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Insulation help in 2x6 rafter slanted ceiling


mdlorenz's Avatar
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11-06-08, 06:18 AM   #1  
Insulation help in 2x6 rafter slanted ceiling

Trying to decide how to insulate the slanted ceilings in my bedroom of my 1.5 story cape. Take a look at these diagrams. The areas I'm looking for guidance is in the 2x6 rafter areas. Have gotten quotes, but from everything I read seems like it wouldn't be too difficult to do myself...



Quotes differ from $3000 to pack them with dense pack cellulose (no ventilation), to $5000 to pack with foam board, (no ventilation), to leaving ventilation, & packing as much foamboard in there as possible. All quotes are from professionals, but all are so different, I'm not sure what to go with.

Look at this diagram below.


If done this way I would get around R-30-35, & still maintain air movement. Would I be better getting an additional 1.5" of insulation in there & sealing it off from any air movement? Also wondering if this is something that wouldn't be too difficult to do myself. It's only about a 25' x 4' area...

Also anyone have any experience w/ prodex? Insulation ~ Insulation Products ~ Reflective insulation ~ Foil insulation ~ Building Insulation ~ Radiant barrier ~ Insulation4less.com I found them on the internet, but don't know anything about it.

They have a couple diagrams which look a lot like what I'm trying to do. I think what I'm thinking of doing would be a combination of the 2 with foamboard in between the rafters, & also on top under the sheetrock.


Any guidance or help is most appreciated....

 
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11-06-08, 04:16 PM   #2  
Hi mdlorenz, I'll give this a try. Opinions differ and I often have to consider no ventilation, but whenever possible I always go with proper venting. You didn't say what climate this is in, but judging from the r-values you are up north. I'd go with your 1.5" air space as shown, and yes with moderate abilities you can do this. One thing not shown is the fitting between the rigid foam and the rafters. You want this air tight. The best way to do that is to cut your fillers small enough so you can fill the gap with straw foam. Once there it's not going anywhere, plus it makes cutting the fillers much easier. The most important detail is making sure no inside air can leak through anywhere. The insulation and the vapor (moisture) barrier must stay in contact and be continuous, with the vapor barrier on the warm side of the insulation. Now if you tell me you are Arizona, I will have to change your diagram. You show strapping over the last one inch layer of foam, I'm assuming it has a foil surface for your radiant barrier. The foil Bubble Foil products you asked about are primarily radiant barriers and somewhat controversial. Again, opinions vary. I have found limited applications for them, but I am in cold country. I see no real advantage for you with them, but properly applied on the inside, no real harm. If you want to use them, post back and we can swap some e-mails, as I don't want to encourage a lot of people to play with them. They are a vapor barrier and often get placed in the wrong locations. Also their claims are not often realistic for all applications. I'm not sure if I answered all of your questions. If not I will try again. GL Bud

 
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11-07-08, 05:39 AM   #3  
In Vermont

Yeah I'm in vermont. Gets friggin cold up here.

Other then the fact that we really need more insulation in these spaces, there was evidence of some moisture in there last winter, so I'd like to stop that. I'm pretty certain that for the last 20 years (house built in 1980) the roof was somewhat unvented. Before the ridge vent was installed (3 years ago w/ new roof) and still there, the attic has 2 gable vents, & the crawlspaces have gable & soffit vents., it's just that it seemed they didn't plan on any venting in between the 2 areas. (in those slanted ceiling areas).

One guy up here a reputable contractor w/ great references said that he'd like to let any moisture dry up in there & then seal it up tight, & get as much r-value in there as possible... I've just heard so many differing opinions I don't know which one is the right one...

Going back to your suggestion... by straw foam, do you mean that spray foam stuff... (ie. great stuff, handi foam...etc) that comes in a can? I was thinking that I'd fit the foamboard as close as possible to the space in between the rafters, wedge it in there on top of the furring strips, blow foam in the cracks & then tape over that... Repeat for the 2nd 2" board. Should these 2" peices of insulation be without the foil facing? & then the final piece of insulation on the outside of the rafters have the foil face, facing the warm room, taped at the seams to create one continuous vapor barrier? Any reason to use a plastic vapor barrier?

Thanks for the help.

 
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11-09-08, 01:36 PM   #4  
Be sure to seal all electrical wires, vent pipes, or spaces around a chimney (metal flashing around the chimney). yes the great stuff. Sometimes it is much easier to cut them small and fill with foam. getting an air tight fit all the way around is time consuming. only the last layer of foam needs to have the foil. needs that air gap. I've seen too many roof leaks and when sealed up tight, the moisture is going to be trapped in there and you may not know it until you have some real damage. plastic not needed if you tape up that last layer of foam. Finding all of the sneaky places where air can leak in is a challenge. When I run a blower door test, builders and owners are always amazed, and a 1980 house, they weren't even trying to air seal. Air seal everything in the basement as well, as air that leaks in ultimately leaks out. If we assume your house is tight for the year it was built, it would have an air change rate (ACH) of 0.5. That's changing all of the air in your house every two hours. Sounds drafty, but 0.35 is the limit. Some of that air is going to make it into your attic spaces and thus the ventilation. Good luck Bud

 
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11-09-08, 05:41 PM   #5  
Your place is very similar to a number that I've been working on recently, so the problems will be very comparable.

