Home insulation

[Pointedstick]

PS,

Using your place in New Mexico as the "alternative" to some super expensive home out east, do you mind telling us what the FMV is?  The reason I ask is that I find this stuff is usually pretty scaleable, and I'd imagine your mortgage wasn't significantly higher as a portion of total housing expenses than someone living in a condo in Manhattan, but perhaps I'm way off here.

Either way, though, it seems to me that this is very individual by individual, with a LOT of people out there with net-worths & lifestyles & housing preferences that would make their mortgage a pretty tiny portion of their retirement expenses.  For others, it's perhaps a BIG piece.

I've never had a mortgage. I bought the house for $98,000 in cash. Since then, I have put about $30k into it and am nearing the end of the home improvement tasks. Its stable monthly cost (property taxes, insurance, and utilities, but not including home improvement) is on average $223/mo. About 40% is property taxes and the rest is homeowner's insurance and utilities. I dream of the day when I build a bulletproof waterproof moldproof masonry fortress that needs no homeowner's insurance, has no utility bills, and is essentially maintenance-free. That was originally the plan but I'm glad I bought a conventional house first to learn the skills to do a better job on the dream fortress house once I get it going after ERE.

I have no idea what the house's FMV is. Based on neighborhood trends, I'm guessing I've done better than broken even on the money I've put into it if I were to sell, but frankly I don't care. I don't include its value in my net worth. It's a place to live, not some kind of freestanding savings account.

[Pointedstick]

Pointedstick, I'd like to build that sort of house as well!  Let us know what building technology you settle on (although you might choose something suitable only for high desert climates).

There are many options. The literally dirt cheap approach is appealing--stabilized adobe bricks, compressed earth blocks, or rammed earth. Such materials work exceptionally well with insulation sandwiched in the middle of the wall, and the blocks on both sides in a mixed-but-more-hot-than-cold climate. In fact my amateur modeling shows (hopefully truthfully) that a mass-insulation-mass wall is the thermodynamic ideal for this climate.

A "massy insulator" like AAC is the second best, and also works very well in my climate. I have been severely tempted to build my own walling product as I've learned more and more about construction and the thermodynamics of mass and insulation. In fact, I even have you on my "PP forum people who might be interested in such a venture" list.

[Mountaineer]

I dream of the day when I build a bulletproof waterproof moldproof masonry fortress that needs no homeowner's insurance, has no utility bills, and is essentially maintenance-free. That was originally the plan but I'm glad I bought a conventional house first to learn the skills to do a better job on the dream fortress house once I get it going after ERE.

Are you going to build to withstand 22, 243, 308, 375, 460, 50 cal, 20mm, depleted u238, missile, or heavier duty?  And, waterproof to what depth?  I'm thinking a missile silo design could be ideal, except for the mold proof part. 

... M

[Greg]

I would think without the air layer (since air is a good insulator), you'd then need to have thicker walls to compensate and that'd be more expensive. Plus you then have a crawlspace in the walls for your children to hide in. Double plus.

[Pointedstick]

Yes, please keep me on that list! 

I'm curious why the insulation layer placed between two thermal mass layers would be superior to having all mass either inside or outside the insulation envelope. 

It looks like you could leave the outside and inside surfaces essentially as is though, with rammed earth.  Is that correct?  In other words, no need for drywall on the inside or stucco on the outside?

Being able to leave off or drastically reduce the interior and exterior finishing steps are big advantages of the mass-insulation-mass sandwich wall. But that's not all.

Keep in mind I am in no way a professionally trained scientist, I'm just a monkey with an amateur interest in thermodynamics and heat modeling.

The reason to place insulation between two mass layers is that it essentially cuts them off from each other, causing them to react to the temperature of their adjoining spaces independently. In the summer, mass is beneficial because with sufficient thickness, it substantially eliminates heat transfer through the wall during the day, but it is detrimental because once the heat does penetrate through (after many long hot sunny days for example), your whole house is hot and the AC will need to run practically 24/7 to keep up. In the winter, mass is beneficial if you have an oversized heating appliance or use solar heat because it can soak up that heat without overheating the interior, but it is detrimental because it is constantly losing that heat to the outside, especially at night.

