I'm building a small house on a budget. Texas has the worst property tax rates in the country, and I have to submit the blueprints to the taxing authority. Anything outside the footprint shown on the plans is not taxed. Seems like a small thing, but if your perimeter is 120 feet, an extra 6 inches is 1/2 of a room to pay taxes on, not to mention the loss of the space. Doing a truss wall or other exterior method allows the extra insulation to be placed on the outside of the structural wall, which does not affect the interior space, unlike ICF, double stud, or other techniques that eat floor space. This may considered a small thing, but it really is considerable, since it these ridiculous taxes go on forever. Any suggestions for the best products to build an outside sandwich in a brutally hot and humid climate?

I have heard of people in some areas successfully arguing that the 'heated square footage' is the inside dimensions, not the outside. It's pretty hard to define the core of an ICF wall as 'living space'. That being said, the chances of successfully arguing your case are slim indeed. When I list my completed ICF home with the county upon completion, I'm going to use the inside dimensions for square footage and challenge them (politely) to prove otherwise. It seems ironic that they want us to build super-efficient homes, but then make us pay a penalty for having thicker walls.

That's what is bothersome to me, too. There are all kinds of incentives for energy saving building. I'm even doing a rainwater catchment system, and it is free from sales tax, and can't be taxed as an improvement.

On a few older threads. some folks said that the SF involved in a thick wall is minimal, but it's NOT minimal! The tax on that "solid" room is many thousands of dollars.

I'm going to try that, but I also not hopeful of any success. It makes no sense to punish good building practices.

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On my small home, the walls add up to 153 square feet- 12% of my total square footage. To me, that is significant.

Using 2x4 construction with open-cell foam cavity fill (~R9-10) and putting 2" of foil-faced polyiso (R13) on the exterior yields a ~R20+ whole-wall R value at the same wall thickness as 2x6 construction that yields only ~R14 whole-wall, with all thermal bridging factored in. If you put 1/4"-3/4" of vented air space between the siding and foil facer you get another ~R2 of performance out of it due the high IR reflectivity/low emissivity of the foil facer. R20 is a pretty good wall-R for a TX climate.

Key to both moisture & thermal performance is air-sealing both the sheathing AND the exterior foam. If outdoor air infiltration isn't well controlled in summer, it can lead to high moisture levels in the wallboard in an air conditioned house. Open cell foam works well for air-sealing the sheathing to the studs, but don't ignore the seams between doubled-up top plates or between the bottom plate & subfloor, subfloor & band-joists, etc. It's a lot easier to lay a bead of caulk or can-foam down as you assemble it that it is to air-seal it later.

Most of TX is in US climate zone 2, and an R20 whole wall would be well above code, and may even be above the most-optimal from a net-present-value of future energy use savings point of view (depends on your anticipated heating & cooling energy costs) but it's still a winner on comfort. If you wanted to back off to 1.5" (~R9-10) on the polyiso it's still not a deal breaker, and gives you room to use 1x furring over the foam without exceeding a standard 2x6 framed-wall thickness. For the mid-point of cost-effectiveness estimates on R-values see table-2, p.10 of this document:

www.buildingscience.com/documents/reports/rr-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones

Note all R-values are "whole assembly", with all of the thermal bridging of framing & fasteners factored in.

What you DON'T want do to is back off to 1" on the exterior foam and long-nail the siding through the foam, since the thermal bridging of 10,000 nails undercuts the performance considerably. Through-screwing exterior furring to the studs with pancake head timber screws (eg Fastenmaster HeadLok) 24" o.c. and nailing the siding to furring-only minimizes the thermal bridging. With 1xs furring you only get 3/4" of fastener bite, and you may want to consider using ring-shank nails for the siding for better retention.

In a TX climate the placement & type of windows, the roofing type, and attic insulation type/depth are the biggest factors on cooling loads. West facing windows need to be minimized and have a very low SHGC (Solar Heat Gain Coefficient.) East facing windows can be a bit bigger without affecting the peak loads much, but it will still affect total air conditioning energy use, if not the AC sizing. Using blown cellulose in the attic is more effective than low density fiberglass blowing wools, since fiberglass is translucent to infra-red, and radiated heat off the roof deck will penetrate the upper couple inches of fiberglass, making the temperature of the fiberglass 2" into it a higher temp than the attic air(!), so you're insulating against a higher temp with fewer inches of insulation.

Tile roofs (terra cotta or concrete) insert a time delay on the peak loading from roof gains due to the thermal mass of the roof, which lowers the both the peak & average AC loading. Short of that, if composite shingle, it's worth seeking out a CRRC-rated "cool roof" shingle with solar reflective index (SRI) over 40, which limits the roof deck temp to something reasonable. Ultra-high shingles tend to be very light in color and look like crap after the algae & dust settles in over time, but there are reasonable ranges of color that won't have those issue, that are still reasonably high SRI. You can search out shingles of the color and min SRI using the CRRC's product search function:

http://coolroofs.org/products/results/search&keywords=shingle/

Also important, keep the ducts & air handler completely inside of the insulation & pressure boundary, not in the attic, even if it means framing the walls with 10' (or even 12') studs to accommodate the ducts & mechanicals. The additional framing insulation & siding costs are pretty low, and it takes quite a bit off the AC sizing requirements, and allows you to better air-seal the ceiling, reducing air infiltration (== high latent loads in much of TX.)

