Advice on adding insulation under small stick built house

Hello,
I have a 600 square foot house in SW Colorado.  It gets soooo hot in the summer and not the warmest in the winter. It is high elevation, semi-arid climate on a hillside that is mostly a pile of rocks.  It is built on piers, and the space under the house ranges from 18" to 4'.  The floor has sandwiched insulation,  and there is large plastic sheeting on the ground underneath.  I,m not sure if the plastic is for moisture issues, radon, or mud prevention when accessing the pipes.  The pipes are heat wrapped and insulated but the only protection around the outside of  the crawl space is corrugated metal sheeting with large gaps.  
I've been researching and brainstorming ideas on how I could add some solid insulation with natural building.  I will most likely be doing the work by myself or with limited help.
 I've considered straw bale, but the tight access under one side of the house makes that seem like a real challenge to intstall and thouroughly plaster to keep mice and bugs out.  

I like the idea of using the abundance of rocks here as thermal mass and insulating it.  I considered making long thin wooden boxes that sit under the outside edges of the house that i could fill with straw wattle and then use stone and lime mortar on one or both sides.  These would be easier to install from the outside of the house and could be covered by the metal sheeting.  But I wonder if the wood would trap the moisture in.  
A swedish skirt seems to be prohibited by the rocky and sloped terrain.  
Any ideas or advice greatly apprecieated!
 

This sounds like a fantastic project, M.  
For starters, what are the house dimensions?

[The house] is built on piers, and the space under the house ranges from 18" to 4'

Would you tell us a little more about the piers? Maybe take a photo of the support system. I'm thinking about cellars/basements/crawlspaces; could an enclosed airspace surrounded by rock/mud wall help keep the wind out and the air temp more stable? Do you have space directly under the floor to attach insulation? How is the underside of the flooring (potential cellar roof) constructed? Maybe insulating around the airspace will make straw or other material less necessary as fill.
I'm very intrigued and looking forward to watching your thread!

Thanks for your response.
The dimensions are roughly 55' x 35'.
The outer edges are large beams that insulation can be attached to. Between the Crawlspace ceiling and the floor is fiberglass insulation. It is very well put together and not drafty. I will include photos. Thanks!

Photos

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If I wanted a quick fix for insulation I would use square bales of hay or straw on the outer perimeter behind the metal sheeting.

For the one side that has the tight access maybe some bales could be cut and rebound. Or maybe use the rock and mud there.

Since you have the metal sheeting, all sort of items could be used behind the sheeting and not be seen.

I have seen old clothes bound into bales, cardboard bales, newspaper bales, cotton bales or maybe even wool bales.

Ok thanks!  Does anyone have experience with putting plywood around straw bale and then stone and mortar over the wood? Does it cause rot issues?

Use lime mortar rather than Portland cement mortar so moisture is not entrapped which will cause rot.
I have some experience with retro insulating buildings;
- the sandwich insulated floor is about the best you can do, unless the insulation  is packed in hard.
- maybe preventing draughts going under the house by limiting the size of openings may be enough.
 cold wind sweeping past the floor can suck the heat out, keep some openings to reduce risk of rot.
- draughts within the house are common.
- The roof itself may not have insulation and there are ways to help that.
- Check the ceiling insulation exceeds requirements, because that is an easy area to improve.

The photos are really helpful M. They confirm that the place is "...very well put together."
While waiting to see if, "...anyone has experience with putting plywood around straw bale and then stone and mortar over the wood" (I do not), I am wondering about the original problem that you identified in your opening post:

It gets soooo hot in the summer and not the warmest in the winter

I'm thinking that, since heat rises, and there is no heat below the house to rise and warm the building in the winter, cool ground air may not be the cause of the cold winter house.
How do you currently heat the house?
As John alluded, is it possible that the heat is escaping by rising through the roof?
Similarly, I'm thinking that too much heat in the summer has a cause, like high altitude sun being absorbed by the house (dark roofs and walls absorb while white walls reflect sunlight). The heavy cooler air under the house in summer probably just sits under the house giving you a lovely cool crawl space. But that heavy air won't rise into the house and cool it without a fan or siphon to draw it up. I don't know much about heat siphons but I'm curious if those who are experienced with such methods would offer their opinions.
Would you consider other ideas besides/in addition to floor insulation? If you are convinced that the floor is the problem, please share your reasoning. I am an amateur builder and eager to learn if I am missing something.