Thermally breaking the wall into two distinct mass sections--interior and exterior--with insulation solves both of these problems, as long as the wall is designed properly.

In my climate, the exterior mass is constantly heating up and cooling down. Even now during the summer, its outside surface might start at 60 degrees in the morning and heat up to 130 while the sun is striking it in the afternoon. This causes an enormous heat flow. All that heat gets stored in the exterior mass as it passes through, eventually hitting the insulation layer. If the exterior mass is sized correctly, by the time this happens, the sun has gone down, and then the exterior surface cools off, reversing the heat flow and causing the exterior mass to lose heat to the exterior. The insulation prevents the heat from being lost to the interior. The mass on the other side ensures that even if small amounts of heat are lost to the interior, they get diffused into a huge mass and neutralized from a comfort perspective. The interior mass, being protected by the insulation in the middle and the size of the exterior mass, stays at more or less the same temperature. Any heat that it stores can be evacuated by opening the windows at night. Ground-coupling the walls works too.

When the weather turns cold, you now have a huge interior mass that's capable of storing a lot of heat and won't lose it to the exterior mass very quickly because of the insulation in the middle. This means you don't need to precisely size your heating appliance the way you do with a light insulated stick framed house. You can have a huge 100,000 BTU furnace, or a wood stove, or a pellet stove, or whatever, because if it produces a ton of heat, it won't overheat the inside; the mass just soaks it up. When you turn it off or the fire dies down, the interior mass radiates the heat back to the interior, and the insulation in the middle prevents much of that heat from being lost to the exterior. Same with solar heat. Any sun that shines through the windows just gets stored, and doesn't overheat the space.

So as you can see, having insulation in the middle is a compromise for a mixed climate. In a cold climate, a mass wall should have nearly all of its mass on the inside, the better to store heat. In a hot climate with air conditioning, a mass wall should have nearly all of its mass on the outside so that most of the daily temperature cycling takes place within the mass and not inside the room.



Greg, air is only a good insulator if it's perfectly still. The principle behind insulation is to trap air within a lightweight matrix where it can't move much, limiting its convection and conduction capacities. An open air space is not much of an insulator since the air will heat up, start convecting within the space, and conducting its heat to the other side. When you fill an empty stud cavity with fiberglass or cellulose or some other insulator, what you're doing is segregating the air into tiny pockets that can't easily touch each other.

[Mark Leavy]

All good thinking PS.  Nothing raised my "bad science!" alert.  Which is rare

[Pointedstick]

All good thinking PS.  Nothing raised my "bad science!" alert.  Which is rare

Phew, I'll accept that vote of confidence! I am a total amateur at all of this stuff, but it's interesting enough to me that I want to keep learning more and refining my understanding. Am I a dork or what?

[Pointedstick]

Desert, you're on the same track I was on a year ago, asking the same questions I was. Martin Holladay's article (your first link) is highly biased, as he lives in Vermont, an almost-always-cold-and-cloudy climate where thermal mass is near useless. He thinks I'm crazy.

You're sort of but not entirely right about your preliminary conclusions, but only for uninsulated mass walls. Nobody builds those anymore. But that said, if they're thick enough, they can be highly comfortable in the summer. The southwest is littered with buildings like this and waking inside when the sun is blazing and it's 100 degrees results in a shockingly high degree of comfort. It's all about mean radiant surface temperature. A wall surface at 70 degrees with an interior air temperature of 80 is far more comfortable than the reverse, which is what we get in lightweight drywalled air conditioned structures where we cool the air but small amounts of heat are constantly entering the house through the thinly-insulated walls and heating up the drywall.

The mean radiant temperature of surfaces is one of the biggest overlooked factors in comfort by nearly all but the Passivehaus people. They recognize it but conclude that the only way to achieve it with insulation is to have like R-50 walls, which is pretty much true. But R-50 walls are ridiculously thick and expensive, which is exactly the criticism levied at mass walls when they can achieve the same thing if insulated correctly.


I'm convinced that the mainstream construction and building science fields are missing something, and I come up against resistance every time I bring up these points to experts through the unimaginably amazing majesty of the internet that permits a layman like me to speak with them personally.

