superduty
New Member
Posts:42
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| 04 Apr 2011 12:25 AM |
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I am going to be building a house in Southern California (the hot part of Los Angeles, where summer temps are regularly over 95, and many days over 100). The plan is to do a vented attic (Los Angeles Building and Safety hasn't quite warmed up to unvented attics). Anyhow I have a couple of questions with regards to the attic. The house is a single story.
Code requires a roof sheathing with radiant barrier on the bottom, so there will be a radiant barrier. I also plan to use a metal roof, such as a standing seam variety. At this time my plan is to use blown in cellulose in the ceiling joists for insulation. Is this a good plan? Should I do some spray foam below the cellulose? Is there a significant barrier in doing the foam first? What type of foam is recommended?
Secondly, what is recommended for venting the attic? There will be soffit vents all around, but I am wondering what is the BEST method of "releasing" the hot air at the roof peaks. Dormer vents, or is the ridge vent considered good. It would seem the ridge vents is the "cleanest" aesthetically. But I am willing to go with some other type of venting if it is considered more efficient. I unerstand there are formulas for how much venting is needed, and we will compute that. Right now I want to know what is the most efficient venting system.
Thanks
Jack |
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acwizard
Basic Member
Posts:265
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| 04 Apr 2011 02:05 AM |
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Being from your part of the woods, ridge vents work extremely well but I do not know of any for metal roofing that would lend itself to a home. All custom homes which I have been involved with have soffit vents around entire perimeter and dormers located as high up the roof slope as possible .The net free ventilating area shall not be less than 1/150 of the area of the space ventilated. There are some exceptions . Are there hvac ducts being installed in the attic. |
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jonr
Senior Member
Posts:5341
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| 04 Apr 2011 08:24 AM |
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Building wrap on the floor of the attic might be much less expensive than spray foam.
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Alton
Veteran Member
Posts:2164
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| 04 Apr 2011 09:42 AM |
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You state that the "Code requires a roof sheathing with radiant barrier on the bottom, so there will be a radiant barrier." I do not know how your code is worded but I would think that a radiant barrier would work the best if it were placed with an air space betweeen the roofing and deck. A radiant barrier on the bottom will not reflect, it might reduce the amount of re-radiation of the heat from the decking to the attic by being Low-E. If the code will allow it, insulated ventiliated roof decking should be a much more effective way to insulate the attic. One could build his own by using furring strips and then also include a radiant air barrier with an air space in the proper location. |
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Residential Designer &
Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu Use email format with @ and period .
334 826-3979 |
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jonr
Senior Member
Posts:5341
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| 04 Apr 2011 11:50 AM |
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it might reduce the amount of re-radiation of the heat from the decking to the attic by being Low-E Yes, that is what it does. You can also use Dupont's AtticWrap to create ventilation channels. |
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superduty
New Member
Posts:42
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| 04 Apr 2011 10:00 PM |
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acwizard - I was looking at cor a vent for a ridge vent with standing seam.
jonr - when you say building wrap the floor of the attic, what do you mean? Put wrap on top of the ceiling joists, then insulate on top of that? I have never heard of that method. Is that a common practice? And then put insulation above that? Alton - I believe the code requires a product such as plytanium According to the linked website - "Thermostat plywood sheathing reflects up to 97% of the sun's radiant heat." jonr and alton, can you elaborate about the ventilation channels and why/how that is a good thing? Any links to a detailed description or prictures of the ventilation channels? Thanks for your guidance. |
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jonr
Senior Member
Posts:5341
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| 05 Apr 2011 09:41 AM |
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http://www2.dupont.com/Tyvek_Weatherization/en_US/assets/downloads/AWSBK14127.pdf
But I'd discuss these things with your builder. He is the one to make it work and warranty it.
