Drawbacks to unvented attic (spray foam)?
Last Post 07 Nov 2013 03:44 PM by Dana1. 23 Replies.
Printer Friendly
Sort:
PrevPrev NextNext
You are not authorized to post a reply.
Page 1 of 212 > >>
Author Messages
seiyafanUser is Offline
New Member
New Member
Send Private Message
Posts:63

--
03 Nov 2013 09:39 AM
I have a raised ranch (26x49) in upstate NY and I have been looking for insulation contractors to add more insulation to the attic, because what I have now is less than R-20. Besides the old school methods like adding cellulose or spray foam the attic floor, several contractors have mentioned about spray foam the attic slopes and gable ends, which would completely seal off the attic. I did some research online and found this vented vs. unvented topic has been debated to much depth yet the answer seems to be inconclusive as to which one is superior. In any rate, this is going to be a hefty investment for me and once the insulation is installed it can't be easily removed. I understand this topic probably has been long debated in this forum, but to avoid "buyer's remorse", could someone please educate me one more time? What are the things I should be aware of if the attic is completely sealed off with foam, I figure it would be better off to learn them now than after the job is done :) From multiple quotes I received, spraying the slopes is cheaper than the floor (because you have to first remove all the existing insulation), and it's comparable to adding cellulose. I don't have any HVAC duct in the attic, and I don't plan on using it as a storage in the future. So if you think I should NOT do spray foam to the attic slopes please let me know as well. :) Thank you!
jonrUser is Offline
Veteran Member
Veteran Member
Send Private Message
Posts:4242

--
03 Nov 2013 11:01 AM
Without HVAC in the attic, cellulose should be lower cost, provide more insulation and not create any moisture or ice dam risk.
arkie6User is Online
Veteran Member
Veteran Member
Send Private Message
Posts:1073

--
03 Nov 2013 11:14 AM
If you don't have HVAC ducts in your attic, there is no way you will ever recoup the cost of spray foam. The better alternative is to blow in cellulose. An even better alternative is stabilized cellulose where a small amount of water is added to the cellulose which activates an added adhesive to consolidate and bind the cellulose fibers. Advantages are less dust, better air sealing, and less weight on the ceiling below.

I just got a quote for R49 stabilized cellulose sprayed in my attic (~2200 sq ft) for $0.72 per sq. ft. It would cost more than that for only R7 of spray foam (1" of closed cell foam or 2" of open cell foam). The same guy that quoted the cellulose also sprays foam and said he charges $1.00 per sq ft for 1" of closed cell foam or $0.50 per sq ft for 1" of open cell foam. He said there are very few applications where the use of spray foam is justified from an economic standpoint, but he still gets homeowners that want it regardless of the cost, so he provides the service.
seiyafanUser is Offline
New Member
New Member
Send Private Message
Posts:63

--
03 Nov 2013 03:19 PM
Posted By arkie6 on 03 Nov 2013 11:14 AM
I just got a quote for R49 stabilized cellulose sprayed in my attic (~2200 sq ft) for $0.72 per sq. ft.


That's so amazingly low price. I did a calculation myself, if I buy cellulose from Lowe's and do it myself, for R49 I would need about 100 bags and that's still around $1 per sq. ft.
Bob IUser is Offline
Veteran Member
Veteran Member
Send Private Message
Posts:1016

--
03 Nov 2013 03:23 PM
how thick? Cellulose is typically about 3.5 per inch; around here goes for .25-.40 per inch (or "per board foot")installed
Bob Irving
RH Irving Homebuilders
Certified Passive House Consultant
seiyafanUser is Offline
New Member
New Member
Send Private Message
Posts:63

--
03 Nov 2013 03:44 PM
R49 is about 15 inches (from GreenFiber website) If I go with cellulose then I will have to do air seal first. Some contractors here charge over $1000 to air seal a 1200 sq ft attic, I thought this is way too much, what do you all think?
arkie6User is Online
Veteran Member
Veteran Member
Send Private Message
Posts:1073

--
03 Nov 2013 04:15 PM
My quote was from Rosebud Insulation in Arkansas. He said he averages 30 - 40 homes per week in this mostly rural area. He buys cellulose insulation by the tractor trailer load and he uses Applegate Stabilized Cellulose exclusively. Per the Applegate website, R49 is ~14.5" initial depth and ~13.5" final settled depth.
seiyafanUser is Offline
New Member
New Member
Send Private Message
Posts:63

--
03 Nov 2013 05:17 PM
Interesting, I have never even heard of any contractor using Stabilized Cellulose in upstate New York.
LbearUser is Offline
Veteran Member
Veteran Member
Send Private Message
Posts:2276
Avatar

--
03 Nov 2013 05:38 PM
So what is the actual OTD price of the cellulose at 15" thick and 2,200 sqft of attic space?

