The Going Green Home - Part 2: Insulation

(The Going Green Home - Part 1: Introduction)

Photo by Jude Hill. Licensed under CC BY-NC-SA 2.0, via Flickr

Insulation is nominally simple. It's usually fluffy or foamy, airy stuff that fills your walls, ceiling, and floor and keeps the heat where you want it: inside in the winter, outside in the summer. Insulation reduces the need to keep the heater (or air-conditioner) going full-tilt 24/7. Big energy-saver... if not life-saver, depending on where you live!

Insulation is, naturally, actually more complicated than that. One piece of vocabulary you should understand is R-value, a measure of how well a material is able to resist the flow of heat. The higher the R-value, the better. A type of insulation with R-8 per inch is a more efficient way to insulate your home than one with, say, R-3.5 per inch.

A bigger R-value means less energy used for heating and cooling, and less energy generally means fewer carbon emissions to your name. According to this (admittedly industry-created) online calculator, increasing a 140-square-foot tiny house's insulation from R-12 to R-16 reduces its environmental footprint by 227 pounds of greenhouse gases per year, assuming it's heated with electricity that comes from a coal-fired power plant, as 39% of US electricity does. That's the equivalent of conserving 11.5 gallons of gasoline per year. It's something!

From what I've heard, some of the pioneers of the tiny house movement kind of slacked off when it came to insulation. I heard one tiny houser (was that on a blog or at the PAD workshop? I don't remember... dagnabbit) recall that during their first tiny house winter, icicles formed on the inside of their tiny house! So insulation is definitely not just about greenhouse gases or even fuel bills; it's about comfort and safety, too.

Even in these more enlightened times, though, tiny houses are generally not insulated as much as full-sized houses. I live in the Department of Energy Zone 4, and for a full-size house here, one environmentally conscious recommendation is to insulate the walls to R-30 and the roof to R-60. That would require between 3.75" and 8" of rigid foam insulation, or 6.5" of mineral wool, in the walls and twice that in the roof. Actually, more than that; those are whole-wall R-values, meaning that they take into account the wall studs and rafters, which connect the inside and outside via a "thermal bridge."

With thicknesses of that nature, do you see where issues might arise with tiny houses? The thicker the walls, the smaller the interior space, and we can't add to the exterior house size without running into DOT restrictions.

But it's okay.

In my Pacific Northwest, Zone 4 climate, the tiny house consensus seems to be that 2x4 wall construction and 2x6 floors and roofs--and as much insulation as you can fit in those 3.5"/5.5" spaces--are actually sufficient. As it turns out, with the kind of tiny spaces that tiny houses represent, you can relax a little on the insulation front because their size makes them so easy to keep warm. I think it's true, too. When Dad and I stayed at Caravan Tiny House Hotel, we noticed that our little house heated up crazy fast. It seemed like even body heat made a difference. We actually had to open the windows!

So: insulate, but there's a balance to be struck between space and wall thickness/insulation. Unless you live in a hot or very cold climate, you can create a well-insulated, energy-efficient home without bending over backward with insulation.

The kind of insulation you use, beyond its R-value, could also make a difference in your house's eco-friendliness. But it's no simple task to determine which is "best." 

As will become clear by the end of this post, I am not expert on this subject... but for what it's worth, I'll be using mineral wool in my 2x6 framed floor, polyisocyanurate rigid foam insulation in my 2x6 roof (polyiso is the highest-R-value type of rigid foam insulation), and structural insulated panels (SIPs) for my walls. Let's compare their pros and cons.

Plastic-based foam, like polyiso, is a great insulator (up to R-8.7/inch), but it takes a lot of energy to produce, is not biodegradable, and is made from petroleum products. Certain kinds, like extruded polystyrene (XPS) and some closed-cell spray polyurethane (SPF), are especially bad because potent greenhouse gases are added to them to puff them up, releasing some of those gases to the atmosphere in the process. Environmentally, there's bad and good. And bad.

Mineral wool is produced by melting down rock and spinning it like cotton candy, so you'd think it would be a very energy-intensive process (molten rock! Oooh!). However, its production actually only takes a fraction as much energy as the production of some of the plastic foams. And its production diverts slag rock from the waste stream, it is chemical-free, and it has a decent R-value (R-3.7/inch). Plus, it's fire-resistant, water-resistant, easy to work with, and pretty readily available. Some people think it's pretty awesome.

Photo: D. Fitswell. Licensed under CC BY-SA 3.0, via Wikimedia Commons

SIPs are generally made from expanded polystyrene (EPS), which is pretty icky stuff, environmentally. However, SIPs provide some of the best whole-wall R-values possible because there are at least half as many thermal bridges than in regular stud construction, as well as less infiltration of outside air and zero convection cells inside the walls. Most of my information about them is from the industry, which is unfortunate, but... well, what they say makes sense to me.

Photo by USDA. Public domain, via Wikimedia Commons

There are myriad other insulation choices, too. Sheep's wool and recycled cotton--AKA recycled blue jeans--are two prominent players in the green insulation market. They hold promise as natural, renewable, biodegradable insulation materials. Moreover, blue jeans insulation diverts denim scraps from the waste stream. Pretty cool! But... I'm not using them in my house.

This Building Green article about blue jeans insulation points out difficulties in installing the insulation, and correct installation is critical if you want the insulation to do its job. Another article, on the Fine Homebuilding website, argues that the chemicals used to grow cotton and create denim are a strike against its green credibility, as is its poor performance when damp. Plus, it's pretty danged expensive, compared to relatively benign materials with similar R-values, like mineral wool.

Photo by Paul/Magic Foundry. Licensed under CC BY 2.0 via Wikimedia Commons

A few tiny housers, like Evan and Gabby and Kate from Naj Haus, are using sheep's wool to insulate their floors. I couldn't justify the expense when there is so little objective, critical information out there about its performance and durability. It should have pretty good insulating power--about the same as other fibrous, batt-style insulation. I buy that it has environmental and health advantages over some synthetic materials... although livestock--including wool--production is not the most eco-friendly thing on the planet, nor always the most ethical. But what about moisture damage? Pests? Settling and shifting? Fire? Plain ol' degradation? (Update: This Australian article confirms that sheep's wool is good on the environmental front, but may have issues with durability and maintenance.)

With all the pros and cons everywhere, it's confusing and difficult to figure out what is truly the most eco-friendly choice. I can't find an objective, peer-reviewed life cycle analysis that really compares all my insulating options. The best I can find is this one, which compares paper wool, mineral wool, and flax fibers (in order of lowest to highest environmental impact--yes, flax was the worst).

But how useful is it to be so meticulous? That same article states in its summary:

It must be recognized that insulation of buildings saves more than 100 times the environmental impacts associated with the production and disposal of the products used for insulation.

Since I can't access the full article without buying it, I'm not really sure where that figure is coming from. But there's a chance that we shouldn't feel bad about using a horrible, plastic-based insulation material, if it insulates our houses better than the "green" choices can. But... it still seems right to try to use insulation that won't poison the planet. Right?

Oh boy, choosing the right insulation can seem like this...

Ultimately, my personal decisions on insulation have come down to a combination of environmental concerns about the material, the desire to insulate my house well and save energy, ease of sourcing and installation, and budget. Got to admit, budget is a big one. At some point, I have to just cross my fingers and hope I'm also doing right by the planet.

I kind of wish I'd studied green building science!