Wall insulation — new build
Hello. We have been sent here by knowledgeable people from gardenweb home building forum. We will be building in Gainesville Fl, 2A region. The state minimal insulation code is R15 wall and R30 ceiling. Having built in Florida in the past with same ‘R’ , we want to up the R value. A bit of our living area history- we have lived in some harsh climates. From 8,000 ft elevation in Cody Wyoming to the northern Arizona desert to southern Az desert to southern humid Florida. We have built three homes. One to Florida code (Sarasota), one in Phoenix Az and one in Cody Wy these two exceded code insulation which made each far more comfortable and extremely low electric costs. (All electric utilities). We again want R30 walls and R40 ceiling. These numbers have served us well in both 110 degrees to minus 40 degrees. This time will want concrete clock build as the 1967 home we live in now has been undamaged through 2 hurricanes that demolished the stick home next door and the stick home three doors away, while each neighbor’s block home was undamaged except for neighbor’s oak tree that fell through the roof. Irma a few months ago did much damage in our semi rural neighborhood but our block homes stood. Also termite consideration. Due to concrete block build which this house has only 1×3 inch on interior side with no indulation- just drywall. We are wondering how to get the R value we want- even 2×6 studs will not allow batts 7nless we are not aware of a higher R value product of batting. Spray foam does not seem to be as effiecient as first thought when it first came out. Please respect our want of higher R values. We have built two homes above minimum required code and the comfort is so much more!! Monthly electric bills were next to nothing. What would you suggest to achieve this R with a concrete block home. Wife has severe asthma. Thank u.
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Replies
Edi and Stev,
1. You asked that we "please respect our want of higher R-values," and I will do so. However, I feel duty-bound to inform you than in Florida, investing in R-30 walls is not cost-effective. You would get a much better return on your investment by specifying high-performance windows, or by using the money you would have spent to upgrade from R-15 to R-30 walls to pay for a PV system.
2. That said, this is America, and you can build the house you want.
3. The usual way to insulate a concrete block wall (also called a CMU wall -- CMU stands for concrete masonry unit) is with rigid foam. To achieve R-30, you would need about 8 inches of EPS, or 6 inches of XPS, or 5 inches of polyiso. Green builders generally avoid the use of XPS, because it is manufactured with a blowing agent that has a high global warming potential.
4. You can install the rigid foam on the interior side of the wall or on the exterior side of the wall. From a building science perspective, the outside of the wall is preferred. It's even possible to split up the foam and put some on the outside, and some on the inside.
5. You might want to consider the use of EIFS. Most EIFS contractors won't want to install 8 inches of rigid foam, but you might get lucky and find one who agrees to 5 inches. EIFS is a combination of exterior rigid foam and synthetic stucco.
6. You might want to consider the use of insulated concrete forms (ICFs) instead of a concrete block wall. Most ICF manufacturers make components that have a lower R-value than R-30, but a few can achieve what you want. A company called Logix produces high-R-value ICFs. Here is a link to an article that mentions Logix ICFs: New Green Building Products — January 2009.
R30 between studs isn't the same as R30 continuous insulation. The thermal bridging of the framing brings that down to ~R20 "whole wall", after factoring in the heat that is transferred through the wood.
With a concrete masonry unit (CMU) wall you also get a signifcant benefit from the thermal mass, providing both higher comfort and lower energy use, but only if most of the insulation is on the exterior of the CMU. Putting the insulation on the interior isolates the interior from the thermal mass, reducing the benefit.
To hit a comparable whole-wall R with continuous insulation would take 3-3.5" of rigid polyiso on the exterior, but to hit comparable energy use and comfort only takes 2", or 3" of EPS. The R-value is only R12-R13, but it's a continuous R12-R13, and the thermal mass is 100% inside the thermal envelope moderating the temperature swings.
Alternatively, building with reinforced poured concrete in a minimum ICF (insulated concrete form) puts 2" of EPS on both sides of the thermal mass, and it's more robust against hurricanes than a CMU wall. Some vendors start with 2..25" of EPS , per side, others 2.5".
A code minimum "mass wall" in zone 2 is R6 if at least half the foam on the exterior, making the 2" polyiso or 3" EPS exterior foam 2x code minimum, and a 2" + 2" ICF something close to 3x code minimum. Even in central AK a 2.5" + 2.5" ICF would beat code minimum.
In a similar vein, using aluminum faced roof decking an 5" (R28.5-R30) of 2lb roofing polyiso above the roof deck, and a "cool roof" membrane or shingled roof with an SRI (solar reflective index) greater than 50 would outperform R60 on the attic floor by quite a bit. Vented attics in FL make no sense at all, since it brings more moisture into the building structure than it purges. Vented attics are also considerably more prone to roof deck failures under hurricane force winds. With an unvented attic the detailing necssary to get good performance out of ducts & air handlers housed in the attics drops considerably, yielding higher whole-system performance than typical.