I am assuming it will be around $3k - $5k


jonrUser is Offline
Veteran Member
Veteran Member
Send Private Message
Posts:4242

--
03 Nov 2013 06:07 PM
I wonder how much air flow there is through 15" of cellulose or stabilized cellulose. It may not be worth it to remove the old insulation and air seal. Or just lay Tyvek over the old insulation (to block air trying to flow through the insulation), tape down the edges and seams and then pile on new cellulose.
seiyafanUser is Offline
New Member
New Member
Send Private Message
Posts:63

--
03 Nov 2013 07:08 PM
Okay, going back to my original question, if cellulose costs the same as open cell foam on the attic slopes, is there any reason NOT to use foam?
FBBPUser is Offline
Advanced Member
Advanced Member
Send Private Message
Posts:992

--
03 Nov 2013 09:11 PM
There may be reason not to insulate the underside of the roof deck.
- you add many cu. ft. of conditioned space to the house.
- therefore you also add sq. ft. to the building envelope.
- you may be introducing multiple layers of insulation and vapour barrier, each with its own dew point.
- depending on the amount of foam, you may increase the amount of icing that develops on the roofing material. There is always some heatloss and under some conditions, snow will trap that heat losses and allow the underside of the snow to melt. When it runs down and hits the eaves, it will cause icing. This does not happen with a vented attic.

I don't see foam and cellulose approaching the same cost. We just did 15" of cellulose at .90 cents per sq. ft. and closed cell on the walls of the same job for 1.00 per sq. ft. inch. That's in Calgary in Canadian funds but not that different the some of the other prices quote in this thread.

BTW even though I buy quite good, it only cost me 10% more to have the insulator do the job then what it cost me to buy the product uninstalled. He buys by the truckload.
seiyafanUser is Offline
New Member
New Member
Send Private Message
Posts:63

--
03 Nov 2013 09:27 PM
Would 8 inches of open cell foam (R30) not have the icing problem while allow some moisture to escape?
arkie6User is Online
Veteran Member
Veteran Member
Send Private Message
Posts:1073

--
03 Nov 2013 10:31 PM
Posted By Lbear on 03 Nov 2013 05:38 PM
So what is the actual OTD price of the cellulose at 15" thick and 2,200 sqft of attic space?

I am assuming it will be around $3k - $5k



In my case, the R49 stabilized cellulose in the attic (~14.5" initial depth) is $0.72 per sq ft installed.  For 2200 sq ft, the OTD price is $1584.
LbearUser is Offline
Veteran Member
Veteran Member
Send Private Message
Posts:2276
Avatar

--
03 Nov 2013 10:59 PM
Posted By arkie6 on 03 Nov 2013 10:41 PM
Posted By Lbear on 03 Nov 2013 05:38 PM
So what is the actual OTD price of the cellulose at 15" thick and 2,200 sqft of attic space?

I am assuming it will be around $3k - $5k



In my case, the R49 stabilized cellulose in the attic (~14.5" initial depth) is $0.72 per sq ft installed.  For 2200 sq ft, the OTD price is $1584.

I thought the 2,200 ft² was just the attic width x length and it did NOT include the depth of the cellulose, which is 14.5". Cubic feet would be the better calculation.

If $1,600 gives you an entire attic full of 14.5" deep cellulose, that is a great deal.


smartwallUser is Offline
Advanced Member
Advanced Member
Send Private Message
Posts:579
Avatar

--
04 Nov 2013 10:06 AM
I'm in upstate NY and the Green Fiber plant in Hagaman NY makes a stabilized cellulose. I'm picking some up this week. Why would you remove the old insulation? Is there a floor in the attic? If your not going to use it for storage then remove it and blow new celly over the old insulation. The cellulose should cost you about $800 retail for 25lb bags, the product at Lowes and the Depot are Green Fiber but not stabilized and the bags are about 18 lb each.
seiyafanUser is Offline
New Member
New Member
Send Private Message
Posts:63

--
04 Nov 2013 03:59 PM
If I do spray foam of course I will have to remove all the existing insulation, otherwise for cellulose I will leave them and add it on top. So far all the foam contractors in my area ask for $2.50 to $4.00 per sq. ft. for open cell foam. :(
robinncUser is Offline
Basic Member
Basic Member
Send Private Message
Posts:482

--
04 Nov 2013 07:26 PM
I've had several roofers tell me that if you have asphalt  shingles with an unvented roof, you will only get about half life outta the shingles verses a vented roof.
Dana1User is Online
Senior Member
Senior Member
Send Private Message
Posts:5462

--
05 Nov 2013 12:20 PM
Posted By seiyafan on 03 Nov 2013 09:39 AM
I have a raised ranch (26x49) in upstate NY and I have been looking for insulation contractors to add more insulation to the attic, because what I have now is less than R-20. Besides the old school methods like adding cellulose or spray foam the attic floor, several contractors have mentioned about spray foam the attic slopes and gable ends, which would completely seal off the attic. I did some research online and found this vented vs. unvented topic has been debated to much depth yet the answer seems to be inconclusive as to which one is superior. In any rate, this is going to be a hefty investment for me and once the insulation is installed it can't be easily removed. I understand this topic probably has been long debated in this forum, but to avoid "buyer's remorse", could someone please educate me one more time? What are the things I should be aware of if the attic is completely sealed off with foam, I figure it would be better off to learn them now than after the job is done :) From multiple quotes I received, spraying the slopes is cheaper than the floor (because you have to first remove all the existing insulation), and it's comparable to adding cellulose. I don't have any HVAC duct in the attic, and I don't plan on using it as a storage in the future. So if you think I should NOT do spray foam to the attic slopes please let me know as well. :) Thank you!

