condense heavily. In most other contexts an R-7 window would be very good most of the year. The moving target you have inquired about makes solid suggestions difficult. You will have to decide just how likely or how long you will find yourself in the coldest conditions. And how much inconvenience/misery you will put up with.
Regarding your heating choices. Not sure if wood stoves with really tight external air feed will be enough for the most extreme option you propose. You do not want to be causing external air to infiltrate if it is -40 to readily. I have no idea about diesel heating. I do know the local excavators cuss a lot about the fuel conditions it gets down to -20 or worse. You might want to try message boards in Alaska to get real life input from those that know.
skibow,
How big is the Tiny Home? The smaller it is, the less the difference matters.
Good points, onslow...good reminder to focus on the big picture (like windows). Malcolm, approx 9'x27' interior space (243sqft).
skibow,
I w0uld mainly tailor the building assemblies to making sure they were safe (that is, do not become risky in any of the climates you are contemplating moving it to), rather than optimizing them to converse energy. In such a small house, the heating and cooling loads are too low to make the difference between assemblies designed for zones 6 and 8 meaningful in terms of energy use.