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Musings of an Energy Nerd

Five Energy Nerd Classics

Energy, the future of housing, heating system design, and choosing foam insulation.

This photo of a one-wheeled tractor is from the early days of Energy Nerd columns. If a farmer can build a one-wheeled tractor, can a carpenter build energy efficient houses that don't rot? (Answer: Yes, we can)
Image Credit: Erik Andersen

Martin will be back soon. Honest.

Until next Friday, please enjoy some classic Energy Nerd columns from the early days of Green Building Advisor.

Energy Use Is the Most Important Aspect of Green Building

Here, Martin sticks a stake in the ground and takes a stand on what really matters in Green Building.

Slums of the Future

Do the McMansion developments of the housing boom represent tomorrow’s slums?

Simplicity Versus Complexity

How to design a heating system: Keep it simple.

Understanding R-Value

The inverse of U-factor, R-value is easier for regular folks to wrap their heads around. Here, Martin explains what it is and what it measures.

Beware of R-Value Crooks

Now that we understand how R-value works, Martin exposes some companies that misrepresent their products with inflated R-values. There are even some ‘client’ testimonials in the comments section.


Yes, We Can

Energy Nerd reveals his underlying positive attitude here with a throw-down to the ‘It’s Impossible’ crowd.

Please enjoy these Energy Nerd Classics; in fact, let’s play a game: see how many comments we can add to these six articles so that his inbox will be full when he returns from his family vacation.

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  1. Danny Kelly | | #1

    A great collection of articles
    Rereading these articles brings a few questions to mind.

    Most of our early training for Energy Star always stated that fiberglass insulation must be sealed on all six sides AND that it MUST BE IN CONTACT WITH THE AIR BARRIER. In one of Martin's articles, he mentions an air space contributing to total R-Value of the assembly. Is the whole fiberglass must be in contact with the air barrier theory correct? Seems to me as long as you have a well sealed air barrier on all sides that an air space would not hurt the performance. From what I remember, I believe they said this is where convective loops would occur thus reducing the performance.

    The two areas that have always confused me with this concept is 1. Conventional attics insulated at the ceiling level with blown-in or batts does not have an air barrier on the top - only sealed on 5 sides - so how does that apply in this situation? 2. In a bonus room situation over a garage. Say you have 16" Trusses or TJIs as your floor system - if you install batts there is an obvious air gap - again, insulation not in contact with the air barrier. We are instructed to block under the knee walls in this situation, which makes sense, but can't you also extend the air barrier along the total length of the floor system top and bottom and insulate the entire cavity? Just extending the thermal envelope but same theory I believe. Sometimes this could be much easier especially when you have ductwork, plumbing and exhaust vents running through the floor system. Seems like in this scenario, the insulation is traditionally installed in contact with the floor sheathing above. In certain instances, it seems to make more sense for the insulation to be in contact with the ceiling below. It would create an air space within the conditioned envelope increasing R-Value and especially if you have plumbing in the floor system, your plumbing could be on the heated side of your insulation. We seem to run into this alot when you have a bathroom over top a covered porch. Seems the typical detail is to keep the plumbing below the insulation and keep the insulation towards the top. Insulating these pipes does little good - if there is no heat source within the pipe insulation then it really isn't doing much good. We have recently started installing 1/2" foam with taped seams on the ceiling below trying to better air seal the floor cavity but have been thinking of moving the insulation down - anyone see anything wrong with this approach?

    Some interesting comments on radiant barriers. Seems like the consensus in the building science world is that radiant barriers are not much good. We have recently used the radiant barrier roof sheathing and I have been very impressed with the results. I guess the jury is still out if it will help or not. I do not think a radiant barrier would replace doing a good air sealing job at the ceiling level but when installed in conjunction with a well air sealed ceiling, I think it has its benefits, especially if the HVAC equipment is in the attic which is common here in NC. Our attics get 140 degrees here in the summer, I have not meaured the temp. in our radiant barrier attic, but it feels closer to 100. I would think that the HVAC system would perform much better in this situation and would prevent some heating the insulation and ceiling joists through radiation which is the main source of heat transfer in an attic. Have had several people tell me they do not do any good - does anyone know of any research on this? If the new energy star requirements are going to require radiant barriers, they have to do some good.

