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Spray Foam in the ceiling


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Shane's Forum Posts: 12
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By Shane in San Antonio, TX on 10/16/2009


I'm thinking about spray foam insulation on top of the sheetrock in the attic, not underneath the decking as most conversations here have discussed. Would there be any benefit or harm in spraying foam (closed or open cell, either one) between the ceiling rafters (again, not the roof rafters)? You could then still vent the attic "normally" as it would not be encapsulated. Seems as if it would seal up any loss of conditioned air leaking out of the living spaces into the attic.


Have not heard of anyone doing this though, so there must be a reason.  Fire hazard maybe, if too close to fixtures? Would it be too heavy and cause problems with the sheetrock?  Maybe it's not cost effective to do this?

We will be using spray foam in the walls and just wondering if any experts out there have ever seen it used as mentioned above?

Thanks.

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By Aquaman in Houston, TX on 10/18/2009


It is all about cost effectiveness to me. Why spend the extra money and then still vent the attic?  I don't see why spray foam on the ceiling would not work but the whole purpose of spraying the roof rafters is to seal the entire attic and protect the a/c equipment and the duct work.  The question is legitimate but why go to the expense when it defeats the purpose of the application?


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By Shane in San Antonio, TX on 10/19/2009


I think I am one of those old-fashioned types who's not sold yet on encapsulating the house from roof to floor.  I've talked to several roofers about it and all except one thought that it probably wasn't a good idea to spray foam on the roof decking for where we live in the south - they think it will shorten the life of the roof deck.  I guess that's what brought up the question about spraying the foam on the ceiling rafters instead of the roof rafters and keeping a vented attic.  The thought was if you've made up your mind to have a vented attic, then what is the best (not necessarily cheapest) way to keep the attic and the living space separate?
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By Aquaman in Houston, TX on 10/19/2009


If the roofer's logic is taken forward, then you shouldn't spray foam in the walls against the exterior wall sheathing either (unless you use foam board with brick) as that is wood "decking". But I hear where you are coming from, Shane.  I too am not totally convinced of the spray-foam application and I am turned off by the extra cost.  The new solar-powered roof vents work really well to suck that hot air out of the attic, by the way.
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By DCG in Lago Vista, TX on 10/19/2009


Don't let a roofing contractor give you advice on insulation, unless he's telling you that the insulation will somehow impact his work/warranty.

I did a foam roof deck because I believe that I'll be paid back over time in terms of energy savings.  I wouldn't foam the ceiling and do a traditionally vented attic because I'd lose a good portion of my efficiency.

Before you decide, do two things:

1) On a given day, within the same hour window, visit a traditionally-ventilated batt- or cellulose-insulated attic.  Measure the temperature.

2) On that same day, visit the attic of a foam-insulated house.

As a bonus, find both that are new construction and do not yet have running HVAC systems that could impact the attic space temperature.  Let the attic temperature guide your decision.

Think about this also, when it's 100 degrees outside, my old cellulose-insulated and vented attic could exceed 120-130 degrees.  I'm then piping all of my HVAC air through that space in conduit...

In my home currently, the attic temperature rarely exceeds 15-20 degrees above room temperature.  It's much more efficient to pipe cool air through the attic.


Radiant barriers work by reflecting heat/radiation - it goes right back up to the roof deck/shingles.  Foam may have the roof deck retaining more heat - that wouldn't surprise me, and I know there were some shingle manufacturers that wouldn't warranty on foam-insulated roofs. However, last I heard, the industry was shifting away from that warranty issue.  I think it was more of an "unknown" than a definite problem.

I wanted a metal roof.  I built when steel prices were high and couldn't afford it.  My next decision point was a 20-yr. vs. a 30-year shingle.  I chose the 20 year.  Why?  Because I live in Texas and an exceptional composite roof lifespan might be 15 years.  Our roofer even said that I'd be throwing away money on a 30-year shingle because the hail would kill the roof long before heat or shingle quality would be a problem.  Keep that in mind...

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By Shane in San Antonio, TX on 10/19/2009


Thanks, guys, so much for your input.  Definitely given me some things to think about.  I probably need to research some more into foaming the roof deck.  Want to get this part right, because I feel it is the single most important thing as far as energy efficiency in the home goes.  Thanks again.
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By Lars in Granbury, TX on 10/19/2009


Shane - I think your idea is OK, especially if you get the AC out of the attic.  If you can make room in your floorplan for a 2'x2' closet, you can install a vertical air handler.  Some of the ducts might be able to run "under" the ceiling rafters if you use soffets.  Those ducts that run in the attic should be placed "on" the ceiling drywall as much as possible.  Then foam away! Good Luck 
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By DCG in Lago Vista, TX on 10/19/2009


Just watch stuffing an air handler in too small of a space.  Just because it can fit, doesn't mean it should fit! The major objection I've had to air handlers in a small space is that smaller air returns = lots more noise. 

You can't hear the AC running (at all) in my current house.  We have two air returns. Previous residence had an air handler in an upstairs closet and a medium-sized return.  It sounded like a train when running.