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Alton
Veteran Member
Posts:2164
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| 05 Apr 2011 11:40 AM |
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Jack,
From what I remember about Plytanium, the foil has little pin holes in it so moisture will not be trapped in it. And the foil is located on the bottom of the sheet. That is, the foil does not face the sun - it faces downward toward the attic. Regardless how the code reads, it can not change the physics of how the board reduces the attic temperature. For Plytanium to reflect heat, the foil would have to face toward the sun and then have the proper air space above it. In other words, Low-E is at work instead of the heat being reflected. The foil does not reflect - it simply keeps some of the sun's heat from being re-radiated from the Plytanium to the attic below. (Dana1, correct me if I am wrong in my understanding or terms.)
For other builders not located where the code dictates Low-E roof decking, more heat can be rejected by using foil and an air space facing upwards. In fact, in Florida where the sun is also hot, one recommended way to reflect the heat consists of the following: Shiny mylar is draped loosely over the rafters or roof trusses. Then a plastic square cut U-channel is snapped over each rafter which pulls the mylar down to maintain about 1.5" of air space and also pulls the mylar tight between each rafter. The plastic U-channels hold the mylar in place until the roof decking is installed. Spray foam can be applied to the bottom of the myalr thus preserving the air space above for reflection and ventilation. |
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Residential Designer &
Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu Use email format with @ and period .
334 826-3979 |
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Dana1
Senior Member
Posts:6991
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| 07 Apr 2011 05:17 PM |
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I though CA Title 24 only insisted on radiant barrier if "cool roof" materials suitable for the roof pitch & CA climate-zone weren't being used. If anything cool-roofs are more effective than radiant barrier anyway. There are many shades of metal roof finishing that are still Title 24 compliant cool-roof. See:
http://www.energy.ca.gov/title24/coolroofs/
http://www.coolmetalroofing.org/
http://www.coolmetalroofing.org/content/index.cfm?pageId=10
http://www.consumerenergycenter.org/coolroof/
Convective venting has only a minor affect on roof deck & attic temps, and even less so under a "cool roof" - it's primarily a moisture-purging measure more suitable for colder climates. But mounting the steel roofing on purlins/furring in a manner that can vent ensures that the roof deck can dry toward the exterior, and would be the moral-equivalent of a vented attic & roof in a SoCal climate. |
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superduty
New Member
Posts:42
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| 07 Apr 2011 11:51 PM |
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Posted By Dana1 on 07 Apr 2011 05:17 PM
I though CA Title 24 only insisted on radiant barrier if "cool roof" materials suitable for the roof pitch & CA climate-zone weren't being used. If anything cool-roofs are more effective than radiant barrier anyway. There are many shades of metal roof finishing that are still Title 24 compliant cool-roof. See:
http://www.energy.ca.gov/title24/coolroofs/
http://www.coolmetalroofing.org/
http://www.coolmetalroofing.org/content/index.cfm?pageId=10
http://www.consumerenergycenter.org/coolroof/
Convective venting has only a minor affect on roof deck & attic temps, and even less so under a "cool roof" - it's primarily a moisture-purging measure more suitable for colder climates. But mounting the steel roofing on purlins/furring in a manner that can vent ensures that the roof deck can dry toward the exterior, and would be the moral-equivalent of a vented attic & roof in a SoCal climate. For the life of me, I now can't find where I read about the radiant barrier sheathing requirement. Dana1, are you saying that utilizing the radiant barrier is kind of a waste if you have a cool roof? When you say "convective venting" are you referring to stuff like eve and ridge vents? If you mount the steel roof on the furring, are you suggesting to eliminate the vented attic? |
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Dana1
Senior Member
Posts:6991
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| 08 Apr 2011 11:03 AM |
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Yes, using a radiant barrier IS a waste if you have a Title 24 compliant cool roof.
Title 24 allows for the use of radiant barrier &/or higher-than-code R-values to be used, or an engineering energy analysis based on building & site-specifics (shading factors, actual reflectivity & emissivity of the roofing materials,etc.) when using a non-cool-roof material.
