culvert hobbit home and rmh

hi everyone

i am in the process of planning a small cabin like underground home for next summer.

i have access to a 8foot diameter x 20foot metal culvert. it will be buried into a slope like a daylight basement.
a quick overview of my project so far.

the floor i was planning would be an earthen floor, so it could act as a thermal mass and follow to corrugation at the same time.
i have access to good compaction sand and clay directly on the land.  a depth between 12 to 16 inches would be acceptable at the deepest spot, depending on the next point. it would provide acceptable floor space and head room.

as for heating i was considering a rocket mass heater with the mass being underfloor. i dont find much info specific to daylight basement (even less for culvert home) related to RMH, anyone had tried something related or foresee some important issue with this design.  still tinkering on whether  the chimney should exit behind , buried insulated pipe peaking in the field above, or going back and forth inside exiting on the daylight side.
any venting or pressure issue would arise from such design?
i have red a long time ago ianto evan's book and i am currently reading the RMH book by erica and ernie.

As for insulation i was originally thinking of closed cell foam on the outside, but one of my friend is close to commercialization of a ceramic paint (reflect most infrared) i proposed myself to be Guinea pig for his product, i could report on that later on.
as for drainage the land is naturally very well drained, and was planning on clean rock/french drain for peace of mind,  the back wall and any potential opening would be sealed and a membrane applied.

if anyone has some insightful thought about the difficulties that i might encounter i am all ear.
my current planning issue concern mostly the RMH part, i tried to give enough detail to provide good idea of the project, if it is too cluttered and i should split some of the info into different post feel free to guide me in  the right direction.

thanks for the brain power of the community
Bruno

Putting that RMH below grade makes a lot of sense?   Floor space will be a premium with only that max dia. of 8 ft.

IR reflection only works if there is an air gap from the painted surface.  If anything touches the paint it no longer works, because once something is touching it any heat transfer is through conduction rather than radiation.  It is very common for radiant barriers to be used incorrectly where they add cost but offer zero insulation because they don't have an air gap.  If you decide to use your friends paint make sure you research and actually understand what radiant barriers actually do and how they are to be used.   When used incorrectly in a worst case scenario they can actually cause faster heat loss than nothing at all.

I personally try to avoid using foam because of cost and environmental issues.  But I think in your case a closed cell spray foam on the outside of the culvert may be a good choice.


I think putting a rocket mass heater in the floor could be a good idea, although I would put down a layer of rocks or washed gravel before putting in the mass for the heater.  That way any moisture that may make its way in can flow through the rock and exit out the front to get it out of the house.

thanks for the lead on the IR barrier, i will double check this one with my friend.
actually since everything will be underground, i was questioning the necessity of insulation in the first place or if insulation should be all around the pipe or maybe just on top half. especially if i end up going close cell foam.
heat sink due to water movement should pretty much be a non-issue in my case.

I think you need insulation, otherwise the ground will constantly be trying to suck the heat away and cool it to whatever the ground temp is there.  In the summer that will keep it cool, but in the winter I think it would be hard to heat with no thermal break between the walls and the earth.

You definitely want insulation all around, maybe not a lot on the bottom but continuous. I would investigate the cost to r-value of different things like styrofoam board before settling on a type. As it would be tricky to insulate the bottom outside of a culvert and you will need a level floor anyway, putting a few feet wide of insulation inside under the floor thickness would make sense.

If properly insulated, you will have very low heat loss, so a small RMH would be sufficient. A 6" J-tube would be more than enough (if oversized, just run it less often), but if you are handy and good at precisely following plans, a 4" batch box could take up less space above the floor. See the cyclone batch box built at Wheaton Labs a few years ago. That one had an issue with overheating the space due to thermal lag, but a continuously occupied non-drafty space would mitigate that, and an inspection access panel built into the side of the bell would give some instant heat to minimize lag.

if i go close cell foam route, my idea would have been to insulate it outside the dugout hole before final placement. if using styrofoam board do you think laying it flat underneath the culvert and straight up could encapsulate effectively some thermal mass outside the pipe?

i was pretty much settle on the 6'' j tube, it would be my first RMH build so the 4'' seem like a flip of coin for me.

as for drafting the RMH my first choice would be a straight run underfloor all the way to the back end and venting it through the ground in an class A chimney. my line of taught was that being at ground temperature it would help with startup during the heating season. (probably making it hard to light up in summer, dont think it would be an issue tho )

I think spray foam sprayed on the outside of the culvert before you put it in the hole is probably the way to go.  But it will cost more.

Your other method would work(assuming the styrofoam can be in ground contact and is waterproof/ water resistant.  Although I would question how much thermal mass you want in a building of that size.

With either method I would put a bed of washed gravel at the bottom of the hole to give water a place to drain.  And once the culvert was set and back filled I would drape rubber roofing over the culvert and 10 feet in all directions then cover that in more dirt.  That way any water coming from the surface is directed away from the culvert.  I would also do minor earthworks as necessary to prevent water run off from flowing towards the culvert, And to prevent water from puddling near the culvert.

I would want some sort of ventilation/skylight/egress window at the end of the culvert buried in the hill.  Without lots of ventilation it may become very stale and humid in there in the summer.

I can't offer specific suggestions about the rocket mass heater.  My experience on them is limited to reading and videos I have watched.

Bruno, I'm having a lot of trouble picturing your plan.