Specify how you want the venting run, if you're using a contractor.  Seems like most contractors today just hang the venting from the roof rafters.

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By Ignacio in Houston, TX on 10/20/2009


I would say maybe on that one. The issue is really one of appearance. I have a shingle roof that is almost certainly 30 years old, maybe more. It is so old that it is down to the fiberglass in places. It still doesn't leak, however, I do have to go up and roofing-cement-down 'flyers' that have come off a few times a year. It isn't pretty. I think that's the major reason to choose a 20- versus 30-year shingle is that most people won't tolerate the old appearance of a 25-year-old roof.

FYI, I don't change my roof because the foundation of the house is also shot and if I want to redo the roof, then I start the whole money cascade of fixing the foundation in a house that I'm not sure is worth saving.

>>"My next decision point was a 20-yr. vs. a 30-year shingle.  I chose the 20 year.  Why?  Because I live in Texas and an exceptional composite roof lifespan might be 15 years.  Our roofer even said that I'd be throwing away money on a 30-year shingle because the hail would kill the roof long before heat or shingle quality would be a problem.  Keep that in mind...">>

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By David in Lancaster, TX on 10/20/2009


dcg, very good advice!
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By tobe in belton, TX on 11/3/2009


Shane,

That's the same question that I had. I called and talked to a DIY foam company and they stated just stay away from the lighting fixtures.  I cannot find anything that says it want work but I'll keep checking until I am ready to foam.

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By David in Lancaster, TX on 11/3/2009


One of the benefits of foam insulation is that it seals the surface you apply it to as well as insulates. If you spray it on the ceiling instead of the roof sheathing, you'll still have leakage through and around the can lights, and your ductwork and possibly your HVAC unit will be in an unconditioned (and very hot) space. If you spray the deck, you can supply and vent your HVAC unit and water heater (if they're in the space ) to the outside and have a better functioning system with no leaks. I don't know your exact situation, but I just wanted to give you something else to chew on. Good luck!
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By Martha in Arlington, TX on 11/4/2009


While I am definitely not an expert on the subject, I have spent a lot of time reading up on this subject.  It seems to me that the latest science on the subject points to "seal the attic" and "vent the roof".  That is the mantra of Peter Pfieffer.  He is a leading architect out of Austin and one of the first in the state to have a net-zero energy home.  There was a great article in Fine Homebuilding last year about his house.

I'm fairly certain that this is the way I am going to go on my ICF house:

1) Solar board decking, with membrane instead of roofing felt.
2) Metal roof.
3) Vented ridge, exposed rafters, blocking with a 1"gap covered by insect screen between each rafter.
4) 6" dead space between underside of decking and a mesh (similar to stucco mesh--something cheap) stapled to the rafters.
5) Open-cell foam sprayed on the mesh (and bottom of rafters).  Probably at least 4-6".
6) No ductwork in the attic.  I may go with ductless AC or geothermal---haven't decided yet.

We'll see if it works!

Martha

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By DCG in Lago Vista, TX on 11/4/2009


Toby,

The solution I found to foam and recessed lighting:

1) Box them with adequate air space.
2) Buy a special recessed lighting fixture that is designed for use with insulation that does not allow for air passage... You'll need to contact manufacturers directly.

Residential construction code (at least in my area) is not currently up to date on this issue. Electricians (and experienced designers/architects) didn't get it right either... It's easy to make a mistake here.

In my home, I simply removed them and went to a surface-mount light.

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By tobe in belton, TX on 11/5/2009


Martha,

I built my house with ICF and love it.  I worked with a company that sells ICF and trains you how to erect it using your friends and family and cutting down on the labor cost.  If you would like the information let me know.  I did the metal roof and used solar-backed sheathing under the metal.  But it never occurred to me to use mesh on the bottom of the rafters - something to think about.  By the way, the gas or propane tankless water heater works great.

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By Joe in Hermiston, OR on 11/6/2009


Tobe,

Which ICF system did you go with? I'm still in the planning stage and am trying to track down people with actual results and opinions. Thanks for your time.

Joe


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By Martha in Arlington, TX on 11/9/2009


If I could add my two cents... I used Nudura and loved it.  I went to the training course and felt very comfortable setting the first two courses of ICF by myself.  I decided to get help after then because the door and window bucks (2x12 treated wood) were just too heavy for me to handle.  We're finished with the ICF portion of house and so far I'm very happy.  Now, it's just like building a conventional house.

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By Lance in Brock, TX on 1/28/2010


Martha,

I saw just read your response about Peter Pfieffer and his article in FHB. I thought his article recommended spraying an inch of closed-cell foam on the ceiling followed by open-cell foam directly to the underside of the rafters.  Has he changed his recommendation? I am trying to decide if I am going with foam on the drywall of the ceiling or on the underside of the rafters. A spray-foam installer I talked to in Fort Worth area said about 80% of the homes he does he just puts a one-inch layer of closed cell foam directly on the drywall ceiling and then comes in with blown-in fiberglass.