Yes, soffit & ridge venting is convective ventilation (as distinguished from gable end cross-ventilation or active fan type ventilation.)
Metal roofing flat on the deck is a vapor barrier- a vapor barrier with lots of air-leaks, but still effective where in contact with the roof deck/roofing felt, inhibiting the roof deck from drying. Metal roofing mounted on purlins allows the deck to dry into the gap (even through #15 felt, which is a semi-permeable vapor retarder), and with or without designed-in venting can dry (but clearly better if it's designed to allow for some cross-ventilation or convective ventilation) From a roof deck moisture control point of view venting between the roof deck & roofing is BETTER than venting the underside of the roof deck, since that's the side with the higher moisture drives, and makes going with an unvented attic nearly a slam-dunk. It's usually easier to air-seal the roof than the attic floor, especially if you have ducts etc in the attic.
If going with an unvented attic approach with no ducts or mechanicals in the attic, putting an inch or two of foil-faced iso board above the roof deck and below the purlin-gap can be significant thermal performance enhancement, since the foil facer puts a radiant barrier on the exterior, and the depth of iso provides a thermal break over the rafters/trusses. But since the foil facers are also a vapor barrier, the rest of the stackup has to allow the attic to dry toward the interior. The rest of the insulation can either be at the roof deck or attic floor (and any gable-ends). Air sealing the roof deck with closed cell foam at all seams & edges (with particular attention to ridges and where the roof meets the exterior walls and any plumbing stack penetrations) can detail the roof deck as the primary air-barrier, which is usually easier than air-sealing the attic floor/upper story ceiling perfectly. (You still want to dry though.) With an inch of iso above the roof deck you have ~ R6 thermally breaking the rafters, and if you then did a cellulose only fill on the attic floor to a minimum of 7" you'd be at R30+ (which IIRC would be code for LA?). But cellulose is cheap- if you can install enough to get 3" over the tops of the truss chords or joists (easy with 2x4 chord trusses, not so much with 2x12 joists) you'll get another significant performance boost by the ~ R10 thermal break over the framing.
If you have any ducts or air handlers in the attic you'll be better off with a combination of 2" of iso (~R13) above the roof deck and 6-10" of half-pound density open cell spray polyurethane foam applied to the underside of the roof deck, and as an air-seal. (If you use higher density open cell foam it should be rated more than 1 perm at the applied thickness. Half pound foam is typically ~ 2-2.5 perms @ 10".) Alternatively, and inch of exterior iso foam-detailing the roof deck as an air barrier and insulating the roof deck using high-density sprayed "blown in blanket" wet spray fiberglass (eg JM Spider) or wet-spray cellulose to bring the R up to code (or higher) works. This is more expensive than an attic floor application, but reduces condensation potential on the AC ducts, puts the ducts inside of conditioned space making any leakage or insulation gaps on the ducts less of an issue.
See also:
http://www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems/view?topic=doctypes/reports
(most of that is concerned with cold-climate stackups with no exterior foam, but should give you some reassurance. And inch of exterior foil-faced iso is at LEAST the moral equivalent of the inch of interior ccSPF in a SoCal climate, but not for cold climates. With exterior foil-faced iso the structural wood is partly inside the thermal boundary and inside the vapor barrier, with much lower swings in temperature & moisture content. In SoCal the air-conditioned interior is drier than the exterior air, on average, and the roof deck will rarely/never drop below the dew point of the conditioned space air, unlike what happens in cold climates.)
http://www.buildingscience.com/documents/reports/rr-0301-unvented-roof-summary-article
http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1496-05.pdf
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superduty
New Member
Posts:42
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| 08 Apr 2011 02:08 PM |
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Dana1, do you think the vented or unvented attic is the better way to go in a hot climate like So Cal? As mentioned my plan is for a standing seam metal roof, which I believe will be a cool roof. I also think the current plans will call for hvac ductwork and possibly the furnace/air handler in the attic (or it may be a pkg unit on the roof - but more likely a split system).
