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khariV

Attic / Roof Insulation

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I'd like to talk insulation to pick the brains of those in the know.

 

The attic in my house currently has no insulation at all, a legacy of the failed renovation contractor engagement.  The house has a cathedral ceiling so the "attic" is really just the underside of the roof and is fully open to the rest of the house.  There is no enclosed and sealed attic really.  The red part is the what has no insulation currently.  Actually, there's no insulation on the floor area of the attic either, but that's not really an especially exciting discussion.

insulation - house profile.png

 

 

The rafters are 12" deep and 16" on center - nothing oddball there thankfully.

insulation - empty.png

 

I'm going to install new insulation myself, but I think I need a bit more information on the right and wrong way to do it.

 

The first option would be to install faced R-38 insulation.  I'm pretty sure that I'd also need to install baffles to ensure the proper gap.  That's Option 1.

insulation - option1.png

 

While that would work, I've been told that I really ought to investigate reflective insulation.  That's where things get complicated.  From my research, I'd use something like this.

46a88e7d-9105-4e0f-81ca-07de3b9d9011_100

http://www.homedepot.com/p/Reflectix-16-in-x-25-ft-Double-Reflective-Insulation-with-Staple-Tab-ST16025/100012574

 

The installation instructions on this say that it needs a 1" air gap to be effective and that it can be used as a vapor barrier with unfaced insulation.  The first problem, and it's my problem, is that Lowes has a 30% off insulation deal on now that doesn't include unfaced R-38, so I'd be using kraft paper faced and it would look like this.

 

insulation - option 2.png

 

Well, that doesn't give the required gap between the paper and the reflective insulation.  Does it matter?

 

In my research, I also see that in NC, you can install the bubble reflective insulation directly against the roof decking, again as long as you have the 1" air gap beneath it. That brings me to Option 2a.

 

insulation - option 2a.png

 

I'd still have to put in baffles on top of the reflective insulation and I'd still be using faced insulation stapled to the rafters.

 

 

So, those are my choices.  I'd like some input from anyone that knows more than I do on this topic - meaning probably just about everyone :D

Here are my questions.

 

1. Should I bother with reflective insulation in addition to the fiberglass?

2. If I do use the reflective insulation, is it OK to use with faced insulation (Option 2)?

3. If I go with option 2a, do I still need a plastic sheet vapor barrier on top of the faced insulation (This is where the green line would be in diagram 2a)

 

As always, thank you all a ton for your experience and advice.

 

kdv

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Sorry I can't help you, but what a great post, detailed and pics ..... You should have no trouble getting what you need with the type of info you provided !!!

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Where do you live? Climate zone is a huge factor in the correct method.

Cathedral ceilings are a little more sensitive than a regular attic.

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3 hours ago, overanalyze said:

Where do you live? Climate zone is a huge factor in the correct method.

Cathedral ceilings are a little more sensitive than a regular attic.

 

North Carolina. I looked up the required thickness on the zone map and it says to use R-38 for cathedral ceilings. 

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R-38 could be correct but the type of insualtion is also very important. Read up on vented vs non-vented.

http://buildingscience.com/documents/digests/bsd-149-unvented-roof-assemblies-for-all-climates

Lots of good info on that website. Condensation is a very big concern with fiberglass and vaulted roof assemblies.

If you want to maintain a venting space, nail up 1x1's in the corners of the rafter bays. Then cut 1/4 plywood to fit in between the cavities. Nail/staple that to the 1x's. Now have a few inches of closed cell spray foam applied or install ridgid styrofoam snuggly up against that before adding fiberglass.

If you go the styrofoam route, be sure to use expanding foam between the seems and tape them with housewrap tape.

Do you have any can lights in the rafter bays?

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Sounds like good advice@overanalyze

My father in law put that reflective sheet up when he did this to his attic space. I don't think that's a bad idea.

But, you don't want the reflective sheet against the roof, it acts as a vapor barrier and being right up against the unconditioned space will form condensation. Unless you put another vapor barrier closer to the conditioned space.

You can go with your 2a option if you put another vapor barrier. If you really solid good insulation, do as overanalyze suggested and put at least 1.5" spray foam under the baffles to create a good r10 plus the vapor barrier, or use the rigid styrofoam, then put your batts in. If you don't want to spend the money on those options you can always put plastic sheeting under your batts.

