Let me start by saying I like comfort despite a relatively high tolerance for misery.
In my teens I spent more weekends in a tent, or a hand built “survival shelter” than at home, I have slept wet and cold more nights than I can count, but the coldest, most miserable I have ever been was living in a poorly insulated house in Northern Mississippi. The warmest I have ever been was in a small stone cabin in the Adirondacks heated with a wood stove (the downside was the trips to the outhouse in -20 degrees)
I also hate high utility bills, for a lot of reasons, but mostly because they are unpredictable both month to month but also over the life of a house. Investing in a well- insulated, tight house can fix this, but it also raises some challenges. I want to touch on some of these in this post, as well as some of the myths.
MYTH #1 – You need expensive windows to have an energy efficient house. I will probably talk about this more at some point, but I want to put this out there right now, it doesn’t matter how great your windows are, they still suck. They are holes in your wall that heating exits in the winter, and enters in the summer. Your money is better spent on insulation. Don’t think that’s true? Ask a building weatherization expert, and they will tell you the same thing - If your windows don’t leak air they are good enough. Did you buy really expensive windows already? Great, you still need insulation.
MYTH #2 – Small houses need less insulation. Sure, if you are going for a really inefficient house, great, do it, and you might see savings in your electric bill, vs. a typical apartment or house, but I don’t want a tiny house with the efficiency of a 1940’s Dodge power wagon (although if you have a Dodge Power Wagon sitting around that you want to give me, I would gladly take it off your hands).
MYTH #3 - Old builders know best. Building technology is a moving target, and the last 30 years have seen some of the largest leaps in building technology since the Romans figured out running water and concrete (and that took them hundreds of years). There are a lot of changes that building science has taught us. Science is the key here: people building things, testing them, and taking them apart years later.
MYTH #4 – “I can’t screw this up that bad, right?” Wrong. Some of the risks from a house that is not insulated following building codes and modern construction standards can range from sickness to death…yup, death. (Yup, you will have to read down to catch the details*).
My take on insulation: Put your money where it counts. I like stuff done right, but let’s face it, sometimes we have a budget that limits what we can do. We need to understand where and when to cut corners. This can be really challenging because sometimes even design professionals have disagreed over what is best, acceptable, or just a really bad idea. But at this point, within the architectural and building fields, this uncertainty has pretty much gone away do to the research that exists. So, rather than address all housing construction, I am going to limit my comments on insulation to the tiny house world.
Your Roof assembly is not the place to cut costs. Your insulation is more critical than you think – put your money here! (and not because heat rises). I am going to assume that all houses will have low slope roofs(that’s what some people call flat roofs) or cathedral ceilings, where ceiling height is at a premium, and building height is constrained. This means that tiny house roof assemblies will be very difficult (I would argue almost impossible) to ventilate. We call a roof vented with exterior air a cool roof, and this is how most houses are built to prevent ice dams at eaves. This can be done for cathedral ceilings, but is difficult, and requires vented soffits and ridge, and insulation must be held a minimum of an inch from the roof sheathing. - This means less R-value in your roof. This leaves three viable options for your roof.
ROOF INSULATION OPTIONS
1. Structural Insulated Panels (SIP) I think for those who have considered this option, they usually think it’s an all or nothing proposition. This doesn’t have to be the case, and there are some real benefits to going with a SIP roof.
First, inch for inch SIPs will give you as much R-Value as any other roof assembly but with a tighter envelope. Second, SIPs go up quick, so, in many cases, can allow for a roof to go on in less than a day. The last advantage I would like to point out is that SIPs offer unique structural potentials that in many cases lend themselves to a tiny house. SIPs are a type of stress-skin diaphragm that can provide additional structural integrity. It is important to point out that this additional structural advantage must be detailed properly; however, the inherent nature of the SIPs is a much stronger assembly than a stick built roof, particularly regarding dynamic loads that a tiny house experiences while trailering. So, SIPs for the roof provide more insulation, a tighter envelope, and faster assembly on site.
2. Rigid Foam Sheathing with spray foam to fill gaps. This option is a similar R-value to closed cell spray foam, more likely lower in material cost, but requires more labor. Translation: if you are not paying for labor, it will cost less, but it’s a lot of work, and you will need to use a combination of spray foam and foam cement to get a tight fit. While this will not be as airtight as spray foam, it is a good option for the roof assembly.
3. Spray Foam Insulation. There are two options that might seem confusing, but really are not that complex.
- Open cell spray foam has a much lower R-value per inch (R3.5 per inch) than Closed-Cell Spray foam and rigid foam. While it does have some real advantages over batt insulation, particularly with its ability to seal air infiltration and prevent cold spots, for a roof cavity, this will mean a much lower R-value than Options 1 and 2.
- Closed cell spray foam brings a higher R-value (as much as R6 per inch after it cures (higher at initial install) and it seals openings in the envelope making a tight skin. The final deciding factor that I would say makes closed-cell foam the best choice over either open cell foam or rigid foam sheathing is the added rigidity the structure of the framed walls get.
