WOW could use some advice Attic Foam

Reply

  #1  
Old 11-11-08, 03:20 PM
H
Member
Thread Starter
Join Date: Nov 2008
Location: Coastal NC
Posts: 4
Received 0 Votes on 0 Posts
WOW could use some advice Attic Foam

We have moved into our new modular home this past month. Now its time to start finishing the 2100SqFt "bonus area". Insulation is the current venue. I got an estimate from a local insulation company for "conventional fiberglass insulation" of ~$1700. This sounded kinda cheap, and I was happy about that. Then my HVAC guy suggests I call a company that does spray foam insulation, so, of course, I do. Like the other company, a man shows up and takes measurements and says various things about this and that, but at the end this guy says $11,150.

The spray foam guy then explains that his insulation will save me money on my HVAC system (about 1 Ton/1kSqFt), and that the savings in the long run would also be tremendous. He then said something I am having a really tuff time with: He said, "Our foam has an R rating of R21/inch and we will be using an average of 2.5 inches giving you an effective R-Value of R50 or better.

I can't find anyone on the internet claiming R21/Inch, most claim R7/Inch to R8/Inch. This insulation will go directly against the roof from soffit to ridge-vent and directly on the end walls & dormers. Foam Suppliers on the Internet indeed suggest R49 for this application, however, they are suggesting 7 inches of foam, not 2.5 inches. At 7 inches of foam for the area I have DIY kits run from $15k to $20k.

My questions are as follows:

1) Is foam really worth the extra expense?
2) Is there a magic foam out there that really has an R-Value of R21/Inch?
3) If this guy's foam is really only R7/Inch is 2.5 inches really enough ((R17.5)?
4) If R17.5 is really enough, then why is R49 reccomended?

I sure hope someone can help me through this. $11,500 is A LOT of money; at $0.10/kWh this is ~115,000kWh before any savings really begin to occur. Even if it saved me 1000kWh/month (a very BIG if) it would mean a 10-year break-even point.

Thanks in Advance!

Hunter
 
Sponsored Links
  #2  
Old 11-11-08, 03:41 PM
Gunguy45's Avatar
Super Moderator
Join Date: Dec 2007
Location: USA
Posts: 21,119
Received 3 Votes on 3 Posts
Well, I won't even try to answer anything other than this....R21/inch??? I don't think so. I think he may have meant R21 for 3 1/2 inches, standard 2 x 4. He's a salesguy.
 
  #3  
Old 11-11-08, 04:09 PM
H
Member
Thread Starter
Join Date: Nov 2008
Location: Coastal NC
Posts: 4
Received 0 Votes on 0 Posts
GunGuy,

Thanks for the response. No, I really could not believe there was anything that had an R-Rating of R21/inch, but I had to ask. As far as what he said, it is exactly as I stated. I have never investigated R-Values (other than the obvious: higher is better); however, I have done a fair bit of work with heat transfer involved in insulating hotwater tanks & pipes. The thermal conductivity of Urethane foam is 0.021 W/mK and it seemed unlikely that one Urethane foam would exhibit a 3 fold greater R factor than any other Urethane foam. Furthermore, despite claims of R7/inch by most sales agents, technical documents seem to approximate it at closer to R5.6/Inch on the high side and a bit below R4/inch on the low side. I guess i need to do some reading about what R-Values mean in terms of thermal conductivity. Sigh. I was hoping for easy answers.

Thanks,

Hunter
 
  #4  
Old 11-11-08, 04:12 PM
T
Member
Join Date: Aug 2000
Location: USA
Posts: 15,834
Received 0 Votes on 0 Posts
There are some considerations re: using spray foam insulation (SPF), such as impact on asphalt shingle warranty. See here: Foam Insulation

You may also want to discuss impact on HVAC system with the HVAC contractor. You would likely get buy with a smaller unit in addition to savings on heating/cooling.
Spray Foam Insulation, Insulation, Cellulose, Fiberglass, Las Cruces, El Paso, New Mexico, West Texas, Amarillo, Lubbock, Albuquerque, Santa Fe

I think what the SPF installer meant was that R21 can be achieved in 2x4 wall voids.
 
  #5  
Old 11-11-08, 04:39 PM
B
Member
Join Date: Oct 2008
Location: New England
Posts: 10,512
Received 37 Votes on 34 Posts
Hi Hunter, First, maybe this will ease the pain. Where I live, we pay $0.18 per KW . Anyway, to the details. Just got an estimate of $1.05 per board foot. That's 12" X 12" by one inch thick. A rough estimate of your rafter area and the $11,150 and I get about $1.50 per bd. ft. So not really too bad and 2 1/2" is typical. I agree with Gunguy, salesman, YOU ARE NOT SUPPOSED TO THINK. Another question is do you want a "hot roof" I know the foam guys are pushing the soffit to peak concept and maybe it works down south, but it is a hard sell up north. It's a given that the r-21 per inch is a myth, even r-8 is pushing it. What do you have for rafters? 2X6 2X10 ?? How are you finishing the inside? Dollar for dollar, I would go with the fiberglas and then a 1" or 2" layer of foil faced foam, tape it tight then strap it, and add sheetrock, with traditional ventillation. Let's see what others have to say, Bud
 
  #6  
Old 11-11-08, 05:12 PM
H
Member
Thread Starter
Join Date: Nov 2008
Location: Coastal NC
Posts: 4
Received 0 Votes on 0 Posts
twelvepole,

Thankyou for the links. The SPF installer was very specific about 2.5 inches of his foam in my 2x10 rafter space would provide the equivalent of R49. He stated his foam had the equivalent of R21/Inch. I assure both his words and my understanding were quite clear. I simply could not believe his claims.

