I want to insulate and protect under my house with something that will hold up for quite some time.  My current thinking:

1) Dig a trench directly under the exterior wall and compact 3-4" of gravel at the bottom
2) Hang Hardiebacker board off the floor joists down to the gravel in the trench.  Use strips to overlap and join ends on interior side
3) Glue and screw 2" rigid foam to the hardiebacker with offset seams
4) Trowel mortar over the foam
5) Add a bit more gravel and regrade to drain water away

I have never done something like this before, any suggestions would be appreciated.

I think the foam is going to want to move and crack your mortar. I'd put the foam on the inside of the back board. 

I feel that if it is well glued it will all move as one if it moves.  Tamping it in will hopefully help.  I got the idea of mortar over foam as that is what the local builders are doing on some new construction.  The difference being their foam is backed by foundation.

You can probably expect the ground outside your skirt to freeze and heave, while the ground inside will do less if any. Thus, there may be a push inward during the winter which the hardibacker will not resist, and mortar on foam on the outside will be even more subject to strain than if the mortar were covering the hardibacker directly.

Hmm, you guys are definitely talking me out of it.  What other lower effort options are there over doing the whole concrete footer masonry wall approach?  Maybe I just need to bite the bullet...

I have a couple questions that may help us get this figured out:

Is the mortar in your plan to make it look pretty? 
How is your building held off the ground now (it's on a trailer, piers, etc)?
What kind of soil do you have (sand, clay, gravel)?

My initial thought is to use something that won't rot and have it reach from the building down to the ground.  Then cover it with something pretty.  And insulate the inside of it.  Most importantly, account for any frost heaving that you would get.

I'd consider using fiber cement siding as the sturdy material that also looks decently pretty (paintable).  Then glue insulation on the inside.  I'd assume that you will have frost heaving so I'd put something on the ground that the siding can slide in and out of.  Or behind.  That way the system can move and will still block most of the cold air.  See crude sketch below.  The dark red is the fiber cement siding, the blue is the Styrofoam, and the purple square is a timber or other material that can move up and down while the siding/insulation stays put.  Note the gap between the siding/insulation and the ground to allow for the frost heave.

You also should consider insulating the floor joists of the house.  It's probably more important to insulate that than it is to skirt in the perimeter.  Otherwise your house heat will constantly be bleeding down into the crawlspace and to the ground below.  I'm not sure about building codes in your area but preventing air infiltration up into the house is also important (vapor barrier or house wrap).  The heat in your house wants to rise and leak out any gap in the ceiling.  Any corresponding gaps in the air sealing at the floor will just help the air escape upwards.

On my small cabin on piers, we insulated the floor joists.  We covered that with 1/4" plywood and made sure to have no gaps greater than 1/4" (for mice).  We didn't skirt it so that it would stay open and critter-free.  It was 18" off the ground which we had been told was the minimum amount to make it unattractive to critter habitation.  In 7 years of owning it, we never had critters living under it or mice getting into the floor.  The floor was cold when it was 0 degrees F outside but I could have reduced that with thicker floor insulation (we used only 5" of fiberglass).

Hope that helps...

The mortar primarily is there to protect the rigid foam insulation from UV but also makes it look better.


Soil is sandy loam for the first 18", then sand and rocks below. It drains well.

The building is actually a small old farmhouse with an unfinished basement foundation (purple) set back about 2' from an old perimeter foundation with no footer (green).  Right now I have wood 4x4s (yellow) supporting the floor joists from the top of the inset foundation.
The perimeter foundation is cracked and dislocated on the left side of the house where it was previously back graded.  I only need to really address this one side. I could just seal the floor joists like you did, but I do have plumbing in that section.  My fear would be that the plumbing would be more likely to freeze and I would loose the easy access I have now.

Would it be feasible to remove and replace that portion of the perimeter foundation?

Totally feasible, I was just looking for an easier option. I may end up just doing more concrete.

So the building is currently sitting on a frost protected (purple) foundation wall on 4x4 posts.  The old foundation wall has no footer and one side is tipped in.  I'm guessing the old foundation was fine due to your well draining soil but now that the new foundation was dug and backfilled right next to it, it is no longer going to be structurally stable enough for you.  That's just a strong hunch on my part.

How about if you just skirt from your nice purple foundation wall up to the bottom of your floor joists and then insulate and seal the underside of the floor for that 2' part that is outside of the skirt?  Then you don't have to worry about frost heaving since the purple wall won't move.  And all the plumbing that is inside of the purple walls will still be accessible.  You can still insulate the entire floor and then seal the perimeter envelope and get a decent result.

In my cabin I built boxes down around the sections of plumbing that were below the joists.  In the shower drain area I had a lightbulb as a heater under the floor.  The drain in the shower glowed which was a bit eerie. 

i think your original idea is great.  I would, however, as Tyler suggested, give up the little extra thermal mass of the hardiebacker by putting the foam inside of it, and skip the step  of mortaring to save time.  would the simple gray backer board meet the level of aesthetics you are going for?   the worst frost heaving will do in this case is aesthetic damage by causing the skirting to deform, and if you put the gravel and ensure good drainage so the area stays as dry as possible, you will minimize or eliminate the frost heaving. also perhaps you could lay some foam nearly horizontally on top of your gravel about 16 inches down in your trench, (going out from your skirting) on the outside as frost protection, (you can read about 'frost protected shallow foundations') 
also i think Mike Jay's idea of using the new foundation and insulating the floor outside that is a great option.  It all depends on what your objectives are and what aesthetic and structural standards you want to reach.  If you do mortar the foam it might be good to do a little test first to see how good the adherence will be.

