Replies

1. Expert Member
   Dana Dorsett | Dec 16, 2014 06:04pm | #1

   In Austin TX a vented attic will be more humid that an unvented attic due to the high summertime dew points. This is true in all the gulf coast states within a reasonable day's drive of the gulf.

   This compilation of published research results is worth a read, if you're still on the fence:

   http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1496-05.pdf

   Having sufficient make-up air for a gas-fired furnace can be an issue though, and your instinct for making them go away and sealing it up tight is a good one.

   " I do not know anything about how humid the attic is in the summer except that based on the homeowners statement I believe it to be in the 80s."

   If that means north of 80% relative humidity, that's credible with open vents letting all the humid summertime air into the cooler-than-outdoors attic. The dew point of 80F /80% RH air is 73F, which is a pretty close to the mid-summer mean outdoor dew points in Austin, according to weatherspark.com datasets. With the vents open it SHOULD be about 80% relative humidity. With the vents sealed up the attic air's dew point will track that of the conditioned space below. The dew point of 76F / 50% RH air is 56F, so assuming the attic was running around 80F when the conditioned space is 76F, the RH up there would then run about 45%.

   When sealing the place up it's good to verify it with blower doors before the air-sealing crew packs up.

   BTW: Don't let anybody talk you into removing the attic floor insulation. While you can't count it as part of the total R-value from a code-minimums point of view, it does no harm by being there, and DOES provide additional thermal benefit, however marginal that might be.

2. 
   charlie_sullivan | Dec 16, 2014 09:37pm | #2

   Dana and Heidi,

   I think you are both suggesting replacing the furnace with a sealed-combustion unit so that you don't need vents to provide make-up air for it. But since neither of you said that explicitly I thought I'd check. I think that is a good idea, if you are willing to spend the money to replace the furnace. (Do you even need a furnace? Sounds like a good location to heat with a heat pump.)

3. 
   Hpw19 | Dec 17, 2014 02:20am | #3

   Yes, replacing with sealed combustion was only option I was made aware of. I do note that Austin has a 30% credit for heat pumps. Can these be put in an attic? Of course hat is where existing duct work is also. Would we need our air conditioner units anymore? Also note there are 2 furnaces to replace I think bc of size of house... Would one heat pump suffice? Thank you Dana for your well thought out response. I think the insulation companies want you to believe that if there is a smell when you seal up your attic it is because you had smelly stuff in your attic, including leaving old insulation. Am I understanding that that would be only reason to leave floor insulation? Thanks!!

4. 
   Hpw19 | Dec 17, 2014 02:21am | #4

   *correction: remove floor insulation

5. GBA Editor
   Martin Holladay | Dec 17, 2014 05:27am | #5

   Heidi,
   The only reason to install spray-foam insulation on the underside of the roof sheathing is to create an unvented conditioned attic. The idea is to bring the attic into the conditioned space of the home. If this is done, it makes no sense to leave open vents.

   Of course, as the other comments have noted, the work that was performed in your attic can't be done if you have any atmospherically vented appliances up there. The spray foam contractor should have known that. An error was made.

   The solution is to replace the existing furnaces with sealed-combustion furnaces, and then seal up the vents. The work is described here: Creating a Conditioned Attic.

   If you want to swap your furnaces for air-source heat pumps, you can probably do that instead. If you do, you will end up with one or more outdoor units on small concrete slabs behind your house. You will still need one or more air handlers in your attic; but in this case, you won't have any combustion appliances in your attic (assuming there is no water heater up there). It would still be a good idea to seal up those vents if you go this route.

6. Expert Member
   Dana Dorsett | Dec 17, 2014 03:53pm | #6

   Heat pump solutions can work very well in an Austin TX location. Be sure to run real heating & cooling load calculations for sizing- it'll probably be oversized for heating if it's sized for the peak cooling load, but probably still within reason. Make sure that the calculations reflect that the air handler & ducts are fully inside the insulation of the house, and that it's a conditioned attic, since ducts and air handlers in attics typically adds a full ton or more to the size of the compressor in your climate. Being slightly oversized for the cooling load wouldn't be a disaster, but if that makes it 3x oversized for the heating load it's not so great. Calculating it carefully and sizing it aggressively to the smaller size does more for comfort & efficiency than dumb "a ton per 500' of conditioned space" type rules of thumb that reliably oversize it 2x for even the cooling load.

   The existing gas furnace is also probably 3x oversized for the actual loads, if experience is any guide. While that's not an efficiency disaster for a gas furnace, it becomes a comfort issue, with the noise and wind-chill of higher than necessary air volumes, and overshooting the thermostat set point. When replacing HVAC equipment it's an opportunity moment for right-sizing it. Equipment that runs longer but fewer cycles lasts longer and is more comfortable. By not oversizing you won't be able to use 15F setbacks (or off) when you head off to work and still get the house up/down to temp within 15 minutes, but right-sizing it and using shallower setbacks (or "smart" thermostats that figure out when they need to start the cool-down/warm-up ramps) doesn't negatively impact energy use.

   Unless you've had bats pooping up there for years &/or major mouse nests, the old insulation isn't going to stink. Even if it smells a bit at 80% relative humidity that's as likely to be from the fungus growing in the wood due to the high humidity as anything else, and that will change once you go unvented, letting the air conditioning dry it out. Only if it's a major stinking rodent mess would it be worth getting rid of it, but if it were that bad you'd probably have noticed it already.

7. 
   Hpw19 | Dec 18, 2014 12:18am | #7

   Do you guys mind ranking my options?
   1) spend 9500 on 2 new furnaces + cost to seal and test
   2) remove $5000 of foam (worried about performance in vented attic), seal what are probably leaky ducts, and make it someone else's $15000 problem in 10 years (perhaps even mine) when the carrier furnaces go out, should they want a sealed attic. Don't know about cost of removal, or if I could even find anyone qualified to do
   so.
   I guess one other consideration is maybe if we finished the job could the house get a marketable/valuable green energy rating. It is 20 years old, but there are energy efficient windows, tankless water heater, and certain other areas are spray foamed such as 800sf addition, crawl space, master attic, etc.

8. 
   Hpw19 | Dec 18, 2014 03:43am | #8

   **My understanding is 2 heat pumps would also probably run the same. (Plus I wonder if they're on the ground if they have sane trouble as a furnace would getting hear to second floor?)

9. GBA Editor
   Martin Holladay | Dec 18, 2014 05:04am | #9

   Heidi,
   I think that you should seal up the attic and replace the atmospherically vented furnaces (with either new sealed-combustion furnaces or new air handlers designed to handle one or more air-source heat pumps).

   Removing foam is difficult, and if you are worried about odors, it is especially difficult -- because getting every last little bit of foam is the hardest part of the job.

   Can your house get a "green energy rating"? I have no idea; that depends what types of programs are prominent in your area, and what the rules of the program state. Investigate local programs and find out what is required to get such a rating.

   In my book, installing ductwork and HVAC equipment in a vented attic is never green. But I'm not the one who defines "green" in Austin.

10. 
    Richard Beyer | Dec 18, 2014 12:13pm | #10

    You may be interested in these two articles...

    https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/11899/open-cell-vrs-closed-where-draw-line

    http://scholar.google.com/scholar_case?case=7789310905996211888&q=icynene&hl=en&as_sdt=8006&as_vis=1