Hmmmm, really great point! I'm not positive it is the base of the house causing the heating/cooling issue.  I guess I assumed it because of it being on top of a hill with major gaps in the siding to the crawl space. The summer heat is more of a problem than the winter cold.  I heat the house mainly with a small wood burning stove, and supplement with a baseboard heater and a portable electric oil heater. I use fans in the vaulted ceilings  on low to push the warm air back down. The majority of the outside of the house is covered in corrugated metal: roof and sides with some wood.  There are 2 skylights that beam sun in and roast the place.  I had to cover them last summer.  This is my first year in this house so I am still learning how the seasons are.  I don't know what kind of insulation the ceiling has. I will see if I can find out that info .   It is covered in wood T&G, so not so easy to access.   There is a small loft on one side of the house, above the woodburning stove that collects heat year round.  The house sits in full sun.  I love my little house so much and have been working hard to make it more efficient and less energy consumptive.  I am definitely open to other avenues of solutions.  Thank you!

Nice project.  I believe you'll benefit greatly by 'normalizing' your crawlspace with a long lasting material and it might as well be rock and mortar.  Leave an access door that can be shut tight and observe crawlspace ventilation recommendations (you'll only need a few 4x16" vents and you'll want to keep the plastic vapor sheet in place.  So for the cost of sand, cement,  and elbow grease, you'll reduce the heating requirements a noticable amount (10-15% would be my expectation)
As far as the summer heat, shading or painting the metal white would go a long way.  Ceiling insulation probably should be at least R49.  Attic should be well ventilated... perhaps a ridge vent along both peaks and many soffits.  I would consider a evaporative cooler or even a heat putif you can catch a deal and a tax credit... I'm cheap and use a swamp cooler.
I consider Swaddle and hay bale a temporary solution that would create forever  maintenance requirements.... and you said, "I love the house".  I'll bet you could finish the crawl space enclosure with rock fairly quickly if you started collecting rocks   and piling them along side the wall.... it's going to take some effort but it is also the cheapest material and already on site.
I enjoy reading about projects at "builditsolar. com"... I think you'll find some good ideas there.
Good luck.

The roof can be the single biggest factor in summer heat load, unless there are significant windows exposed to summer sun (skylights are a prime example, and I would definitely rig retractable or removable shading for those.)

From the picture, it seems like the loft would have a vaulted ceiling so no attic to insulate. Does the ceiling feel hot on sunny days in summer? Is the t&g wood on the ceiling applied to the bottom of the roof structure, or on top of roof beams/rafters? If the latter, your best bet might be to remove the (metal?) roofing, add some framing on top of the existing and insulate with the most efficient material you can get, and reinstall the roofing (assuming it is in decent condition.) Making it a "cold roof" with free ventilation between roofing and deck would help mitigate heat buildup. If the roofing is not great, replacing it with white metal roofing would make a real difference in summer heat load.

For winter, obviously improving the roof insulation will help a lot. I would build solid walls for the crawl space, with flexible sealing to the underside of the house walls so you don't have to worry about frost heave. A few vents for summer humidity control (perhaps close all but one or two in winter), and one entrance for maintenance access, along with full ground coverage with the plastic sheeting, will help keep the place warmer. If the tall side of the crawl space faces the winter sun, I might consider glazing that side so there is some actual heat gain below the floor in sunny weather. As long as summer sun does not hit that glazing significantly, you will have a net increase in comfort. Shade panels to cover the glazing when appropriate may help.

With drafty sheetmetal skirting, I cannot imagine radon being a factor in the plastic on the ground. I think humidity would be a minor consideration too, in a "semi-arid" climate, though if sealed up it may be more of a factor. I have a feeling simply staying out of the dirt during construction and maintenance was the primary factor.

If you enclose the crawl space tightly, I would find out if radon is a common issue in your area, and if so, get it tested for.

I live in Bendigo Australia which has very hot summers.
I work in the area of making small homes liveable.
From the photo I notice a few extra things that may help you;
- catch the water out of the spouting in the front in a 5000 gal water tank located somewhere. My signature has a link.
- Cover the skylights and relevant windows on the inside with a reflective film like this one renshade-reflective-rolls-for-hot-skylights
- install a 'safari' roof or wall panel to reduce the penetration of heat from direct sunlight, I will explain that.
- install a temperature controlled exhaust fan high in the roof space or the loft or both. Power operated are better because they work when its dark and still hot.
power operated roof vent
 https://www.ecohome.net/guides/3630/why-roofs-need-to-be-ventilated-how-to-install-roof-ventilation-vents-fans-methods-building-code-us-canada/
- Do you have a flat plate solar water heating system? They would work well there .

Safari roof design.
This is a system I created after observing similar systems on homes in the tropics, and on some vehicles.
It is a secondary roof or wall panel which is not weatherproof and fits over existing weatherproof surfaces.
There are two varieties;
- single layer no insulation
- double layer with reflective insulation.
It could be described as a shade panel for a surface.
They are constructed to shade an existing roof or wall and have air flowing between the two.
1 Fit 50mm metal battens running up the roof or wall to the surface.2
2  Install a good quality reflecting insulating material to the battens
3 Attach a second lot of 50mm steel battens at right angles to the first layer, IE horizontally
4 Attach the new roof or wall sheeting to second lot of battens.