If I had time I would have foiled the southern exposure but foaming the 2x4 walls and roof rafters answered most of the concerns raised here. All good points but we use a hot roof in Minnesota often since it allows us great flexibility throughout the installation of mechanical systems from heating to cooling and venting plumbing without concern for sealing, thermal bridging or shoddy installation.

If I were in Texas I would do the same but for a properly installed steel or tile roof.

Dana1, all of a sudden I feel better about making decisions on this build.

I'd planned to go ahead and pop for a lighter-colored metal roof (e83-r59) since I'd really like to do a Rainwater Catchment system. Would tile be a better choice than good quality metal?

I know a lot of folks think foil doesn't do much, but I'm a big fan of the stuff for this climate, and I've used it a lot for various projects in the last decade. I'm tempted to do under truss foil and all the cellulose I can afford in a sealed attic. I assume the foil is overkill for the cool roof, but what if I do the concrete roofing? Need it, or no?

A point in defense of assessors: They don't make appointments and they don't have the right to come inside. That leaves them the measurements they can make, namely the structure's exterior. They'd have serious legal issues if they switched back and forth on a case-by-case basis.

What's more, it's silly to lament the space lost to thick walls and then ignore the many design approaches that maximize living space, or at least avoid space killers like the two-story entry foyer, his and her baths, her separate closet for shoes.....

Todd, +1.

I've found a big problem looking at floor plans because of this. If you're really shopping smart, think about the stupid wastes-of-space. A logical closet is long and skinny, against a wall, and the space you stand on to use it is in the room, usable, not an empty, unused standing-hole in the closet. Foyers are kinda dumb and so are unneeded hallways.

I don't want wasted space, but I still think appraisal districts should legally be allowed to deduct the energy saving mass from the appraisal. If, by law, they can deduct things like solar setups, wind energy, and expensive rainwater systems from the appraisal, why not walls? All kinds of energy improvements are being given a tax pass.

Check out notsobig.com for ideas, floor plans and directories of pros who make the most of limited space. And keep walls in perspective, The typical texas home has walls eight inches wide: 4.5 for studs and coverings plus 3.5 for brick ledge. Upgrading to 2x6 and an inch of exterior foam adds 3 inches to make 11, but is that a deal killer? Suppose the kitchen/living room area in your home is 20x30. Thicker walls would subtract 17.5 SF from that living space, or three percent. But eliminating the interior walls that divide the rooms and set off a pantry gets you almost half of that space back. Plus open floor designs make houses feel bigger. Another approach: I have a deck on top of my garage that adds a very nice 450 SF to my 1600 SF house for half the year. But I am still assessed at 1600 SF.

There are answers.

Hey, Todd, I've looked at your house before (Happy Houseversary, btw) and really like it a LOT. I'm within reasonable distance of block, if I can find an experienced installer in North Central Texas.

Is there anything you would change about your choices?

It is heresy on this site, but my mistakes, primarily in complex hvac, have been overemphasizing technology. Build a small house with proper attention to attic insulation. windows and air sealing. End of story. AAC was not a mistake -- for me anyway. I wanted DIY and stucco, but no way would I put stucco over wood. Your results may vary.

" Build a small house with proper attention to attic insulation. windows and air sealing. End of story" Toddm you have it dead right. I would add good bath fan, range vent and HRV Cheers, Eric

Posted By Robertson on 03 Aug 2013 04:59 AM
Dana1, all of a sudden I feel better about making decisions on this build.

I'd planned to go ahead and pop for a lighter-colored metal roof (e83-r59) since I'd really like to do a Rainwater Catchment system. Would tile be a better choice than good quality metal?

I know a lot of folks think foil doesn't do much, but I'm a big fan of the stuff for this climate, and I've used it a lot for various projects in the last decade. I'm tempted to do under truss foil and all the cellulose I can afford in a sealed attic. I assume the foil is overkill for the cool roof, but what if I do the concrete roofing? Need it, or no?

With a high SRI there's little additional advantage to going a high mass roof.  A high quality high SRI metal roof is the right approach here.

And with a high SRI roof AND cellulose the additional benefit of foil would actually be tough to measure, even if you instrumented the  whole place! With a concrete roof + code-min cellulose the advantages of the foil also disappears significantly into the noise- it might be measurable if you wire up the place like a laboratory experiment, but from a bang/buck point of view you'd be better off spending the foil money elsewhere. The situation where foil has real benefits are retrofits where there simply isn't space for IRC 2012 code-min R, the ducts are above the insulation, and the roofing is high-gain and new enough that it won't be due for replacement in decades.

I'm glad to not screw around with the foil. I think I finally have my head wrapped around this. THANKS!

Do a 2x4 framed walls with a 6" nail base SIP panel to exterior side of studded wall.
Use SIP's for roof.

www.buysipsonline.com