WALL INSULATION OPTIONS
1. Structural Insulated Panels. As with the roof, inch for inch SIPs will give you as much R-Value as any other assembly but with a tighter envelope. Second, they go up quick, with less on-site labor and the use of wall SIPs also offer unique structural potentials that in many cases lend themselves to a tiny house. SIPs are a type of stress-skin diaphragm that can provide additional structural integrity. It is important to point out that this additional structural advantage must be detailed properly, but if you are at the point that you are building your walls and roof out of SIPs, the manufacturer will be providing those details. The inherent nature of the SIPs is a much stronger assembly than a stick built wall, particularly regarding dynamic loads that a tiny house experiences while trailering. So more insulation, a tighter envelope, faster assembly on site. The drawback is that panels are sheathed on both sides, so wiring and plumbing must be run through panel chases.
2. Rigid Insulation with spray foam gaps filled. This is a good option for R-value, but working around studs, plumbing, wiring could be tough. One option is to sheath the insulation on the exterior of the house (this would be best done in combination with another type of insulation such as open cell spray-foam or batt). Rigid insulation on the exterior also provides a thermal break to prevent thermal coupling in the wall assembly, which is a significant benefit.
3. Spray Foam Insulation The real benefit of spray application in walls is its ability to seal at window and door openings as well as to seal around wires and pipes running through walls. My choice here would be closed cell spray foam – higher R-Value, added rigidity to the structure and the ability seal the wall assembly make it the prefered choice.
4. Batt Insulation. There are 3 basic options for batt insluation.
- Fiberglass. Fiberglass is the lowest R-Value per inch, but that aside it has some real drawbacks. A small tight house means condensation entering the wall assembly is a huge issue that can propagate mold in the wall cavities as well as reduce the effectiveness of the insulation. In addition to these issues, I am going to throw in the fact that rodents love this stuff, and anyone who has taken apart a vintage travel trailer has seen the evidence.
- Rock Wool. Higher R-value, but still lower than foam options and doesn’t have the issues with moisture that fiberglass does. However, condensation is still a potential issue that needs to be managed in the wall cavity with a vapor barrier.
- Alternative (green) batt – these range from wool to blue jeans. While I don’t have any experience working with these materials, R-Value moisture and pests should be considered.
FLOOR INSULATION OPTIONS
1. Structural Insulated Panels. For most tiny house projects the built-up framing is a lot of additional structure that is serving very little structural purpose. Detailed properly this could be a viable floor decking option. While I have not used this in a tiny house floor, I have used it in other floor assemblies, and think it might work well in some house applications.
2. Rigid Insulation with spray foam gaps filled. This seems to be the most reasonable option for tiny houses constructed without a vented floor cavity. If you are sealing the floor, particularly with flashing sitting on the frame, closed cell foam is the best option.
3. Spray Foam Insulation. The real issue I see with this option is access to the floor cavities, getting a sealed floor cavity makes this a great option though.
4. Radiant Barrier Insulation. This is a product that is full of controversy, and I take a middle-of-the-road position on it. Radiant Insulation (the foil bubble wrap looking stuff) claims really high R-value (as much as R-18) and seems too good to be true, except for the price, and then you really want to believe it works. I spent a week camping in the desert with a radiant insulation material to shield my tent from the sun, and I noticed a huge difference, but that was with the foil exposed to reflect the direct sun, and was not in anyway scientific. I have talked to several people in the vintage RV restoration world who swear by this type of product. Keep in mind that my 1974 Airstream has only a 1.5 inch thick wall.
I dont think that you should depend on this for your R-value, but if you choose to use it, keep a few things in mind. First you need to read how to install it, and if you don’t do exactly what they say, plan on getting a much lower R-Value. Also, the stuff you get at Lowes and Home depot is glorified bubble wrap. Those bubbles are where the insulation comes from, so you better believe bursting them will make it much less effective. This also means I would expect diminished effectiveness over time.
The product that I like best of the ones I have seen is called Prodex. Rather than using bubble wrap, it is about ¼” of closed cell foam with a radiant barrier on top. The upshot is that you can nail and staple through it. They also specify how to use the product on the outside of a building envelope which means, if you get nothing else from the product, it will do a great job as a thermal break, and can also act as house wrap.
*Finally, MYTH #4 explained. Tight houses mean less air is leaking out of your house. This “leaking” is bad for thermal efficiency, but living in a sealed jar isn’t all good either, and could be dangerous. Many conventional building materials off-gas all sorts of chemicals. This can cause a range of short-term and long-term health problems. A recent example of this is with the post-Katrina FEMA trailers. In addition to new materials chemical off gassing, your indoor air quality is impacted by things like mold, dirt, and even rodents (and their excrement).
We have seen a range of these issues in the press in the last few years. I really don’t think we need to fear these, but we need to build smarter, especially with Tiny Houses. We also need to consider those who might own our houses later in its life cycle. Maybe you plan on using it occasionally, but then you sell it to someone who lives in it full time. The chemicals you put in your house might not be an impact to someone in the house for short term stays, but the daily impact could be much more significant.
The other issue you need to be careful of with tight envelopes is making sure you are properly venting your combustion appliances as well as your waste lines and tanks. Putting a carbon monoxide detector in your house if you have combustion devices is a must. See this article for more details.
Thermal Bridge – http://en.wikipedia.org/wiki/Thermal_bridge This is an important concept, and often an overlooked issue.