Mike Jay wrote:So the building is currently sitting on a frost protected (purple) foundation wall on 4x4 posts.  The old foundation wall has no footer and one side is tipped in.  I'm guessing the old foundation was fine due to your well draining soil but now that the new foundation was dug and backfilled right next to it, it is no longer going to be structurally stable enough for you.  That's just a strong hunch on my part.

How about if you just skirt from your nice purple foundation wall up to the bottom of your floor joists and then insulate and seal the underside of the floor for that 2' part that is outside of the skirt?  Then you don't have to worry about frost heaving since the purple wall won't move.  And all the plumbing that is inside of the purple walls will still be accessible.  You can still insulate the entire floor and then seal the perimeter envelope and get a decent result.

In my cabin I built boxes down around the sections of plumbing that were below the joists.  In the shower drain area I had a lightbulb as a heater under the floor.  The drain in the shower glowed which was a bit eerie. 

I like Mike's idea, though you may have some uses for the unfinished basement in which case I would insulate inside the yellow and purple sections, at least down to the frost line, and insulate under the cantilevered floor as much as possible (fill the joists and place rigid foam over that, seal joints and edges). Skip making the tilted old perimeter foundation look nicer, or place large, free rocks there.

I just thought I'd add something for the other interested folks out there, but you might not like it as it's not permanent, although long-lasting, environmentally very, very good, and cheap:  bales of straw placed around the perimeter of the home.  If you do some research, you'll find that straw bales are extremely thermally efficient.  They are also safe from burning (the compacted straw stems are not able to get oxygen to burn well), and can be tucked under the bottom of the house and then hidden by anything you can come up with...decorative picket fencing, aluminum panels, trellis panels....

Parker Free wrote:I just thought I'd add something for the other interested folks out there, but you might not like it as it's not permanent, although long-lasting, environmentally very, very good, and cheap:  bales of straw placed around the perimeter of the home.  If you do some research, you'll find that straw bales are extremely thermally efficient.  They are also safe from burning (the compacted straw stems are not able to get oxygen to burn well), and can be tucked under the bottom of the house and then hidden by anything you can come up with...decorative picket fencing, aluminum panels, trellis panels....

awesome idea, though highly subject to attack by a conventionally polluted mindset : )  perhaps some salvaged roofing metal to screw onto the floor joists for easy removal and bale replacement in 20 years.   the drier the space, the longer the bales will last.  if placed on top of some foam laid on the ground, maybe they last a lifetime... i haven't had the honor of working with strawbales....do they insulate well if they aren't plastered, or is there excessive air infiltration in that case?  I like this alternate line of thinking...

Maybe I am missing something with what is being asked so I am going to share my thoughts anyway. We use to own and live in a mobile home for 4 years. We found many problems that if we ever built above ground we would do things a bit differently then what was done in the place  we lived.

We found we wanted at least a 3 -4 foot crawl space. This allowed us to get underneath for repairs or adjustments much more easily. It can be used for cold storage or just storage.
To keep plumbing lines from freezing an electric heat tape was best . If off grid and energy is important then wrap the lines with that black foam insulator that is split down the middle plus put roxul between each joist.
We also found it was best to use sheet metal to form a skirt with a couple areas that comes off easy so one can crawl under or retrieve and or place items for storage. If it is really cold where one lives then one would use the roxul insulation on the inside against the sheet metal skirt as well.
Why roxul? because it is easy to handle and keeps it shape. It is fire proof and mold resistant.
If mice  or rodents are a problem get some cats or set up some traps.
The other reason for being 3-4 feet high is because we were always under the trailer adjusting the supports. After the spring thaw the supports would always shift. Maybe not enough the first year or 2 but for sure by year 3.

I am an advocate of using wall-to-wall carpet and padding that has been torn out to replace. This is the system I used for the North wall of my greenhouse. I used some squares of milk jug plastic for washers and screwed to layers of carpet padding to the bottom of the beam then a layer of carpet on the inside and outside of the beam.  Keeps the cold out for the winter and can be rolled up and tied for ventilation in the summer. Cost: a box of screws and the hauling the carpet home from a house down the road. Durability: after three years it still looks the same except some alga on one corner which gets wet more often.

Corey Schmidt wrote:

Parker Free wrote:I just thought I'd add something for the other interested folks out there, but you might not like it as it's not permanent, although long-lasting, environmentally very, very good, and cheap:  bales of straw placed around the perimeter of the home.  If you do some research, you'll find that straw bales are extremely thermally efficient.  They are also safe from burning (the compacted straw stems are not able to get oxygen to burn well), and can be tucked under the bottom of the house and then hidden by anything you can come up with...decorative picket fencing, aluminum panels, trellis panels....

awesome idea, though highly subject to attack by a conventionally polluted mindset : )  perhaps some salvaged roofing metal to screw onto the floor joists for easy removal and bale replacement in 20 years.   the drier the space, the longer the bales will last.  if placed on top of some foam laid on the ground, maybe they last a lifetime... i haven't had the honor of working with strawbales....do they insulate well if they aren't plastered, or is there excessive air infiltration in that case?  I like this alternate line of thinking...

Thanks.   Yes, they are highly insulating without any plaster.  The plaster is there for water protection (there are straw bale houses without plaster in dry climates and they keep the inside cool/er) and aesthetics, pretty much.  You are correct in that if they are kept from moisture or moisture is allowed to easily drain, they will last years.  There is basically no air infiltration as the straw is too compressed to allow it.