Notice how in that ORNL study, the benefit of well-insulated mass was always positive, even in the coldest, most challenging climate--Minnesota. In Arizona, the benefit was huge. It was never a drawback.

I'm also convinced that their less-favorable results for a the mass-insulation-mass and exterior mass walls were due to the fact that their insulation layers were way too skimpy: R-1.6 to R-2.25. That's pathetic. The lousy fiberglass batts in most of our houses' walls are R-13, by contrast. That little insulation will be a drawback in nearly any wall with has diurnal temperature fluctuations. With 3 or 4 inches of interior polyiso or XPS, depending on the climate (polyiso for Miami, XPS for Minneapolis), I suspect the results would have been very different. Still, the exterior-mass-interior-insulation wouldn't be my choice anywhere. That much insulation would be more useful outside the mass or in the middle of it.


P.S. Insulation R-value does vary with temperature; it nearly universally rises at temperatures lower than 75f and falls temperatures higher than that, except for Polyiso foam, which exhibits the opposite trend. This is due I think to the molecular properties of the trapped gas. Most insulation traps air, which moves more slowly at low temperatures than high ones. But the trapped gas in polyiso foam is the blowing agent used to create the foam rather than air; apparently this gas has different molecular properties from air. I think I saw somewhere that it freezes at low temperatures or something. Radiation absolutely plays a role in insulation performance. I haven't done a ton of research on this yet but I suspect radiation is a bigger deal for foams--which consist of regularly-sized air cells that have substantial open air space for heat to radiate through--than it is for materials with more irregular structures like loose cellulose, where conduction is the primary transfer method. Fiberglass is probably also more vulnerable to radiative heat transfer since the material is glass, after all, which is inherently more IR transparent than other materials. In other words, to cut down radiant heat transfer, it may be beneficial to make your insulation out of a material that has lower thermal conductivity in solid form and then pack it in than it is to trap air in pockets, especially large pockets. This is why dense-packed cellulose is gaining popularity for high-R walls. And if you're going to use insulation that is vulnerable to radiative heat transfer, it makes sense to keep your delta-T as low as possible.

[Pointedstick]

Oh, and mods, could we get this discussion split off into another thread? The original thread is frequently cited but this building science tangent is irrelevant to it.

[dualstow]

This thread has that I've-been-split-off-from-another-thread feel, as the subject doesn't get mentioned for several posts.
Anyway, good time for me to plug "mushroom" insulation - http://www.ecovativedesign.com/

(I didn't use it. I tried to get the contractor into it, though).

[sapperleader]

I am not a expert on it, but we are building a very energy efficient home.  Our home is Insulated concrete forms(ICF).  It is basically a foam mold, where the concrete gets poured between the two pieces of foam.  It is around 8 inches of concrete. 

The house is still being built, but the AC is now being run at night to deal with the humidity(lot of rain this summer).  We have had some really hot days, and the house is going up a half degree to one degree every hour(with all the windows closed).  Basically they turned off the AC at 7am, and by 5pm the house went up almost 7 degrees to 77.  I am really looking forward to seeing how nice it is one we live there and can open and close curtains, run fans, and open windows. I suspect we will only use the AC in the hottest parts of the summer.

Our builder jokes that you can heat the place with a candle

[Pointedstick]

ICFs are excellent; worlds above stick frame construction. High longevity and a good match for a much smaller than typical mechanical heating or cooling system. They're not perfect, though. The mass is placed where it's not as advantageous as it could be, and the insulation is placed where it will be exposed to harm on both sides, so it needs to be covered up just like a wood wall, negating some of the potential savings compared too something like AAC. Also, the insulation is exposed to termites and carpenter ants and other boring insects on the outside; they have been known to nest in it and use it as a conduit to the wooden attic framing. So you can't stop doing chemical termite treatments if termites are a risk in your climate.

[l82start]

In fact my amateur modeling shows (hopefully truthfully) that a mass-insulation-mass wall is the thermodynamic ideal for this climate.

A "massy insulator" like AAC is the second best, and also works very well in my climate. I have been severely tempted to build my own walling product as I've learned more and more about construction and the thermodynamics of mass and insulation. In fact, I even have you on my "PP forum people who might be interested in such a venture" list.

Please keep me on this list as well.

ditto