Onto the most important of my questions:

1) Is foam really worth the extra expense?

After reading through the wikipedia explanation of R-Values, I think R-Values are a typical example of government trying to define science in a completely unnatural way. Ignoring R-Values and moving onto practical considerations. I need to insulate approximately 3780 sqft of roof deck space, or alternately use conventional insulation on knee walls, roof space and ceiling space. (+ additional wall insulation on the ends of the space). Is the effective insulating affect of ~2.5 inch thick foam going to be better than conventional insulation in terms of energy consumption and comfort?

Thanks for the answers!

Hunter
 
  #7  
Old 11-11-08, 05:38 PM
H
Member
Thread Starter
Join Date: Nov 2008
Location: Coastal NC
Posts: 4
Received 0 Votes on 0 Posts
Bud,

hrmm, I actually calculate the insulated area (soffit to ridge vent + end walls + dormers) at ~ 5000 square feet * 2.5 inches = 12,500 board feet = ~$0.89/board foot. Almost exactly what I can buy DIY kits for, LOL. Anyway I would need some strong arguments to go SPF to justify the 6.5X cost difference. Like you, so far no one has brought the benifits to bear in their posts (other than the reduced HVAC size, which I am convinced must have something to do with real insulating properties vs R-Values, but thus far has not been explained.)

Thanks for the advice!

Hunter
 
  #8  
Old 11-11-08, 05:48 PM
T
Member
Join Date: Aug 2000
Location: USA
Posts: 15,834
Received 0 Votes on 0 Posts
Do you know the brand of spray foam insulation? Most manufacturers seem to have a website. I would want to confirm the R21/inch.

According to Advanced Insulation, "There are foam contractors in the marketplace using terms such as “effective R-value,” in order to get credit for the air sealing properties of the foam. We understand these contractors' frustration with traditional R-value measures, but their efforts often mislead consumers. Sometimes they intentionally mislead consumers, which is bad for the industry. For this reason, Advanced Insulation does not use terms like “effective R-value.”

The very same contractors who advertise “effective R-value” will also install less than 6” of foam on the underside of roof sheathing. Without proof, (from third party independent laboratories), supporting applications of less than 6”, we do not recommend R-values less than a 20."
Advanced Insulation - Spray Foam Insulation

More info on R-value: Compare Spray Foam | McGlaughlin Spray Foam Insulation

There is nothing unnatural about R-value (Resistance Value). It is natural for air to flow. R-value is the measurement of a material's resistance to heat flow.

Spray foam offers greater heat flow resistance per inch than fiberglass batt insulation. Surfaces are completely covered, and there are no air gaps such as can occur with batt insulation.

Consult with your local building inspector. He should be able to provide some input and clarify any code requirements re: insulation.
 
  #9  
Old 11-11-08, 07:40 PM
airman.1994's Avatar
Member
Join Date: Dec 2005
Location: VA
Posts: 5,795
Received 8 Votes on 8 Posts
I will add that spray foam is about three times the price in my area.
 
  #10  
Old 11-12-08, 07:08 AM
B
Member
Join Date: Oct 2008
Location: New England
Posts: 10,512
Received 37 Votes on 34 Posts
I used 3,000 sq. ft. for a rough est. to see if the price was in the ball park, (forgot the end walls), so you are correct and $0.89 is a low price, IF you can get past his exaggeration on the r-value. Spray foam does seem to out perform fiberglass, usually attributed to it's air sealing properties, which for a roof may be irrelevant. My understanding of soffit to ridge vent would actually mean no vent? Sealed hot roof, are you ok with that? Another issue not mentioned is a radiant barrier. In an air conditioned house there can be a significant benefit to reflecting the radiant energy before it gets to the insulation. Typically a foil covered plastic sheet looped between the rafters, leaving a gap for the venting above the standard fiberglass insulation. A radiant barrier is another one of those undefined energy savers that has varied opinions. It could act as your vapor barrier, reflect solar energy, and reduce the wind washing effect of the vented air. With 2X10's you have space for about 8 inches of high density fiberglass. I picked up the following quote on a search. "For cathedral ceilings, high density batts rated at R-30 for 8 1/2 inch spaces, and R-38 for 10 inch spaces are available." The hot roof mentioned above (with the foam sprayed directly to the underside of the sheathing) has the long term problem of never knowing when your roof is leaking. Moisture that sneaks in can be trapped and damage the roof, to be discovered only after considerable damage is done. The foam people (they look kind of funny and walk around like robots ) use exaggerated r-values because the approved testing methods do not reflect the actual benefits homes experience when foamed. Since you have the 2X10's, you have the space for a reasonable quantity of fiberglass. I am old and traditional so would stay with the FG on the rafters, and it is probably less expensive. For your gable ends, you probably have 2X6's so again some better fiberglass, "A medium density batt for 2 X 6 inch frame wall construction is rated at R-21." Hope this helps, Bud
 
Reply
Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: