Insulating Hut 2.1

I posted some thoughts on insulation a while back, but now that I’m actually in the process of insulating Hut 2.1, I figured I’d do another post talking about what I’ve ended up doing.

As you can see in the photo above, I’m using R-5 3/4″ thick polyisocyanurate rigid foam insulation boards on the exterior. If you recall, I used that stuff on the roof as well, so basically the whole structure, save for the windows, door, and floor, will be encased in those boards. Normally, insulation goes inside the wall cavities, but I first thought of putting insulation on the outside while working on Hut 1.0, and implemented the idea in Hut 2.0. The reasons for doing so are manyfold:

  1. Putting insulation on the outside leaves the wall cavities available for use. I’ve filled in some of the walls with “shelves”, but I also have the option of filling them in with batt insulation if the rigid boards on the outside prove to be insufficient. The same applies for the space between my rafters, which I may fill with batt insulation later.
  2. Instead of using a traditional air barrier, I’m taping the seams between the insulation panels together so that the insulation panels themselves form an air barrier. The boards on the roof also act as a waterproof layer, in the event that moisture leaks through the roofing panels. This cuts down on building materials, which lowers the financial cost as well as the total environmental footprint.
  3. By wrapping the entire structure in insulation, I am increasing the thermal mass within the thermal envelope. That is, all the posts and beams and OSB sheathing inside the insulation act as a thermal mass, which can absorb heat and release it slowly. Obviously, wood isn’t as effective of a thermal mass as, say, a concrete slab, but I think it counts for something. The downside is that, if I’m heating the structure from a dead cold, it takes longer to warm up, because, for a while, the structure itself is going to be absorbing some of that heat.
  4. Covering every square inch of the exterior in insulation (again, except for windows and doors) prevents thermal bridging. Thermal bridging, in the context of structures, is when heat conducts through structural members, bypassing insulation. For instance, in a traditional 2×4 stud wall construction, only the space between studs typically have insulation, so the studs themselves can conduct heat in or out. While that may not seem like much, if studs are spaced 16″ apart, that’s 1.5″ out of every 16″, or close to 10% of the surface area that’s left uninsulated.

When I first thought of the idea, I thought it was all new and radical, but I’ve since learned that this exterior insulation thing is… well, a thing. For instance, a related and somewhat similar concept is SIP –or Structured Insulated Panel— construction, which is a fancy way to say “insulation boards sandwiched between structural sheathing.” The one interesting thing about SIP is that the insulation boards are glued to the sheathing, thus increasing rigidity and eliminating any gaps. Gaps in insulation are bad, because it could allow for air circulation, which can render insulation moot by carrying heat in or out. For Hut 2.1, I originally tried using a spray-on glue to attach the polyiso boards to the underlying OSB sheathing, but the glue I got didn’t stick too well. So, instead, I’ve been nailing the boards onto the sheathing, and, where possible, through to the 4×4 posts (the gray patches of duct tape in the photo above are where I’ve put in nails). That doesn’t eliminate the gaps, but, in my case, I’m not terribly concerned because those gaps will be inside the thermal envelope, and as I mentioned above, the foam boards should, in theory, form a mostly air-tight enclosure and relatively little of that heat should escape outside.

For the floor, I’m thinking of doing something different as well. Normally, batt insulation would be stuffed in the spaces between the floor joists. But, I don’t like fiberglass, and ever since I saw those cubes of recycled cellulose blow-in insulation at the hardware store (pictured below), I’ve been wanting to use them. The stuff is made of recycled materials, supposedly uses far less energy to manufacture than traditional alternatives, and is pretty cheap ($8 per bag), so it sounds pretty awesome all around. The current plan is to lay down more 2×4 “joists” on top of the existing floor (though running perpendicularly to the existing joists for strength), then put another layer of OSB on top of that, and fill in the 3.5″ tall gaps with the blow-in insulation, which should give me about R-13. Since the insulation will be sandwiched between two layers of OSB, I also won’t have to worry (as much) about critters getting in there or stealing my insulation, which I hear are concerns for the more typical exposed under-the-floor batt insulation.

So, that’s basically the plan so far. I’m considering getting some batt insulation that’s made of recycled materials, similar to the blow-in insulation I got, to stuff in between my rafters. But, I think I’ll hold off on that until I get my stove going, and see how well the existing insulation works (or doesn’t work).

19 thoughts on “Insulating Hut 2.1

  1. Are you sure that you are not a carpenter? That looks great. I’m thinking hot tub or sauna next. Sitting in a hot tub gazing at the stars surrounded by snow.

  2. Ryo, you cover so many bases, I don’t feel I have anything to add. I find your blog immensely helpful, reassuring and entertaining.

  3. Yup, looking good-very professional!!

    You could lay styrofoam 4X8 sheets right on the floor also and save some time. The flooring ply can screw right on top of this. Costs more $$ though…….

    • Yeah, two bags of cell insulation cost about as much as a single foam board. But, I also like that the cell insulation is environmentally friendly (foam boards are pretty bad).

      • Styrofoam is about as friendly environmentally as nuclear waste…..
        Just a thought.

        I live in central Ontario, wood heat, no insulation on the floor. Feet get cold from time to time, but don’t actually miss the insulation at all. Aprox. 20% max heat loss goes through the floor.

        Put down a throw rug….

  4. Actually, weight for weight (or mass for mass 🙂 ) wood is a better store of heat than concrete. By volume concrete wins, of course, but only by about a factor of two (i.e., you can store roughly twice as much heat in a m³, ft³ or whatever of concrete as the same volume of wood for a given rise in temperature).

    I think you’re quite wise to have a reasonable thermal mass inside your insulation as it will make the temperature of the hut a lot more controllable – you won’t have it too cold then, when you turn your back for a minute or two, it’s suddenly too hot. I think this could be a real problem with a small volume.

    Having a lot of exposed wood is also good as it’ll help to buffer the humidity a bit making that more manageable, too.

  5. Perhaps it is the fine blend of fermented barely and hops I am currently enjoying that is interfering with my understanding but there were a few things that puzzled me.

    1- You’re adding depth to the floor? Will the (in swing) door still work?

    2- Are you using blown-in/ loose insulation for the ceiling? 2×4 rafters?

    • 1 – I am adding height to the floor… basically laying another floor on top of the current one. The area near the door won’t have this addition, so it’ll be lower than the rest of the interior. This is actually pretty common in Japan (and perhaps other places), where people take off their shoes before going inside. The lower floor near the door, where shoes are taken off, prevent dirt from permeating.

      2 – I’m using blow-in insulation for the floor, to fill the space between the old floor and new one.

  6. you I have insulation board all on the outside of my 24×24 place. Like you I also am on posts but since I am in an area of cold weather I took the insulation board all the way to the ground and backfilled with crusher dust…keeps it warmer for water lines so they don’t feeze and helps keep critters out. I also have 2 inch styrofoam board in between osb/floor sheathing..makes a huge difference…and is still stable. We put down some new flooring and I did have to compensate for the extra floor foam and use 4 1/2 inch nails..not to mention the pain in actually finding the boards to nail in the first mark them clearly before covering..Looking at your size I would be very suprised if you had heat loss issues..I think your bigger issue will be remembering to close the window when you go to bed due to it being to hot in there. Also, in terms of thermal mass..I was trying to think of a way to create an interior moveable counter that was essentially filled with water that you could wheel into the light or near the woodstove so that all that useful heat gets stored..water is the best for storing energy…took some passive heat courses but our project was too far along when I had some ideas. Looks great! keep up the good work!
    Oh, one other thing..if the place is too sealed up it will create a vaccuum where pressure is always attempting to suck air into the dwelling..usually cracks and windows and doors allow for enough air to ensure air is entering..I say this because if the interior is starved for air you will have a difficult time getting a fire going and sometimes the pressure is so great inside that the fires attempt to feed through the chiminy and the smoke enters into the house…can be even more pressure if there is no wind. Anyway, I digress again..keep up the good work and happy holidays!

  7. Can I assume that you have given up the idea of having a hole in the roof, to let the moisture and heat out?
    The logic of copying the SIPS system, in having the insulation outside, is good. However, you do need to protect the polyisocyanurate with more oriented strand board, or similar.
    I gutted and insulated a home some 33 years ago with one inch thick polystyrene (a very modern idea then) and it worked perfectly saving about 90% of my heat.
    I have moved on since then, and my current job is insulating a flat roof with eight inches of polyurethane sheet, this has gone well, I have a foot or so of snow on top that shows no sign of melting, with an internal temperature of 71/72F.

    Batt insulation is a waste of time and space.

    You were correct to fit an external, wind, rain proof insulation that is almost airtight.

    You cannot fit batts to the same standard and your heat and water vapor will find its way behind them and condense (not good) If you must go down this road at least fit another layer of sealed polyiso on the inside.(key being sealed airtight, keeping in mind that water vapor molecules are hundreds of times smaller than air molecules and will get through the smallest gap)

    Keeping in mind that the inside surface of the polyiso sheet will be the coldest surface (other than the windows) inside the cabin and water vapor will head for it to condense. Followed by pools of water inside the frame.

    • I’d like to second Perry525’s caution about fitting batts inside. Their effect would be to lower the temperature of the OSB greatly increasing the risk of condensation there. I meant to mention this in my comment above but forgot.

      If you do decide you need batts on the inside then a vapour barrier inside them would be a very good idea. It doesn’t need to be another layer of polyiso, though; a simple polythene sheet would be quite sufficient if the joins are well sealed and care is taken not to puncture it for electrical fittings, etc.

