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Cathedralizing attic in climate region 2a: cellulose and Tyvek?

Cathedralizing attic in climate region 2a: cellulose and Tyvek?

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  #1  
Old 06-20-15, 11:09 AM
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Cathedralizing attic in climate region 2a: cellulose and Tyvek?

I have been struggling with the process of cathedralizing my currently vented attic. I'm in the hot, humid South (N Fla), and there are dire warnings from experts about using cellulose, because of humidity. However, I am adamantly opposed to using spray foam, not just for the environmental considerations, but because I'm concerned I'll be one of those people who develops a chemical sensitivity, leading me to being forced to leave my own house because of health problems. As for why I'm cathedralizing it (converted from vented to unvented), I have both A/C ducts in the attic, and still some k&t wiring running along the attic floor. Blowing in cellulose would result in a continued loss of cooling via the ducts - until the house caught fire from insulation around k&t.

I have seen it stressed repeatedly that there needs to be an air-impermeable barrier on the underside of the roof deck - and the only such barrier mentioned is spray foam (or possibly some rigid foam, which I expect would be a bear to apply). It occurred to me that Tyvek is an air-impermeable barrier - and is, importantly, also NOT a vapor barrier. So I'm thinking of doing the following:
- apply Tyvek to the underside of the roof deck, wrapping it around the rafters rather than cutting and applying it between, so as to minimize the likelihood of air leaks.
- attach netting (Insulweb) to the bottom of the rafters.
- have cellulose dense-packed between the rafters.
- (the same treatment will be required on the one knee wall, and at the ends, along with closing off gables and any holes).

While this won't give me the requisite R-30 insulation (2x6 rafters), it should still be a vast improvement. Can anyone alert me to potential problems with this approach?
 
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  #2  
Old 06-20-15, 12:09 PM
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Welcome to the forums! Attic spaces are meant to be at the temperature of the ambient outside air. Your "seal" should be at the ceiling level, allowing the attic to breathe with soffit and ridge venting. This will keep the air moving and temperate. What you are proposing is making an oven of the attic space, containing all the hot air in the envelope.

I would reconsider rewiring the K&T, moving it away from the ceiling joist area, and insulating well at that level, after you seal all the leaks from your living environment, such as light fixtures.

Our insulation specialist will be along shortly to read through your suggestions, so hang in there for his observations.
 
  #3  
Old 06-20-15, 05:10 PM
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Hi Greymule and welcome to the forum,
I'm a bit slow at offering suggestions for those hot humid climates as I live way up here in Maine, 80 is hot for us.

My first thoughts are:
1. A cathedral ceiling doesn't have to be an unvented one.
2. R-30 is code minimum, which gives people the absolute cheapest option, no the best option.
3. The objective for the air impermeable barrier directly below the bottom of the roof is to prevent warm humid air from reaching a cool roof deck. I'm not sure how big of a problem that is in a hot climate.
4. Not on your list and maybe not currently an option, but when the roof shingles get replaced they have ones that reflect a greater percentage of solar heat.
5. Dense packed cellulose has been used for unvented applications, but there are research people who say some codes prohibit it and it can cause problems. I need to reread those articles to see if they are related strictly to cold country or whether they have objections with hot climates as well.

Have you considered maintaining a vented roof and adding more insulation below those rafters?

Bud
 
  #4  
Old 06-20-15, 06:15 PM
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K and T should have been replaced at least 20 years ago.
Cathedral ceilings look cool but have all kinds of issues.
Heat trapped at the top of the ceiiing.
Cracking tape seals because of the heat.
Tyvek is not a vapor barrer!
Think your looking at at least 10 year old info on foam insulation,
 
  #5  
Old 06-20-15, 08:18 PM
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joecaption:

"K and T should have been replaced at least 20 years ago." Of course it should have been. But it is still permissible, AND I'd rather replace it after insulating.
"Cathedral ceilings look cool but have all kinds of issues." This is not a cathedral ceiling. "Cathedralizing" refers to turning a vented attic into an unvented attic.

"Tyvek is not a vapor barrer!" That's the point. I don't WANT a vapor barrier. That would be a recipe for disaster.
"Think your looking at at least 10 year old info on foam insulation". Please provide a more recent link that says something different.
 

Last edited by Greymule; 06-20-15 at 08:37 PM.
  #6  
Old 06-20-15, 08:28 PM
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Thank your for the warm welcome, Bud. I hope that in the future I'll be able to help others, rather than just seeking help myself.

(1); no, but this is not a cathedral ceiling, it's a 3/12 pitch roof, and I WANT it to be unvented. ;^)

"3. The objective for the air impermeable barrier directly below the bottom of the roof is to prevent warm humid air from reaching a cool roof deck. I'm not sure how big of a problem that is in a hot climate." Neither am I, but everything I've found, including building codes, mandates an air-impermeable barrier for my hot, humid climate. As for (4), Joe Lstiburek points to moisture problems, but his article describes flat roofs and cathedral ceilings, and in any case the Tyvek is aimed at preventing moisture problems.
(5) I don't see anything in the building codes that forbids cellulose, but when everyone thinks of spray foam when "air-impermeable" comes up, I could see problems with inspectors.