Thanks

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By DCG in Lago Vista, TX on 1/29/2010


Anyone have a reference or copy of the article?

I'm no expert either. In brick and stick construction on a solid foundation, you usually have some parts of the HVAC system in the attic. I see a substantial advantage to sealing the attic via foam on the roof deck. The HVAC equipment is then living in semi-cooled space and you're no longer piping cold air through 120- to 140-degree attic (summer in TX). 

If you could get all of the HVAC out of the attic, I can see why you might vent the attic and seal the ceiling with foam.  The attic is then a traditional space.

I DO see substantial cost advantage of using 1" closed-cell foam (essentially airtight) then getting the added R-value of cellulose without the expense of foam. That makes a lot of sense to me...

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By Martha in Arlington, TX on 1/29/2010


I'm in a time crunch now, but will look up the article that refers to "vent the roof... seal the attic". Like I said before, I'm not an expert, but I have an engineering background and it makes sense to me to take this approach... especially since I already have ICF on all the walls and gables. It could be a major pain putting the mesh across the rafters, but it seems to me that most things about building a house are painful! I'm probably going to do the attic area above the garage first to see if this method will work. I'll post the reference article later.
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By DCG in Lago Vista, TX on 1/29/2010


Why do you have to put mesh across the rafters? 

(I'm thinking radiant barrier, space for the heat to escape?) - I think there are alternatives.

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By Lance in Brock, TX on 1/29/2010


I found the article. One-inch flash coat of closed-cell foam (for air seal) followed by cellulose in the walls.  Roof: one-inch closed-cell then additional open-cell sprayed to plywood.  Plywood is nailed to 1x4 laths to create an airspace.

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By Dennis in Bowie, TX on 1/29/2010


Hi Shane,

I am the owner of a company specializing in spray-foam insulation.  I won't get into the laboratory dialog like some; I'll keep it very basic.

Spraying foam on ceiling sheetrock is not a bad idea... BUT DO NOT USE CLOSED-CELL FOAM! Open-cell foam will wick off (dry out) moisture and be a good acoustic. Spraying your rafters (under roof sheathing) and making a complete sealed-attic system, then adding inexpensive blown-in cellulose or fiberglass over the ceiling will give you great performance and add very good acoustic effect.

Closed-cell will seal great, but will also give you a great echo chamber. Closed-cell holds moisture and can be a potential haven for mold if you have a ceiling leak. Also, it's rigid and extremely hard to cut away if you have to do a remodel.

Finally... your budget may tell you what is best for you. Retrofitting foam in an existing structure can be very expensive. Be sure you will have it long enough to realize a capital return.


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By Stephen in Corpus Christi, TX on 2/1/2010


To restate what has already been said in various ways in this thread, IF YOU DO NOT PUT THE AC EQUIPMENT AND DUCTS IN CONDITIONED SPACE, THERE IS NO BENEFIT TO USING FOAM!   This rules out any logic in placing the foam on the attic floor. In conventionally vented attic insulation methods, sheetrock in itself is a very good air barrier.  You just need to seal all penetrations to the attic.  This includes wires, plumbing, light fixtures, AC ducts and even between the top plate and ALL wall sheetrock (where practical)  This last step is being considered by Energy Star for a requirement in 2011.  Much more cost effective than foaming the attic floor.  A home airsealed this way (including walls) can be as "tight" as a foam insulated home.   If doing a conventional vented attic, placing the AC air handler down in conditoned space yeilds a 2 to 3% reduction in TOTAL electric usage over having the air handler in the attic (bad idea no matter what all the home designers are doing). 

While in many cases conditioning the attic with foam is the best way to go, I am attaching some info from Florida Solar Energy Center on conditioned (foam) attics which might make some think twice before spending the "big bucks" on insulation.  If you do foam, I recommend a blower door test before sheetrock to verify the envelope tightness.  We usually find several air leaks in the foam which could casuse moisture problems.  Bottom line - spending a few hundred bucks for an Energy Star rating from an INDEPENDENT rater is cheap insurance that your dream home will turn out as planned.

 


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By Martha in Arlington, TX on 3/20/2010


Hi Everyone,

Sorry that I have taken so long since my previous post. The article that I was referencing about 'venting the roof' and 'sealing the attic' is at the following:

finehomebuilding.com/how-to

Since I am located in Texas, I really appreciated Peter's article. And since I am building an ICF house, it really makes sense to me that I should continue the insulated envelope that I have already created in the walls and gables by venting the roof and insulating the attic. I've already decked the roof and have left a 2" space on both sides of the ridge board and I have open rafters wherein there is a 1"x24" netted space in between the rafters located above the top plate of the walls. After placing some sort of mesh on the bottom of the rafters (imagine a vaulted ceiling with mesh instead of drywall), I plan to spray a very thick wall of open-cell foam. I will then have a complete envelope of insulation from ground to ridge and the roof will not get hot because there is air flowing from the top plate to the ridge.

While I haven't seen this exact design anywhere else, I think it should work. If not, I can always try something different.

Martha


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