Which insulation to choose?

ElectraTooSeptember 19, 2012

I'm to the point where I have to choose insulation and I know the knowledgable folks here will offer good advice as they have for me in the past.

I have a bid for blown in cellulose or fiberglass in the walls with cellulose in the attic and batts in the garage for $4500. I also have a bid for OC foam in the walls and blown in cellulose in the attic/batts in the garage for $5800. I don't think the foam quote is too out of line, but I'm wondering if it's worth the extra $1300. I would like an efficient home, but my budget is tight so I want to do what makes the most sense in the long term without breaking the bank now.

Here is some info that may help:

- 3000 sf heated

- 2x6 framing with 1/2" OSB with Tyvek

- Northwest Arkansas -- I'd consider this more of a Midwest climate but we definitely do more cooling than heating here

- Radiant barrier on the roof deck

- Vented roof (mostly turtle vents -- many -- and two copper dormer vents on the front)

- HVAC is 15 SEER 2-ton/3-ton system with an 80% gas furnace and, yes, it's in the attic (I tried, but there was simply no room to spare in the conditioned space and attic installs are typical around here)

My questions:

- Is there any benefit to using the foam? My research would indicate that foam in the walls isn't such a great deal.

- I assume using any kind of foam on the rafters to moderate the attic would be pointless given that it's vented?

- If I go with foam, will I need to invest in an ERV as well? It's really not in my budget now or in the foreseeable future and I would hate to have to suffer with poor air quality and dry skin until it is. Also, I frankly do not want more holes in the roof.

- Can anyone speak to off gassing they've experienced with foam? I'm pretty sensitive to smells as well.

- Is foam really a better sound insulator?

I guess I'm just wondering if the extra cost for the foam is worth it considering all of the other decisions I've already made that are not conducive to a super tight envelope. The blown in stuff seems like it might make more sense but I'd like to hear what others think. Also, if anyone has an opinion on choosing blown in fiberglass over cellulose or vice versa, I'd love to hear it as well.

Thanks in advance as always.

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david_cary

$1300 is pretty cheap. Foam is better than fiberglass for a number of reasons but it usually isn't worth the upcharge for walls. Since yours is so cheap, it is probably worth it in the long run.

Foaming the rafters is only done if the attic is unvented. It would probably have been worth it since your foam is so cheap but I am guessing it is too late.

If you are sensitive to smells, I suppose an ERV is always worth it. Whether you are required to get an ERV depends on other decisions also and not simply foam vs fiberglass.

Each climate is of course different but I wonder what prompted 2x6 walls in your climate? Seems like 2x4 + foam sheathing would be cheaper and more effective.

In a cooling environment, walls are a minor expense. I have a bigger house, 2x4 with batts and I think my cooling cost is $50 a year from the walls (and even that is over estimated). With 2x6 and your size house, your walls might be $20 a year. Going to foam isn't going to save much..

Now if you have significant heating needs, than an 80% furnace is a dinosaur.

    Bookmark   September 19, 2012 at 4:46AM
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Annie Deighnaugh

We went with closed cell foam which was very expensive. Your upcharge doesn't seem like that much. With the closed cell, we needed no vapor barrier. But we are in the northeast so we have heating needs far more than cooling ones.

I can tell you that we did have an odor from the closed cell, which did dissipate over time, but once the sheetrock went up and covered it, the smell was completely gone.

I can also tell you that for sound deadening, it's phenomenal. When the house is closed up, we hardly hear any external sounds. The house is very quiet.

    Bookmark   September 19, 2012 at 9:17AM
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lzerarc

what makes little sense to me here is your equipment is in an unconditioned, vented attic. Your money is better spent getting that inside the insulated shell than foam in your walls. Foam will seal it up nicely, but then again, so will caulk which is a fraction of the price.
I would see if its possible to close your vents and foam the rafters. Or you can talk to your builder if you can separate it off into its own space and foam that.
Even at 1300 difference, I would probably skip the foam and go with caulk/spot foaming your penetrations (electrical, plumbing, etc) and blow in cellulose. Also remember everyone focuses on the walls for air sealing. You also need to air seal the attic or roof. In vented attics, that means your gyp ceiling is now your air barrier. This needs sealed just like the exterior walls. Air tight cans need rims sealed around them, any junction boxes in the ceiling needs sealed above where wires enter and to the gyp, and where your gyp meets interior walls/partitions also needs sealed.
Obviously if you foam the roof deck, you will not need to do any of this.

    Bookmark   September 19, 2012 at 10:51AM
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energy_rater_la

for the mininal upcharge to switch to foam in walls
I can't believe that you'd even come close to the
R-value code required for your area. open cell is R-4
per inch.

besides if you put foam sheathing to the exterior of walls
it is more cost effective see link below:
http://www.buildingscience.com/documents/insights/bsi-001-the-perfect-wall?searchterm=perfect+wall+assembly

on this same site, look for the air tight drwyall approach information.(this is a REALLY good site that you should bookmark and do some reading)

put foam sheathing on exterior of walls as per link,
insulate with conventional insulation, air seal and
air tight drywall to interior.

now for roof..you've invested in a radiant barrier.
if you foam over it..you've lost the rb as it will
no longer have an air space to do its work.

personally, I'd upgrade to higher efficiency gas furnace
which wouldn't require combustion air, and foam the attic.
covers a multitude of errors..leaky ductwork, the 50 IC
not air tight recessed lights, builder errors like open thermal bypasses, and other air leaks at ceiling to
attic which at present is your very leaky air barrier
to attic with extreme temps.

if not upgrading furnace..then foam sealed attic would
require combustion air for all gas appliances.

foam in walls..not a good idea. long payback for something
we do fairly well already. foam sealed attic..usually 10-12
year payback. takes away the problems of air leakage into
house, and puts ductwork and equipment into semi conditioned attic.

IF the house is tight..and foam is used in attic
you would downsize hvac systems.
IF house is tight..you may need fresh air, but until
it is tested for air infiltration..its just a guess.

with efficiency and comfort cost is always upfront
benefits long term. load calcs, duct designs,
blower door testing, ensure that things work like
they should. granted they don't show, but they provide
the healthy comfortable affordable to live in home
that we all want to have.

best of luck.

    Bookmark   September 20, 2012 at 12:03PM
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