I have never seen the reflective sheets used in attic insulation on the many new houses I've worked in, which tells me the professional insulation guys don't have much stock in it

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@overanalyze

That site is amazing.  I've been going down the rabbit hole of attic ventilation, vapor barriers and insulation all morning.  Great for educating myself; not so great for getting anything productive done at work. :D

 

From what I'm reading, I can see that I'm probably asking the wrong questions.  I'm not really up for spray foaming the entire underside of the roof decking, for cost and environmental reasons, so a true unvented roofing system is right out.  I think the foil is out, but also perhaps so are the pre-fab plastic baffles.  There are soffit vents at both ends of the roof that need to be tied into the ventilation.  From a number of articles, I am just not convinced that the prefab plastic baffles will do as effective of a job providing ventilation and keeping the outside air out of the fiberglass as a site-built baffle using foam boards or plywood.  There are no lights in the rafter space to deal with, so I've got going in my favor.  Is there a good reason to use both plywood and foam boards or would a baffle made out of foam boards (sealed of course with foam and tape) do the job just as well?

 

I'm also going to have to re-measure and possibly use furring 2x4s to build out the depth a bit after the foam baffles are in place to be able to fit the R-38.  I'm going to keep reading and perhaps take a trip to the depot to check out the options.  

 

Thank you again for the tips.

 

 

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I will have some time later to respond more in depth. Do NOT do 2 vapor vapor barriers! The Foam is one. After that you only want to use unfaced fiberglass.

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1 hour ago, overanalyze said:

I will have some time later to respond more in depth. Do NOT do 2 vapor vapor barriers! The Foam is one. After that you only want to use unfaced fiberglass.

I look forward to the detailed response, but in the meantime, here's what you're saying.

 

Using unfaced insulation:

roof diagram 2A.png

 

One minor wrinkle (actually it's an $800 wrinkle) is that I can get faced insulation for 30% less than unfaced.  With that in mind, what you're saying absolutely do NOT do is this:

roof diagram 2B.png

 

What if I flipped the faced insulation over and put it with the paper up like this?

roof diagram 2C.png

 

In theory, the two vapor barriers are back to back and touching.  Is there opportunity for moisture between the 2?  I suppose that I could also perforate / slash the paper if that would help.  I don't want to end up with rot / mold from trapped condensation, but the cost differential is significant at this point, so I'd like to figure out my options.

 

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Yes A is the method. Here is another very good read that was updated recently.

http://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling

I don't think there would be an issue with placing the facing against the styrofoam but I would do a bit more research. My biggest concern would be the paper face creating wrinkles between the two layers and creating air pockets that could lead to condensation.

Make note of the chart in the above article. It gives you the minimum R value of the air impermeable insulation (Dow type dense styrofoam) for each climate zone before switching to the fiberglass.

The reason you don't want 2 vapor barriers is it can trap moisture and cause big problems. You still need the ability for the little bit of vapor/moisture to dry to one side...in your case the interior.

Also drywall needs installed afterwards. I hope this helps.

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Yes A is the method. Here is another very good read that was updated recently.

http://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling

I don't think there would be an issue with placing the facing against the styrofoam but I would do a bit more research. My biggest concern would be the paper face creating wrinkles between the two layers and creating air pockets that could lead to condensation.

Make note of the chart in the above article. It gives you the minimum R value of the air impermeable insulation (Dow type dense styrofoam) for each climate zone before switching to the fiberglass.

The reason you don't want 2 vapor barriers is it can trap moisture and cause big problems. You still need the ability for the little bit of vapor/moisture to dry to one side...in your case the interior.

Also drywall needs installed afterwards. I hope this helps.

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This is good stuff, I'm glad you passed this on to us, I have just the basics down through the manufacturers of the spray foam that we use.

.

But the one thing I still don't see is if there are no real benefits to a vented roof, why do it?

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The venting of a roof is more important for attics than vaulted ceilings. Time and research has shown this. It isn't conventional thinking so not venting a roof assembly gets some opposition from old school inspectors and those who haven't done research. I love that building science site because they have years and years of different techniques in practice to watch the results.

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why wouldn't you do otion b with the faced paper towards the warm side (interior) of the house? that is what i have always read and heard. I am currently doing a vaulted ceiling with the pro vent baffles and ridge vent combo. I installed them in my garage and s long as you have enough openings they should supply ample air flow. 

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