    Is part of the anti-radiant barrier a result of shady salesman? I see a lot of people selling the radiant barriers at trade shows and they install them at the ceiling level. I do not think this is a very good way to install a radiant barrier, although it may help a little for a short time, ultimately not getting the best performnce out of it. These same guys are usually also selling solar PAVs at the same time so am a little weary of their building science knowledge. Have also seen these radiant barrier paints and know a few builders that use them and claim they get good results with them. I can't see how a paint could possibly work properly. anyone have any experience with the paint on radiant barriers? We need to be sure that a few bad apples don't spoil the bunch.

  2. GBA Editor
    Martin Holladay | | #2

    Response to Danny Kelly
    You raise a lot of questions -- more than can be answered in depth in one posting. I'll touch on as many of your points as I can.

    Q. Should fiberglass batts always be installed in closed cavities, surrounded by a six-sided air barrier?

    A. If possible, yes.

    Q. What about fiberglass batts installed on an attic floor? Why isn't a six-sided air barrier required in this position?

    A. Two reasons: because "that's the way we've always done it" and because the penalty for the missing top-side air barrier isn't as severe as in some other locations -- for example, the back side of kneewalls. Nevertheless, fiberglass batts perform poorly in this location. The performance of fiberglass batts installed on an attic floor without an air barrier can be greatly improved by blowing a 4-inch (or deeper) layer of cellulose insulation on top of the fiberglass.

    Q. When insulation under a bonus room floor is installed in contact with the ceiling, with an air space between the top of the insulation and the subfloor above, what's the problem? Doesn't the air space provide additional R-value?

    A. In almost all cases, insulation installed in this way performs very poorly, for this reason: it's very hard for most builders to achieve a perfect air barrier in the cavities between the garage ceiling and the bonus room subfloor. In almost all cases, the rim joist area leaks. With exterior air flowing through the air space above the insulation, the performance of the insulation is severely degraded. If you could do a perfect job of air sealing -- which would mean an impeccable job gluing down the subfloor, and a perfect job installing closed-cell spray polyurethane foam at the rim joist area, and gaskets at the perimeter of the garage ceiling drywall -- then the air gap above the insulation would contribute to the R-value of the ceiling/floor assembly. However, these details are far more difficult to achieve than other easier options. In most cases, your proposed design leads to frozen pipes -- which is why conscientious builders in Vermont and Canada usually insulate bonus-room floors with spray polyurethane foam.

    Q. Do radiant barriers work?

    A. Yes, a radiant barrier works, as long as it is facing an air space. The main problem with building air spaces lined with radiant barriers is that they usually cost more than insulation. If a builder installs HVAC equipment and ductwork in an unconditioned attic -- a terrible practice -- then it makes sense to install radiant-barrier roof sheathing. But it would be far better to build the house correctly in the first place, and to locate the HVAC equipment and ductwork where it belongs -- inside the thermal envelope of the building.

    Q. What about radiant barrier paints?

    A. There is no such thing as a radiant barrier paint, since no paint has yet been invented to meet the legal definition of a radiant barrier (0.1 emissivity or less). While marketers of radiant barrier paints are all scammers, some legitimate marketers sell "low-e paint" that performs worse than a radiant barrier but a little better than plain white paint. There are no uses for such a product in residential construction.

  3. homedesign | | #3

    Alternate Location of Air Space

    In certain instances, it seems to make more sense for the insulation to be in contact with the ceiling below. It would create an air space within the conditioned envelope increasing R-Value and especially if you have plumbing in the floor system, your plumbing could be on the heated side of your insulation.

    Have you looked at this Insight?
    You might find figure 7 interesting?

  4. Danny Kelly | | #4

    Thanks Guys
    Sorry Martin - I got a little carried away - I think I thought I was on the Q&A page - but in my defense, Daniel did want us to fill up your inbox :)

    Thanks John - that does seem to make a lot of sense - I thought I invented the idea - ha ha. Oh well - back to the drawing board.

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