Insulation. Questions

txaggieinargyleMarch 17, 2012

We are building a home in the North Texas area and have a couple of options presented to us by our builder.

Dense pack cellulose in exterior walls (2x4) and blown in fiberglass other attic wih radiant barrier on roof deck


Open cell foam on exterior walls and 2" of o pen cell foam on the attic floor with additional blown in fiberglass to bring the feffective r-value in the attic up to R44. Radiant barrier on the roof deck.

Is there any science that shows if the foam on the exterior walls and attic floor only will provide as much benefit as full encapsulation?

We don't do full encapsulation simply from the aspect of cost and we are not sure how the house will "breath" with full foam. What happens when you get a roof leak. We get a damaging hail storm at least 2 times a year in my area. With a normal roof I can find the leak quicky. With foam where does the moisture go in the event of a leak?

Also with foam how to run new electrical and/or communication lines through a dense foam?

Both options get ZipWall sheathing. There is about a $3000 difference between the two options.

Am I wasting my money on the foam? Or is there some benefit to it?


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My pick for a hot climate like yours would be dense pack cellulose in 2"x6" walls + 1 1/2" exterior foamboard, and spray foam attic ceiling (unvented attic). The cooling season is much longer than the heating season and you will definately be able to see the difference between 2x4 & 2x6 in both energy savings and the more solid quiet feel it will give the home. In hot climates, an unvented attic is the only thing that makes sense, especially with HVAC ducting in the attic.

    Bookmark   March 18, 2012 at 2:25AM
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I respectfully completely disagree with live wire oak. That wall system would be complete overkill in a hot climate.

I live in a mixed climate that certainly has greater cooling duration and more recently cost. We have an energy audit that states our walls cost us $25 a year for 4000 sqft of 2x4 with fiberglass batts. Walls matter very little in a cooling climate - in our case with bare basics, it was 6% of load.

Foam does not pay off in walls in almost every situation. Certainly not in your case in addition to zipwall. Foam is mostly good at infiltration and the zipwall is pretty good already.

Now foam on the attic floor is a little more reasonable since any air from the attic is worse since it is hotter and you don't have the zipwall as a barrier. But I generally think you can just do penetrations for less. Going to 44 with a radiant barrier is overkill. At r-30 without a radiant barrier - the attic is also just at 6% of load.

A house performance in a cooling climate is dominated by solar gains.

The next line item is internal gains (nothing to do with insulation).

Next is duct location - obviously they should be in conditioned space.

Only after those things are tackled would you consider walls and attic but know that you are only going after a small number. If your house is 4000 sqft and let's say your cooling cost is twice mine, then your baseline wall cost is $50 a year. Going to foam might save $10 a year. Might. Now going from r-30 to r-44 in the attic will get you $15 a year (and I went to r-40) but you could get there cheaper with cellulose (I think I paid $200). Of course, the real benefit is for winter and that is why I did it.

Live wire oaks proposed walls would easily cost $5000+ for an average new larger house and save roughly $30 a year. Unvented attics are great if you have ductwork in the attic but it is far more cost effective to just get the ductwork out of the attic.

Info to provide - size of house since it matters when saying "is this worth $3000", your motivation (ie cost or something else), your general heating needs, and your utility cost.

Maybe at 110 degrees for 3 months and $.20 a kwh, it makes some sense to upgrade the walls. But sprayfoam is not the most cost effective - exterior foamboard is. Also geothermal becomes attractive at extreme temps.

    Bookmark   March 18, 2012 at 5:16AM
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2x6 is not just about the dollar savings on energy, although that can be decent compared to the usual minimum standard construction in the South. We did 2x6 construction with foam board exterior and glass batts for the cavities and cellulose for the attic on our shop/garage 15 years ago. I figure the attic is around R-40 with the amount of cellulose we packed in. The shop is substantially quieter and more solid in feel than the 2x4 R-16 wall/R 25?ceilinged house (197o's vintage). The train that is 1/2 a mile away is blaring it's crossing horn right as I type. It's sound really carries and penetrates the house. I can't hear it at all in the shop. And I can't hear the various air tools, banging noises, and engine revs from the shop in the house! (And neither can our neighbors, which was an important consideration to us.)

The upgraded walls and insulation also allowed us to use a 1 ton AC for the 1500 square foot space that has 10' ceilings. It gets colder and stays colder in there than it does in the house. For heat, we had planned to do hydronic radiant, but the first winter we just used a couple of 1500 watt electric heaters and it kept it a perfectly comfortable working temperature at around 62 degrees. When it was -5 overnight, and we hadn't run the heat in almost a week, the coldest that the interior ever was was 48. The PEX tubing is still sticking out of the slab in the corner, covered in batting and a tarp, just in case. But the little electric heaters do just fine in heating that whole space and have carried the load for 15 years. I joke that you could keep it perfectly comfortable just switching to halogen lights instead of the fluorescents!

Yes, it might have cost us 5K on the build, which might seem outrageous for a shop build, but a substantial amount of time is spent in that structure. So, it's comfortable. And the difference between the two is VERY evident, summer or winter. And that's why I'll always recommend 2x6, even when the "numbers" don't seem to indicate economic payback from it. :)

    Bookmark   March 18, 2012 at 9:38AM
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Thanks Live_wire and David,

We had not considered 2x6 construction yet with our builder, I can ask him what the premium would be for that. Inevitably it will add some square footage to the build. And probably changes the construction around the window and door openings.

The house is a 3150 SF single story home that faces Northeast. That implies that our backyard faces to the southwest and will get the brunt of the summer heat.

I don't know how to calculate the cooling load but we will be using two 16 SEER A/C Units with natural Gas for heat. Our Roof pitch will be fairly steep 10/12 and therefor create more roof surface area and attic volume.

We keep the A/C on 82 during the day and 78 at night in the summer. We runs fans A LOT. The humidity is not too bad most of the time. We have budgeted for a variable speed A/C that can run on low just to remove humidity and hopefully make the house feel more comfortable at a higher temp.

This last summer was the hottest on record but we did have over 100 days of over 100 degree heat. But even in a normal year it is over 95 degrees by Memorial Day and stays hot until October. We are paying around $0.09 / kWh currently.

One of the reasons I question the efficiency of foam is that our current (rental) home is full open cell foam, 2 story, and 1800 SF with heat pumps for A/C and heat. The house was just built last year. Our average bill is $140 last summer. That sounds great until you know that our previous home was 3000 SF single story (tract home) with no radiant barrier, batts in the walls, fiberglass in the attic (14" approx) and 12 years old ; and it has very similar average cooling bills. The average was about $170 but it was almost double the size home too. The rental home and the previous homes also had trains within 1/4 mile and 300 yards respectively. (though at 300 yards away that train NEVER blew it's horn but did fly though the area at 50+ miles per hour. But the the foam house seems quieter, but the train still keeps my wife up at night.

From what I am hearing from you both what do you think of using the new Zip Wall Product that incorporates foam board into the sheathing and gives it an effective R Value of R-6. I don't know what that costs though.

If I used that Sheathing, Dense Pack Cellulose in the walls, and R-40 worth of blown fiberglass in the attic along with Radiant barrier on the roof and foam in all penetrations.....I may have a very efficient home and save a couple thousand dollars.

It sounds like foam in the walls without doing the roof deck may be a waste of money other than from a sound insulation value. (the new house will have a train 1/2 mile away). Or budget for 2x6 walls in the build too.

Thanks for your input.

    Bookmark   March 18, 2012 at 10:33AM
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I forgot to add that the duct work will be in the attic. How do you get it out of the attic and into a conditioned space without foam? We have no basements in Texas.

    Bookmark   March 18, 2012 at 10:36AM
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In our new build we foamed the rafters in our attic, not the ceiling joists. The attic will now be a semi-conditioned space. We also foamed the walls, we used 2x6 plates, 2x4 studs, 12 inch centers alternately aligned on the two different sides of the plate. Our builder has used similar methods in other homes he built here in the DFW area and while it costs more up front, it pays for itself in about 5 years by the savings on the utility bills. In fact, he even has an engineer that develops these plans that will guarantee your savings, might be worth having your builder ask around. Good luck with your build! One other thing--be careful with the radiant barrier--it can play havoc with your cell phone reception inside your house. We didn't need to use one.

    Bookmark   March 18, 2012 at 11:35AM
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If your ducts are in the attic, then it is worth considering foaming the rafters. I never buy the payback in our climate - it was about 100 years, but you are hotter.

No problems with cell phone coverage here - I really can't imagine that since it is line of sight and the towers are relatively short - ie most of the signal is coming through the walls. But with a 1 story house, I could see it.

You can put ducts almost anywhere. Certainly first floor can be in the slab.

Since backyard is certainly where the windows usually are the most plentiful, that is your real issue. The more you can cover with overhangs the better.

If you want to save money - work on the windows and the ductwork and don't give a single thought to the walls. Your ductwork and windows will probably be 80% of what you can control.

Sure foam is quieter, but if you are doing brick, most likely windows are your real noise entry anyway.

I like Live wire oak's admission that his numbers don't make sense.....

Something to thing about - if you are all electric, your hot water bill is likely higher than your a/c bill. Consider working on that - solar or heat pump. With solar incentives, it is often less than 10 year payback. Heat pump will nearly always pay back if you are willing to live with the noise (or can isolate it somewhere).

Foam on the walls is great but just think of your delta T - it is only 20 degrees on a really really hot day. That is nothing. In a heating climate, delta Ts of 50 degrees are common.

    Bookmark   March 18, 2012 at 12:06PM
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the only way to get ducts out of the attic is in the design
stage. hard to add it later. IF you can design the house
with fur downs for ducts it can be built that way.
to add it in later hardly ever works.
so the option is to foam the roofline.
this moves the air and thermal barrier to the roofline
instead of the attic floor.

this is a checklist from my local utility co-op.
something I'd request you take a look at.

we have built many houses with this wall construction
that are as tight and efficient as houses with foam in walls. and at a lesser cost.

what I'd tell you do is this.

save foam in walls for exterior foam foil sheathing boards
R-7 foil faced. spend that foam in walls $ for the roofline.
install foam sheathing with foil facing out, as described in slemco hand out.
tape all seams, seal all holes.
put conventional insulation in walls.
in a 2x4 wall R-13 plus the R-7 is R-21.
sole plate caulked to slab, air tight drywall.
foam board on exterior of walls stops thermal
conductivity, insulation value of wall
is raised and wall performs.
see perfect wall system.

in the attic..6-7" of open cell on the roofline.
you have to meet code.
2" of open cell foam on attic floor & RB doesn't meet
even adding lose blown insulation on attic floor
to meet code requirements still leaves ductwork in
hot humid attic.
instead create an unvented attic, rafter bays filled with
open cell foam and faces of rafters covered with 1" of
foam, to lessen thermal conductivity.

ductwork is mastic sealed and in conditioned attic.
mechanicals are also placed in attic.

putting RB on roof and insulation on attic floor
will still allow the ductwork to condensate. when R-8
ducts are in an unvented attic, with 130 degree air
surrounding the ducts dewpoint is met and ducts condensate.
add in a vs unit that gets air colder and
ducts stay wet from condensation in the attics in august & september
..even earlier this year if today's temps are any indication!

get some bids
ask for regular install of foam for roofline & then bid
that includes full rafter fill and covering face of 2x's.
don't buy into the double the price for double the product.
companies I work with give the average bid
of 3" open cell (R4x3"=R12...not code)
and then 6-7" open cell. R4x6 1/2"=R26 code for cathedral ceilings.
the better bid is not twice the cost because set up and
installation is covered in initial set up. once they are
there it doesn't cost but product & labor to bring foam
level to meet code.
and remember this is minimum code. the bonus is the
air sealing, but it doesn't figure into R-value
but in air tightness.

once walls and attic are air tight and you've chosen
windows with .30 or less solar heat gain coefficients &
Ufactors, then sizing of the hvac system is the next step.

with improved building envelope more sq ft per ton.
load calcs based on building improvements, and a
tight (not leaky or average) house, tons of units
required are lessened.

these suggestions are based on my 13 years of
energy rating in hot humid La. RH here is a killer.
we oversize...all the time. vs units please both hvac
contractors & homeowners. but air is colder...putting
those ducts in a vented attic, even well sealed ducts
condensate. no way that R-8 duct insulation keeps
ducts from condensating in our attics when they are
unvented attics.

best of luck.

    Bookmark   March 18, 2012 at 12:07PM
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thanks energy_rater!

That was great info.

    Bookmark   March 18, 2012 at 9:32PM
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I agree 100% with energy rater LA. I know a guy that just built a 3k sf home in longview and foamed the attic joists, completely sealed the attic, and foamed all exterior walls. They then cellulosed the interior walls. He left the attic floor open and its a semi conditioned attic. Its so much cooler in the attic.

The old theory of there being to much heat buildup on the roof drying out wood has pretty much been disproven. With the summer we had last year, I am planning on doing much of the same things in our build. It might be a little more on the heating in the winter, but we average 50 vs 175 in cooling in our current 1800sf rental home "damn single pane"

We will also be going with Stained concrete throughout and that should help with cooling.

    Bookmark   March 21, 2012 at 3:07AM
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I am building in central california. It gets 100+ here in the summer. Can I use this build as well engery rater la? We are building with 2x6?

    Bookmark   March 21, 2012 at 3:16AM
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If you think about it, hot air rises and cool air is lowered. Having minimum insulation in your attic floor allows the heat to escape your rooms and get to your rigeline, while the cool air is lowered to your floor, much like the old dog trot southern homes before we had AC units.

We also plan on having reflective/tinted windows that face the south with some overhang. We are also going to plant two shade trees, either redbud, little gem magnolias, or ann magnolias about 9 foot at the corners of the house and more shrubs in the front of the house "our home faces south". We limited the amount of windows facing south in design and added more in the middle back of the home for the back yard view. This should also allow us to capture more cool air and slowly force it through the large front door on cooler days "dogtrot". Just some different ways to cool, also add a tree that will block the light on your AC unit if it sits in direct light, nothing blocking air flow, but that wouldn't hurt.

    Bookmark   March 21, 2012 at 3:18AM
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Cell foam seems to be consistently getting better. Even ways of adding foam pressed in seem to be much better.

One thing with foam, I would cut off a small piece and repeatedly try to light it on fire before living in it, somebody might screw up or try to shaft you "like the Chinese drywall".

    Bookmark   March 21, 2012 at 3:20AM
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agentslim, yes.
building science is advocating the
use of exterior foam sheathing walls for all climates.
that you are 2x6 just increases the total R-value of the
wall assembly.

best of luck

    Bookmark   March 22, 2012 at 12:47PM
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