Attic Insulating Cost/Benefit (Return on Investment)

[CottyGee]

Never mind. I made a gross error in calculating the cost of this. It's much more than I can afford. It's not 1.6X as much as reinsulating - it's 2.3X as much. http://www.herfersparadise.net/portal/modules/Forums/images/smiles/asdf.gif

I had an interesting conversation Friday with an insulation contractor. He quoted me a ballpark figure for adding R30 to my existing ceiling insulation, which would put me at around R49 or so. I asked about radiant barrier, which he didn't think was a very good return on the cost.

But then (PTSU you'll love this!) he suggested I consider cathedralizing my attic - sealing the gable vents, foaming with 1" of spray foam on the underside of the roof, and insulating the underside of the roof with fiberglass bats to get the underside of the roof to R30. So, I'd have the existing cellulose on the ceiling, which has settled from 8" to about 5" (from R30 down to about R19 now) PLUS the R30 cathedral "roof" insulation.

So, air handlers and ducting would all be in conditioned space - he says the attic would tend to run ~10ºF hotter than the house. Presumably, a significant savings from doing this???

So, the cost of blowing in new loose insulation on top of my existing cellulose is "1X". The cost of doing the cathedralizing is "1.6X". Is there any way to guesstimate the payback of doing 1.6X compared to doing 1X??

You guys are so good I'm thinking someone probably knows of a website calculator or just has enough experience to know the answer with confidence.... What say ye?

[jkish]

I had an interesting conversation Friday with an insulation contractor. He quoted me a ballpark figure for adding R30 to my existing ceiling insulation, which would put me at around R49 or so. I asked about radiant barrier, which he didn't think was a very good return on the cost.

But then (PTSU you'll love this!) he suggested I consider cathedralizing my attic - sealing the gable vents, foaming with 1" of spray foam on the underside of the roof, and insulating the underside of the roof with fiberglass bats to get the underside of the roof to R30. So, I'd have the existing cellulose on the ceiling, which has settled from 8" to about 5" (from R30 down to about R19 now) PLUS the R30 cathedral "roof" insulation.

So, air handlers and ducting would all be in conditioned space - he says the attic would tend to run ~10ºF hotter than the house. Presumably, a significant savings from doing this???

So, the cost of blowing in new loose insulation on top of my existing cellulose is "1X". The cost of doing the cathedralizing is "1.6X". Is there any way to guesstimate the payback of doing 1.6X compared to doing 1X??

You guys are so good I'm thinking someone probably knows of a website calculator or just has enough experience to know the answer with confidence.... What say ye?

Since you have HVAC-Calc, can't you estimate new heat gain/losses with increased insulation and moving duct work out of the unconditioned attic?

You could then convert the BTUs saved to dollars based on your energy costs and figure a payback.

Could be a good ballpark estimate.

[CottyGee]

Since you have HVAC-Calc, can't you estimate new heat gain/losses with increased insulation and moving duct work out of the unconditioned attic?

You could then convert the BTUs saved to dollars based on your energy costs and figure a payback.

Could be a good ballpark estimate.
Hey! Good idea! I didn't think of that. I already did the increased insulation - I think it was 1,200 BTUs. Not staggering. But making the change to ducting in the conditioned space is easy, I think. Not sure if the air handlers are grouped in with the ducting in HVAC-Calc.

Thanks for the reply! I'm curious if anyone else has any experience with this...

[gonekuku]

Very good question....

When cooling, keep in mind that with R6 ducts in a 120F attic, you are losing about 25-35% of your BTU's because of the ambient temp in the attic. The next significant loss (related to your question) is the attic R value. Going from R30 to R49 may not bring much ROI.

I crunched some numbers on this. Lets assume a 1000 sq ft attic and 300 ft of 10" R6 duct. (just some numbers I felt like using).

Attic losses:
1000 sq ft 75F inside, 120F attic, R15, 3000 BTU loss
1000 sq ft 75F inside, 120F attic, R30, 1500 BTU loss
1000 sq ft 75F inside, 120F attic, R49, 918 BTU loss

Duct losses:
300 ft of 10" R6 duct, 55F inside, 120F attic, 8450 BTU loss
300 ft of 10" R6 duct, 55F inside, 85F attic, 3900 BTU loss

I feel it's improper to call the attic conditioned space when the rafters are insulated resulting in a lower temp in the attic, it just happens to be cooler. The benfit is that the ducting and the equipment (typically insulated at about R4) in the attic have significantly reduced losses.

I don't have any conclusions... I'm eating lunch... just thought I'd monkey with some numbers with my free hand.

[pstu]

You should be aware that Manual J Version 7, which Hvac-Calc is based on, is known to give a "free pass" to duct losses. Manual J Version 8 is much better, though I won't swear by it. J7 assigns a duct loss for attic ductwork, similar to ducts in crawlspace. This is intuitively wrong (what about radiant energy in attic?) and I believe there has been adequate research to prove it as well.

My own Hvac-Calc report assigned about 9% of total cooling energy to duct losses. I felt honest in adding 10% to that figure based on my hot-humid Houston climate. See no reason why it would not apply even more strongly to the Phoenix climate.

If the price is acceptable, I see a lot of benefit to sealing the attic. Matters little whether you call it conditioned space or not, that IMO is just semantics here. The benefits to duct losses will fully apply, duct sealing is no longer such a critical issue, and the house will surely be more airtight. You can learn a lot on this subject by looking up Lstiburek's work -- he most recommends this type of sealed attic (sans the insulation on the attic floor). You will find lots of research on this. The hot desert climate is where he has most strongly proved this type of construction.

I do fear your contractor has been ignorant when he dis-recommended radiant barrier. But people tend to recommend what they sell.

Best of luck -- Pstu

[CottyGee]

LOL - free hand monkeying - I like it! ;)

The insulation salesman said the attic would run about +10ºF more than the interior of the house. From what you said kuku, it sounds like that might be a load of huey??


Just to be clear, the attic is currently 5" of cellulose. It was 8" for R30 15 years ago when it was installed. It now has settled, and the average depth is about 5 inches. 5/8ths of R30 would be R19 by my calculations.... Unless the relationship between inches of insulation and R-value isn't linear.

[Shophound]

If I lived in Phoenix and had the dough on hand, sealing and insulating the attic, provided the HVAC equipment is to remain in that area, is a no-brainer.

[CottyGee]

LOL - the "dough in hand" part is a bit troubling, but I've already shot our budget to hell with add-ons and so forth. It's not THAT much more to get the attic done this way if the guy's estimate is worth a darn.

[CottyGee]

It's not THAT much more to get the attic done this way
Crap - I just checked my notes and I made a math error when I was talkin' to the guy on the phone. We can forget this whole like of discussion! I'm not talkin' about 1.6X the cost of blowing in more ceiling insulation. It's 2.3X.

Sorry to be wasting peoples' time! http://www.herfersparadise.net/portal/modules/Forums/images/smiles/asdf.gif