My architect and contractor haven't worked with unvented attics. And everything I see from Los Angeles City Building and Safety seems to indicate they want vented attics. I assume it would be possible to convince building and safety to approve an unvented attic, but I am not quite sure of the procedures involved.
Thanks |
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superduty
New Member
Posts:42
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| 11 Apr 2011 12:21 PM |
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Some more information and questions. I need to make a decision in the next 24 hours. The plans are being finalized:
House is a single story so the HVAC air handler and ductowork will be in the attic.
For a hot (not humid) climate like So Cal is vented better than unvented? (remember the ductwork will be in the attic)?
If I go unvented MUST I use purlins below the standing seam roof? (there will be no ice deamming in this climate, the reading I have done seems to point to that being the main reason for the purlins or "venting" above an unvented attic)?
I would rather not use purlins. I would like the standing seam installed to OSB or plywood for a firm walking surface (if need be). Installing purlins then another layer of plywood or osb would be cost prohibitive.
If I go with some sort of ridgid foam above the roof sheathing can the standing seam then be installed directly to the foam board or do I then need to lay another layer of plywood or osb?
Sorry for all the questions. This stuff is so confusing. There is so much to read on the internet and there are soooooo many different variables at play. It seems like there is no one standard recipe or agreement amongst the "experts." |
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Dana1
Senior Member
Posts:6991
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| 11 Apr 2011 05:44 PM |
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Posted By superduty on 11 Apr 2011 12:21 PM
Some more information and questions. I need to make a decision in the next 24 hours. The plans are being finalized:
House is a single story so the HVAC air handler and ductowork will be in the attic.
For a hot (not humid) climate like So Cal is vented better than unvented? (remember the ductwork will be in the attic)?
If I go unvented MUST I use purlins below the standing seam roof? (there will be no ice deamming in this climate, the reading I have done seems to point to that being the main reason for the purlins or "venting" above an unvented attic)?
I would rather not use purlins. I would like the standing seam installed to OSB or plywood for a firm walking surface (if need be). Installing purlins then another layer of plywood or osb would be cost prohibitive.
If I go with some sort of ridgid foam above the roof sheathing can the standing seam then be installed directly to the foam board or do I then need to lay another layer of plywood or osb?
Sorry for all the questions. This stuff is so confusing. There is so much to read on the internet and there are soooooo many different variables at play. It seems like there is no one standard recipe or agreement amongst the "experts."
With the air handler and ducts in the attic it's FAR better to go with an unvented attic with the insulation at the roof deck. Reason: This puts the ducts & AH inside the pressure and thermal envelope, so duct leakage then won't pressurize/depressurize the house driving air infiltration, and the ducts do not pick up heat/coolth from the outdoors. The primary reason for mounting on purlins is the increased the drying capacity of the roof deck to the exterior. (The whole ventilated roof deck to prevent ice-damming thing is a red herring- a band-aid for those conceding defeat on under-insulated roofs with heat leaks, but that doesn't mean there aren't other, BETTER reasons for venting it.) Water vapor can't escape through the metal where the metal is in contact with the roof decking, but it can if there's an air gap. If you don't use purlins that's fine, just be sure that you keep the vapor retardency from the roof-deck inward above 1-perm. Using 2-3" of rigid iso above the roof deck for R12-18, and adding 4-5" of half-pound density open cell foam on the interior of the roof deck gets you to a code-legal R30 that outperforms most other methods due to the thermal break over the rafters delivered by the exterior iso. With half-pound foam on the interior the roof deck can still dry toward the interior. Closed cell foam at those R-values would less vapor-retardent than optimal. In L.A. putting the air-gap there also increases the effective R-value of the stackup: You move a lot less heat from the metal to where the insulation begins if you have even 3/4" of air space. The metal runs hotter that way, but it both radiates & convection cools on the exterior as long as there's at