"...upstate..." is a broad term, and covers three US climate zones. Climate makes a difference- got a zip code?

The IRC has addressed the issues of insulating at the underside of the roof deck, with minimum prescriptive R for non-air permeable insulation to be in direct contact with the roof deck.  There is also strong evidence from WUFI simulations (with competent users, not the garbage-in/garbage-out simulations) that lesser amounts of foam than the prescriptive values can be used if it has sufficiently low vapor permeance.

Open cell foam though not air permeable, is still too vapor permeable at the IRC prescriptive values to be super-low risk without interior vapor retarders.  But vapor retardent paint applied directly to the foam brings it down to ~5 perms, which is sufficient to be protective even of roof leaks, provided you keep the interior relative humidity below 35%.  See Figure  10, page 28 (PDF pagination) of this document.  This is in R49 assemblies in a Minneapolis climate (US climate zone 6) simulating the drying of a  small roof leak over several seasons.  There is R24 (the IRC prescriptive value for zone 6) in contact with the roof deck, with the rest as fiber below.  The blue line is 2lb closed cell foam, the orange is half-pound open cell with a 5-perm paint-on vapor retarder.

Insulating at the roof deck has a negigible effect on shingle life, protestations of the roofing contractors worried about getting hung out to dry on warranty replacements notwithstanding. This has been studied to death in FL, where peak & average shingle temps are dramatically higher than in NY, and the best-guesstimate is that it takes 15% off the lifespan of a 25 year shingle even in a much hotter climate than yours. The COLOR (or more correctly, the solar reflective index) of the shingle makes a larger difference on both peak shingle temp and shingle life.  In almost any climate zone the value of the energy savings over the lifespan of the shingles is far more valuable than the modestly reduced lifecycle of the shingle.

Removing the attic floor insulation is necessary to meet code, but does not in fact create moisture problems in the real world.  The concern is that the semi-conditioned space will run at a mold-inducing relative humidity, but that is something easily monitored and rectified, if need be.  I've just left it in place in my own home (YMMV).

What IS required is that the roof R meet the code-minimum R, and many foam installers will try to give you a " X-inches is all you really need", and they are dead-wrong, on both a legal & performance grounds.  Some states explicitly allow a reduced R value for "compact roofs" or cathedral ceilings- typically R38 where the attic floor R requirement would have been R49, or R30 where it would have been R38. (No NY building codes allow lower than R38 at the attic floor.)

Some cellulose installers will try to sell you on the notion that dense-packing cathedrals ceilings is sufficiently air retardent to be considered not air-permeable for code purposes, and they too are dead-wrong.  But it is possible to use spray or dense pack cellulose (or dense-packed fiberglass) on these roofs if you use a "smart" vapor retarder such as Intello Plus or Certainteed MemBrain as an interior side vapor retarder, detailed as an air barrier.  These vapor retarders are class-II low permeance vapor retarders when the proximate air is below 35% RH (as it would be most of the winter, when the roof deck is cold enough to adsorb moisture from the room air), but go high-perm when the RH is over 40% (as happens inside the rafter bay in the spring when the roof temps are higher, and driving off any winter-accumulated moisture.)

Generally speaking, insulating the roof deck to code-min in a moisture-safe way is dramatically more expensive than doing it at the attic floor in new construction, but if the attic floor is difficult/impossible to air-seal well or you don't have sufficient depth to get the full-R with cheap stuff like cellulose, there are thermal performance advantages to sealing & insulating at the roof deck.

smartwallUser is Offline
Advanced Member
Advanced Member
Send Private Message
Posts:579
Avatar

--
07 Nov 2013 08:59 AM
I don't think there would be too much air flow in a sheet rock ceiling
You are not authorized to post a reply.
Page 1 of 212 > >>


Active Forums 4.1
Membership Membership: Latest New User Latest: Ozrick New Today New Today: 2 New Yesterday New Yesterday: 11 User Count Overall: 28690
People Online People Online: Visitors Visitors: 303 Members Members: 19 Total Total: 322

GreenBuildingTalk

Welcome to GreenBuildingTalk, the largest, most active forum on green building. While you can browse the site as a guest, you need to register in order to post.

Register Member Login Forum Home

Search Directory

Professionals Products

Get Free Quotes

Tell us about your building project and get free quotes from green building professionals. It's fast & easy! Click here to get your free quote.

Site Sponsors

For Advertising Info:
Call 866-316-5300 or 312-223-1600

Professionals Serving Your Area:

Newsletter

Read the latest GBT Newsletter!

Copyright 2011 by BuildCentral, Inc.   Terms Of Use  Privacy Statement  Free Quotes  Professional Directory  Advertising Programs