Why Not Pile On The R-Value Even in Climate Zone 3?
AtticAdventures
| Posted in Energy Efficiency and Durability on
Often I have clients with “problem” rooms (too hot/too cold/inconsistent temp with respect to rest of house). The first step is always air sealing but I have the tendency to pile on (R-60+) the fiberglass insulation. We live in DOE climate zone 3 (Dallas, TX) and the ORNL calculator (http://www.ornl.gov/~roofs/Zip/ZipHome.html) indicates R-49 for attics. Is pouring the fiberglass insulation on these problem rooms (R-60+, 22-24 inches) a silly thing to do? Thank You.
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We pour it on like that here in 4 marine and I think R-60+ in the attic is the right thing to do. Once you are set up and blowing, the extra stuff is very cheap and will pay dividends as long as the building is there. We have gotten a surprising amount of positive feedback from clients who we've done this for, folks who were previously not really tuned into insulation and air-sealing as important steps. They like the comfort and stable temperatures.
Thank You Armando for your comments. I think you are exactly right and I might even add to the "doing things on the cheap" list - poor performing windows. The challenge is how does the humble insulation (or even general) contractor help these people in a cost effective manner.
Here is the short answer in my opinion: Hot/cold room issues in Dallas and most anywhere else is a combination of doing things on the cheap side. Leaky building envelopes, bad insulation choices and installation, HVAC equipment that is designed incorrectly, oversized, badly installed, leaky ducts, flex ducts, equipment in ventilated attics, no return ducts, and no commissioning the equipment. Other than that, must HVAC contractors I’ve talked to don’t have a clue on doing it right or wanting to do it right.
Here is my starting point, if you install more than 1 ton of cooling per 1,000-1,200 sq. ft., you have something wrong either in your building envelope (product and material specification and installation) or HVAC systems designed or installed wrong. Installing huge insullation amounts while having a bad envelope and HVAC system is putting lipstick on a pig, and a waste of money.
Aren’t you glad I gave you the short answer?
Randy,
It all starts with education and wanting to do things right. I know many good contractors, but most folks in the industry refuse to do things right because increases their costs on jobs. We work on the theory that all is priced by the sq. ft., and we “compete” based on that rather than how good our job is. The system will change and contractors will do a better job either by Building Code upgrades and enforcement or by making contractors responsible for the energy bills and home performance of their clients; and the later ain't going to happen.
Randy,
One other point to add: of all of the available insulation types, loose-fill fiberglass is least likely to perform as expected, because it is so fluffy and air-permeable. Often you have to install "R-60" worth of fiberglass to get it to perform as well as R-40 of cellulose.
So Armando, how much attic insulation do you think is correct? You are correct that HVAC equipment is often a big problem, but you also mention envelope and insulation problems. It sounds to me like he is improving envelope and insulation with what he is doing. What are you recommending, assuming he can't get the customer to buy a complete HVAC rebuild?
Expanding on Martin's comment regarding low-density fiberglass:
Fiberglass is somewhat translucent to infra-red radiation, and under a hot roof deck the air in the fiberglass a few inches in will be several degrees HIGHER than the attic air. (Some folks at Texas A & M studied this to death in the1980s.) This means you're insulating against a higher temp (the temp a few inches into the fiberglass) with a lower R (the thickness of fiberglass from the hottest layer down.)
Then during the heating season the high air permeability of fiberglass causes it to give up some performance to convection currents within the fiberglass- the warmed air at the ceiling bubbles up to the top, to be displaced by cooler denser air from above.
Cellulose is far less air-permeable than fiberglass, and is opaque to infra red. Under a hot roof deck the surface of the cellulose is the hottest point in the fiber layer, and it is convection-cooled by the attic air to within a very few degrees, despite the radiant flux from above. The net result is a much stabler R-value and performance over the entire temperature range experienced over a year.
Dana
I agree with you. With fiberglass being the most air porous of any insulation I just dont get its use in an attic. I think summer or winter you have air moving through degrading its insulating value. Then I like the way you put it being "translucent" to radiant heat. So its has 2 strike in the summer.
I have read for so long that in order for fiberglass to perform at its stated r value it needs to be in a 6 sided air tight enclosure yet in an attic we have it open to the top and then we have roof vents. Add a power roof vent and is even worse.
David,
Design retrofits are a different animal, but I always recommend starting with an Energy Analysis. It gives me a chance to educate the homeowner on how to do the job right. Sealing the building envelope and encapsulating the HVAC system, in a ventilated attic, is a priority; and it maybe easier, and cost less, to condition the attic.
For me, I would not work on a new home or remodel design job if the clients are not willing or able to do the job right. It’s always a recipe for disaster, and it would not be a good fit for my business model.
@ David, Armando
Like Randy, I'm zone 3 ... but east by a good amount (of humidity). The house I'm upgrading has blown fg in the attic. It's my thought to vac all of it up, seal the ceiling as much as possible with a flash of a couple inches of spray foam, then pile blown on top.
Both guys recommend I pile the fg back on; one explained that the humidity might cause some issues for the cellulose, and it's more expensive/inch here. Assuming the budget remains the same and the target is high effective R-value, would you prefer on top of spray foam --
more fiberglass
or
less cellulose
I want to keep the vented attic. Thanks in advance.
Armando is spot on. I consulted on one house in Fort Worth TX that was Energy Star certified and has gotten acceptable blower door numbers (it was a very large volume house so E-star acceptable blower door at 6 ACH-50 was pretty easy to achieve) but significantly there had never been a duct blaster test done. The owners complaint was about excessive dust in the home, they weren't complaining about comfort or cost. They had analyzed the dust and it was cellulose.
The builder eventually did spring for a blower door test and the return air ducts in the crawlspace were pulling in lawn mower clippings through the foundation vents and distributing them on the furniture. Obviously this was also loading up the heat exchanger with damp and moldy lawnmower clippings, impacting indoor air quality, energy costs and comfort / humidity performance of the HVAC system.
Adding more insulation to the attic would not have been part of the solution. It's important to start with an audit that includes both a blower door test and a duct blaster test. Many energy audits don't include a duct blaster test as it involves sealing off all the ducts and can be time consuming to perform. Calculating duct leakage from a blower door test is no substitute.
JoeW,
Cellulose is hygroscopic, but I do not know of high humidity affecting its performance; other than rain/leak wetting. Assuming you do not have the HVAC system in the attic, I do like vacuuming all existing insulation, installing the 2” CC foam and then pack the rest of cellulose to meet R38 or more.
You could try sealing with EcoSeal and then pile-on the cellulose; that maybe less expensive.
Perhaps Allison, Carl, Michael or John Tooley know more about that issue in your neck of the woods.
I prefer to pull the FG out of the way, seal the penetrations and top plates with can foam, box any non-IC can lights and then put the FG back as I go and cover all with six to ten inches of cellulose. the cellulose picks up the humidity and skins up fairly soon and helps resist the wind washing that is an issue with the fiberglass.