While I have considered additional insulation, for the reasons I stated initially, I have rejected that option.
 
  #7  
Old 06-20-15, 08:32 PM
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Chandler:

???? "Attic spaces are meant to be at the temperature of the ambient outside air." That is absolutely NOT true for an UNvented attic.

[edit: From what I've read, it's not even true for a vented attic. My understanding is that the most critical function of venting is to prevent moisture buildup. Unless you have a strong fan working constantly, I suspect the attic temp in a vented attic in a Southern summer is going to be considerably hotter than ambient air temperature.]
 

Last edited by Greymule; 06-20-15 at 08:48 PM.
  #8  
Old 06-21-15, 05:38 AM
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Finding a specific article supporting your approach hasn't happened and due to my cold country experience I'm hesitant to say your approach is fine.

However, as you have read, most of Joe's arguments relate to a cold climate or at least cold enough for warm humid inside air to become a problem when it reaches a cold roof. I don't see that as a high probability in N Florida.

The biggest concern I see would be a leak from the outside. Your assembly as described over an unvented attic would have difficulty drying to the inside even with the house wrap and cellulose being vapor open. I have attached the most recent discussion I could find but it appears to repeat much of the older articles. In other words, Joe still doesn't like an air permeable insulation up against a roof deck in an unvented application, and neither do the code people. But again, are they addressing your climate, I didn't see that. In the end, it will be up to you.

As for your ceiling being a cathedral ceiling, it is, and the space below it needs to be brought into the conditioned living space. You don't want to trap that attic between your existing ceiling and the new unvented roof.

Latest link I could find below.
Bud
How to Build an Insulated Cathedral Ceiling | GreenBuildingAdvisor.com
 
  #9  
Old 06-21-15, 10:56 AM
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Bud9051:

I'm coming to the conclusion that my proposed approach is rare at best. But I'm also concluding that it is in fact workable. And the air-IMpermeable, vapor-PERMEABLE barrier should dramatically limit the risk of moisture problems, both inside and out. Add to that that any serious leaks should become apparent in the cellulose, and I'm reasonably confident about it.

Regarding Joe Lstiburek, I recommend everyone read http://www.buildingscience.com/docum...o-in-the-south, especially the first item on the list, Vented Attics:
"So much for the energy concerns. Now lets talk moisture. What? Are you all crazy? The air outside is hot, humid and disgusting. And you want to bring this into an attic where it can diffuse through the vapor barrier-less attic insulation and get to the cold, air conditioned ceiling? What were we thinking! Before it gets there it will see those cold R-6 insulated ducts, fittings, etc. and drip all over. Give me a break. Venting attics in the South was dreamed up by some disgruntled Yankee pissed about the Civil War and wanting to get even. Be sure when insulating at the roofline in humid climates to follow moisture control principles as you would with any insulated wall so that the roof assembly is self-drying".
I chuckle every time I read it.

"As for your ceiling being a cathedral ceiling, it is"; I suppose it's a matter of definition. My house has the typical cross-section of a box topped by a triangle. If I turned the attic into a room (which I can't, because the highest point is 5 ft), then its ceiling (the roof deck) would, I assume, be a cathedral ceiling. But all the rest of the house, the part I live in, has flat ceilings. I would not claim that I have cathedral ceilings, but maybe that's just me.

To integrate the attic more thoroughly into the lower conditioned space, I'll probably just keep the attic access door open.

Thanks for your help, Bud9051.
 
  #10  
Old 06-21-15, 01:23 PM
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Once again, I don't know how southern homes work, but in my north country, if one moves the insulation and air barrier to the roof, the space below becomes part of the house. We run into this often when a home owner closes off a room or the entire upstairs to reduce their heating costs. In one winter they will have mold growing on the walls and ceiling.

Leaving the current insulation and air/vapor barrier at the ceiling level and hoping that leaving the attic access open will integrate the attic space with the house is optimistic. First hot or cold draft and that opening will be closed.

Once again, I'll leave it to you to decide, but IMO, you are pushing into too many unknown applications. I like tried and proven.

But I do appreciate that you have certainly done your share of reading so I'm sure moving forward will be an informed decision.

Best,
Bud
 
  #11  
Old 06-21-15, 05:05 PM
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You seem determined to make a heat trap of your attic, so I'll make one more comment and move on. Vented attic spaces, as you have, now, are meant to breathe. Yes, it is true the ambient temperature and the temperature of the attic will vary, especially in Florida. But with proper soffit venting and ridge venting, this heat is minimized, and the air flow will keep moisture to a minimum. As Bud has pointed out, sealing the attic will mean opening it to the space below, and I am sure you don't want to do that. Having a "sealed" attic will force trapped extremely hot compressed air to press down on your living space and it will eke its way into it by some means.

I am sure your research will lead you to a good resolution of your quest, and I wish you the best in doing it.
 
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