least a minimal amount of slope to the roof. (2:12 or higher.) The metal can take the heat, so that's not a problem, but it'll be noisier in a rain storm than if resting on the roof deck. If the metal roofing is mounted on purlins/strapping there is usually no need to put down a full secondary layer of sheathing over the rigid foam unless the metal is less than 22 gauge (and thus wouldn't be a sufficient thermal barrier against ignition, per code. That may be waived for exterior apps in some locations or for some rigid-foam types, depending on it's fire ratings.) Whether or not you would otherwise need to use a secondary sheathing layer depends on the prescribed fastening method for the metal roofing system. The metal roofing manufacturer may have guidelines for mounting over roof deck insulation, which may also specify types of facers such as fiberglass, perlite or asphalt felt, etc. In L.A. fiberglass faced goods rather than foil-faced would probably be the better choice under standing seam if there is to be no ventilation gap, but ask around, find out what type of rigid foam goods the commercial builders put under standing-seam in your area. From a total cost point of view it's probably easier/quicker (and time=money) to go with nail-base panels purpose-designed with 7/16" OSB bonded to the iso for standing seam, such as Hunter H-Shield-NB (Atlas and some others make similar panels), rather than a DIY approach with foam bonded to secondary sheathing. But if you do your own, staggering the seams of the rigid and the exterior sheathing by a foot is a good practice. A 3" thick panel (nailer OSB included in total thickness) delivers ~R16. (From a design POV in cold climates it's wise to de-rate that to ~R14.5, but in L.A. you can use full rating.) Dow is big on pushing XPS for use under standing seam, but IIRC you MUST use a fire-rated decking/sheathing above the foam, and SFAIK it doesn't come in pre-bonded panels. (XPS & EPS should also be de-rated ~10-15% for use under hot roofing from a design point of view, making for a noticeably thicker assembly.) |
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superduty
New Member
Posts:42
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| 11 Apr 2011 07:08 PM |
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Dana1, thank you very much for sharing your knowledge.
So I did some research today on pricing:
Open cell foam to R30 in the rafters is .50 a board foot. I will have about 8150 sq' of roof. The foam company said R30 is about 8 inches, so that works out to somewhere in the neighborhood of $30,000 to insulate the attic using foam.
I am not sure if it will be cheaper and if so by how much, if I were to do a rigid foam above the sheathing and then less spray foam in the attic.
I called one place for a cellulose price and it was .80 and .90 a square foot for R30 and R38 respectively. The house will have a 5100 sq' footprint so that puts the cellulose price at less than $5,000 for R38.
Is .50 a board foot expensive for spray foam? They use Gaco Firestop
This is a SIGNIFICANT price difference. Much more than I was expecting. What are your thoughts on such a difference in price. I dont see how it can be worth it. I am guessing it will take a LONG time to make up that money spent in energy cost savings?
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jumpingspidermedia
Basic Member
Posts:104
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| 12 Apr 2011 12:13 AM |
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The importance of venting your attic cannot be overlooked. In cold climates, improper venting can lead to moisture buildup which causes mold to grow, wood rot in extreme cases or even ice in the attic insulation. During the summer, stagnant, superheated attic air can dry out your trusses, increase your energy costs for air conditioning and make your house uncomfortable. __________ SIPs |
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superduty
New Member
Posts:42
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| 13 Apr 2011 01:32 AM |
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So, 30k for unvented attic w/ spray foam vs 5k for a vented attic w cellulose?
Is this the typical difference in cost? Is this where a person should spend their money when building a house? How long will it take to recoup that 25k? How are people justifying such a cost difference? Are my numbers/estimates way off? |
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Dana1
Senior Member
Posts:6991
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| 13 Apr 2011 11:02 AM |
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8150' of roof is a lot of roof! (This isn't some tiny shack, eh?) I was presuming (without asking) that it was a typical 2500'' house (and maybe space-starved as the reason for putting the mechanicals in the attic.)