It sounds like you're going to have a 20 foot tube, mostly buried in a hillside:
1. Lengthwise or crosswise?  
2. Will you only be accessing from one open end, or will you be cutting holes for windows/doors in the side?
3. Somehow, you will need some flat surfaces to stand on. Are you going to do them  in stages like extra wide steps? If not, how are you going to preserve your head room?
3a. How tall are you? Have you done the math? Once you put a floor down the pipe, will you still be able to stand straight up?
3b. Remember the roof will also be curved, so just how wide will the area across the top be that you can stand in and not bump your head?

Just a super crazy alternative to consider: Cut the tube into two 10 foot lengths and orient them vertically in your hillside? It will be more complicated as you'll need a very sturdy roof to support dirt and some way to connect the two. https://youtu.be/_Z1STIG88GM

Or cut it in half lengthwise and build a substantial knee wall, so the half tube is a lovely curved roof?

Eight feet is already a very narrow space to work with, and it's only going to be eight feet wide four feet off the ground. I'd hate to have you put a lot of work into this project, only to find that you hate living in the space!

hi
to help clarify some point:

only one end will be open, door and window like a daylight basement.

i will try to avoid cutting as much as possible, it is thick and a pain to modify without a plasma torch, and want to keep it as structurally sound as possible.

if i go as planned for a 12 inches at deepest spot (center of pipe) it gives me 5.3 feet wide floor 7feet of head room at the center and 6 at the edges.
extra space within the curve on each side will be bench and short countertop like table(+-16 to 24 inches).
the sleeping quarter will be in the back end, along the horizontal diameter of the culvert to be able to lay flat across, and storage 3feet high below bed.

so for the buried exhaust from RMH if it can fit inside 12 inches of floor all good, i can go few inches deeper if need be.
i will likely be able to sink the RMH a few inches inside the floor but not all of it ( flooring depth will be limited near the edge of the cross section)
i will likely to have my manifold exiting at a angle , or from the bottom

@ j hillman
i was tinkering with backend skylight i think it could serve as ventilation as well, and would add a lot the livability of the place.
this one still need planning ,potential proximity with the chimney is one of my concern.
washed gravel and french drain is in the plan, probably not needed but so much more trouble fixing than planning. we said ``belt and suspender`` around here.

Your numbers of a 5' 3" floor and 7' headroom  work out if the culvert is 8' inside diameter.  Every metal culvert I have seen is listed by its outside diameter.  You will have to subtract 2x what ever the corrugation depth is.  I assume that will cause your inside diameter to be closer to 7'6" or 7'8".  Which will still allow you a 5 foot wide floor and over 6 foot of head room.   You should double check your measurements before committing yourself too much to the idea.

I have a cattle pass culvert that was replaced because the bottom rotted out.  I plan to do the same as you but for a tornado shelter.  Because the bottom is gone I plan to put in a cement pad and a single row of block to set the trimmed up bottom edge of the culvert on to give me a bit more head room.  When it was still under the road, even with both ends fully open it was kind of claustrophobic when standing in the middle of it, so lots of light on both ends and possibly a skylight in the middle would be a priority for me if it was intended to be living space.

good call for the corrugation, i dont remember if i took the measurement in or out of the sinus , they were more like 1''  deep tho . will definitively double check that one before the final plan.

the pipe now stand in the open , it doesn't feel too dark, but when buried and wall framed it will be another story. i will figure a way to include plenty of  light at least at the other end. and while at it a second emergency egress should also be a good addition.

bruno hay wrote:...and while at it a second emergency egress should also be a good addition.

You may not *have* to follow code where you're building, but this is absolutely one of those areas where the code is totally right. Safe egress from a sleeping area is a good way of extending your lifespan!

I don't know  what your topography looks like. Have you looked at the Wofati buildings done a Wheaton Labs? They use a number of systems to get an exit on the uphill side of their buried buildings. Depending on the shape of the hillside and the orientation to south or east, you may be able to bury the culvert at an angle, still get good dirt coverage, but make both the back egress and the RMH exhaust. Are you planning for a dedicated air intake fixture at the daylight end?

Do you actually *have* to bury it in a hillside? Many Wofatis seem to be built only slightly underground, then they build a pond and dump all the pond dirt on top to bury the building. The use an "umbrella" of insulation and a waterproof membrane one top of the first layer of dirt, so that layer acts as dry thermal mass to help control temperatures. Wet dirt doesn't insulate. Lots of plants help keep the dirt stable on the roof.

are you thinking of this project at wheaton's lab? :    https://paulwheaton.com/allerton-abbey-the-first-wofati/

the area i am thinking of building is actually a big field on a mountain side (+-5 acres). about 10 years ago the county remodel a major road nearby and needed place to ''store'' thousands of tons of dirt . so as an operator it is an actual sandbox for grown up.
so yes maybe the option of having both end open and changing the topography would be an obvious solution, i think i would have places suited for that building path.

If you can have some of both ends exposed, that would be ideal. A hobbit house is very cool, but you don't want it too deep - 15' from the window end would be decent, 20' seems a bit much. All you really need at the far end is enough exposure to put a decent sized operable window less than 4' off the floor for emergency exit.

If doing a 6" J-tube RMH, I would locate the feed farther from the door to give the least chance of a bottleneck in case something went wrong with it. This is another argument for an emergency exit at the back end. I think you could run the exhaust under the floor say 5' toward the back, then turn and run toward the front and up. A 6" system is supposed to be good for about 35' of duct, minus 5' for each 90 degree bend, so 25' under the floor. Keeping the chimney inside the space until it reaches the roof would give the best draft and heat retention. I would make the floor fill of cob, not just dirt, with plenty of rocks included to improve the mass and reduce the cobbing work. I would allow for largely surrounding the barrel with brick or cob once you operate the system and see how much instant radiation you want versus thermal storage.