  8. I would worry about condensation between your polyiso and your osb, rotting out your walls eventually.
    I think there is a good reason why insulation is not typically put on the outside!
    Your eaves are also much too short for the height of your walls, so the bottom half of your house will be exposed to weather. 3 ft eaves should be minimum on any structure intended to last a long time.

    You can read all about the problems moisture and inappropriate construction techniques produce. In New Zealand ‘leaky homes’ has been the result See

    • Damage through condensation is possible, but I’m not terribly worried. The area I’m in is very dry, and things don’t really rot all that easily. But we’ll see in a few years.

      As for the eaves, the walls might get wet, but I’m going to have cement-based exterior siding covering it all up. I doubt rain or snow would get in as long as I seal all the gaps. Again, we’ll see in a few years…

      Ultimately, this is a $2500 structure, not a $100,000 home. If it falls apart in 5-10 years and I have to rebuild, it’s not a huge deal.

      • Right attitude Dude! Don’t take life too seriously.
        In such a small space you don’t even need insulation-if you don’t mind waking up frozen each morning…

        How’s the rifle shooting going?

  9. SteveR, I think you will find when you delve into the New Zealand story, that the problem was caused by badly installed poly foam, which prompt shrank, leaving gaps for the rain to come in.
    Operators that get the mix wrong, the surface temperature and the humidity, end up with shrinking foam.
    In any event, even when things go perfectly foam always shrinks by 15% over the first five years.
    The problem is, that most often the foam is hidden from view and the client never finds out.
    There should be a compulsory infrared camera test after the job has been completed.

  10. What cladding will you be putting outside the PIR? I sort of assumed a ventilated cavity then wood cladding or something but have just noticed that you don’t actually say in this post.

  11. May I follow up on condensation.
    The humidity or not of a region has no bearing on the amount of water vapor inside a home.

    The water vapor inside a home comes from cooking, washing, breathing and sweating.

    This water vapor is held in the air, as long as the temperature is steady, as the fire dies and the room cools, the water vapor turns to condensation.

    Where the warm wet air meets the cold inner surface of the cabin and its temperature drops, you will get condensation and probably with your lack of insulation you will have ice forming on the polyiso and framing.

    Warm air holds more water vapor than cold air. This is OK until the convection within the room,( warm air rising from the fire, hits the roof, a lot of your heat goes through the roof, the remainder, chills, falls down the walls cooling as it drops cold by the time it hits the floor,) causes the water to drop out as condensation.

    However, because of your design, (an open frame) this will dry out again fairly quickly once the fire is up and running and will not lead to any damage. Unlike the normal situation where the water sits inside a wall, where there is no passing warm air or radiation to dry the water, and it starts mold and wood rot.

    I suggest that you buy an infrared temperature gun and use it to scan the inside and outside walls and roof of the cabin. This will tell you at a glance how warm/hot/cold any surface is and encourage the purchase of more insulation, especially for the roof, most of your heat will escape through the roof.

    An infrared temp gauge will cost about $25 on Amazon.

    14 inches of polystyrene insulation is recommended as a means of keeping your heating problem under control and making wood burning less of a chore.

    Keeping in mind the sourcing, cutting, storage, drying, moving, feeding, the better the insulation the smaller the fire, the less wood you will have to prepare and burn to keep comfortable.

  12. Er, yup, other commentators have noticed “ventiltion”, years ago I read in a book it is best practice to heat the interior, insulate the structure and most importantly provide for ventilation.

    That book, 30 years ago at least, was advising one “air change” per hour…(calculations available somewhere I am sure) but you
    can mess with that I am sure. Any sort of wood burner, though,
    and it is time to get advice.

    Thermal bridges are always best avoided as you are doing, but watch out for the “dew point” ocurring inside, or inside the structure.

    I have just added external cladding to my 30x20foot home and I was “told” to provide an open at top and bottom air gap inside the claddding with a 2 inch air gap! Roughly the opposite of what you are doing (Swish Celuka uPVC planking). I ignored that, my precious heat going up a massive chimney, but I did concede by using n expensive vapour breathable membrane on the insulation between the studding facade. It is fantastic stuff
    when carefully fitted to an existing wall…I did a beach hut 39 years ago and it never rotted, shrunk, cracked, warped or discoloured.

    I admire you doing what you are doing, and am skeptical of manufacturers recommendations but ignorant of the possible reasons..I wish they would discuss it like you are.

    Be careful in case it has something to do with staying alive in a fire or not dying from carbon monoxide poisoning. I don’t know,
    but I hope you can take a nap congratulating what you do, and wake to feel relaxed about covering any such issues.

    I am gonna read your blog again, carefully, you have reminded me how little I really understand.

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