Closed cell foam runs ~$1 per square foot per inch of depth for high volume jobs in my neighborhood- 5" would deliver R30, making it 8150' x 5" x $1= $41K if closed cell is used (ouch!) At 5" of closed cell foam is a vapor barrier, and could create a moisture trap if the metal roofing is flat to the deck rather than mounted on purlins. Open cell is a cheaper/better solution. Half pound density open cell foam in those volumes in my neighborhood run about $0.40 per square foot per inch of depth, and it would take ~8" to hit R30, so doing it all in open cell would be 8150' x 8" x $0.40=$26K Apparently you live in a higher-ticket neighborhood. :-)
With the ducts & mechanicals in the attic, air sealing the ceiling/attic interface becomes more difficult (and possibly unreliable over time._ If going the cellulose on floor route it's worth air-sealing that interface with 1-2" of closed cell foam at the ceiling level (~R6-12), and adding the full R30 or more in cellulose above the cc foam, hopefully burying the joists/truss chords by 3" or more as a thermal break. The insulation on the ducts then needs to be R8 minimum, both supply & returns (as opposed to R6 on supply ducts only in a sealed conditioned attic.)
Better still is to move the mechanicals indoors rather than the attic, air-seal well between the attic & conditioned space, even if you have to make the ceiling heights a foot taller to do it. In that scenario you can air-sealing the attic with spot-foam on all electrical & plumbing penetrations etc, at which point blowing in R40-R50 of cellulose is cost-effective. Avoid recessed lighting in that ceiling, (or if you muse, use only fully gasketed air-tight insulation-contact rated fixtures) or you WILL have air leakage into the attic. Unless the attic is used for storage, don't provide an access door, only a service break-out panel that is well-sealed. If operable access doors/hatches are used, get religion about weatherstripping it.
Code minimum R values for L.A. are well below what's cost effective term using cheap insulation. R40, even R50 in the attic isn't unreasonable if using low-cost fiber. But air-sealing to under 2 air changes per hour @ 50 pascals (2 ACH/50) is more important than bumping from R40 to R50. Typical ACH/50 air leakage on houses without air-seal detailing during the construction of the shell &/or blower-door verification/remediation prior to insulating is 3-5x that. Making it air tight with the mechanicals fulling inside the envelope means you would spend part of the difference between an all-foam code-min solution and the cheap cellulose R40-50 on a heat-recovery ventilation system (if not already included in the HVAC package.) If going R50 in cellulose on the attic floor, be sure the framing/rafters/trusses are designed to accommodate the ~14-16" of depth all the way out to over the top plate of the walls ("Energy Heel" trusses, etc.).
If trusses, be sure to watch the chord-loading numbers and loading numbers for the ceiling gypsum relative to the weight of the cellulose as well. Going 16' o.c. rather than 24" with trusses may be necessary, since you're going to be between 1.75-2lbs/square foot static load with a cellulose-only solution @ R50.
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Alton
Veteran Member
Posts:2164
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| 13 Apr 2011 12:38 PM |
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Dana1,
Excellent! I really appreciate that detailed solution to accommodate a large house. |
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Residential Designer &
Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu Use email format with @ and period .
334 826-3979 |
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arkie6
Veteran Member
Posts:1453
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| 13 Apr 2011 02:58 PM |
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Posted By superduty on 13 Apr 2011 01:32 AM
So, 30k for unvented attic w/ spray foam vs 5k for a vented attic w cellulose?
Is this the typical difference in cost? Is this where a person should spend their money when building a house? How long will it take to recoup that 25k? How are people justifying such a cost difference? Are my numbers/estimates way off?
Your costs are in the ballpark. No way I could justify that extra cost. Here is a previous thread on the subject: http://www.greenbuildingtalk.com/Fo...fault.aspx |
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