First, whenever you have a knee-wall, you have to check how the joists run under the area you label with "fiberglass batts".
Almost every place I've see with this construction has open joist bays, allowing the cold air to move through and turn the floor of the 2nd floor bedroom into a big cold space. This is absolutely critical. If you look a thermogram during the winter, this is what you see:


Here's what it looks like if you look under the floor from the back side of the knee-wall:


You can fix this by placing board foam into each of the bays then using the canned foam to seal the boundaries. This is time consuming and tedious and if you are planning on getting any foam done professionally, then you can do this much more simply. Fold up fiberglass batts and shove them into each of these bays so that they form a block. Then have them spray foam that so that it seals each bay air-tight.

If you have flooring behind the knee wall, don't assume that it's sealed underneath of it. Tear up some of it and look to see if you've got a situation like that illustrated. If you don't, and you lay fiberglass on the floor like in your picture, that fiberglass is worthless because the air will travel right through the joist bays. I've seen this 100 times, and without fail, it works out that way.

Next, and this is something I want to amplify to everybody out there, the knee walls should be considered like attic space when thinking about insulation. Since the knee wall is below the very hot roof, it will heat up tremendously and require a very high R-value, just like an attic. R-40 plus. So where you have 2" foam board - that's totally inadequate and besides, doesn't work well. I just worked with a builder who filled the knee wall with fiberglass batts, then adhered foam board to the back of the studs, taking care to do it as well as possible. It still didn't to a great job to air seal and insulate the walls.

Your best best is to have spray foam applied to the back of the knee wall. Fill up the entire cavity, up and over the wall studs. Again - you want the full R-value there. This will also air seal all the outlets and penetrations that will be a big source of moisture movement into the space. It's all good.

While you're in there, spray foam the floor behind the knee-wall (representing the ceiling of the room below). Or, fill it with cellulose or fiberglass. Again - R-40 if you can.

I like the foam board with a channel above it in your illustration if you're going for a vented roof. . The extra inch of board foam to remove thermal bridging of the 2x6's is a great idea. And cellulose in the attic space, maybe 15"-20", will give you good R-value. But if you're going with vented rafters, you're going to have a hard time keeping that channel space clear.

If spray foam isn't in your budget, remind yourself of the long term expenses for heating as well as the comfort issues. Low-R, Loose fill or batts will never do the job that foam will.

There's a recent trend to "flash and batt", meaning spraying a very thin coat of foam, then using batts. This will help with air infiltration but is not a vapor barrier. If you read the foam literature carefully, you'll find that you need 2-3" of foam to be considered a true vapor barrier - i.e. a perm rating of less than 1.

Disclaimer - I have no interest in selling foam. I am only promoting techniques that I know work based on case studies I've been involved in.

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12-02-08, 06:20 AM   #6  
Thanks TedInoue. This is great knowledge. I had a reputable contractor out the other day & he noticed the floors being cold in the 2nd floor. I called a spray foam contractor to get an estimate.. I think I'm going to try to do a little spray foam in the knee wall area floor & behind the wall (& do the folded Batt w/ foam under it to seal off those joists) & then foam board in the rafters, sealing up everything tight.

It's real hard to figure out what actually is the best; 1, for heat loss, & energy conservation, while 2. Doing what is going to be best for the longevity & health of the house/roof...etc (no moisture/rot/mold...)

I get such differing opinions about venting, not venting... Confusing for a DIY'er to figure out what actually is the best way to proceed. I think I'm on the right track, & the more I think about it, the foam board on the rafters won't actually be that tough. & I can leave the spray foam in the kneewalls to an expert.

I'd love to hear other opinions. Always open to hearing other ways to accomplish what I'm looking to do.

 
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12-03-08, 06:43 AM   #7  
Take pictures as you go, it will be interesting to see what you choose. As far as differing opinions, IT IS DIFFICULT. No offense intended to the more experienced members of the board, but the old way of doing things is hard to change, and often for good reason. But the battle between tried and true vs new and wonderful (where no one knows what it will look like in 20 years), will go on. I am gradually pulling together the wisdom of experience and the advantages of new technology to form my own opinions. Of course, I'm old enough so I probably won't be around in 20 years to have to answer for what I decide.

I've just started my project as well, cape, slopping ceilings, knee walls, as well as 3 1/2" of rigid on the outside of the house. I'll let you know in 20 years
Bud

 
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12-03-08, 06:59 AM   #8  
I have a call into a spray foam place. I think what I might try doing if it's cost effective is try to lay down a base of spray foam on the kneewall space floor, & the back of the kneewall itself, & then put a foot or 2 of blown in cellulose (keeping those proper vents fee & clear) & then go with my original plan for the slanted ceilings...

Hopefully I can do the prep before the holiday (spray foam), & do all the labor myself for the rigid foam in the slanted ceilings in between xmas & New years.

 
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12-05-08, 12:47 PM   #9  
Just keeping the updates going...

Had a spray foam guy come over last night. Not as expensive as I had thought originally... & it seems that this would be a worthwhile product if I were to go the route of an unvented roof... I'm giving serious consideration to it.

I'm thinking that since the spray foam acts as such a good air & vapor barrier I could spray foam the slanted ceilings, & leave the space above & the knee-wall space vented. I'm not sure there would be a problem if I were to disrupt the venting just in the slanted ceilings, especially if it were with spray foam.

Then I'm thinking I'll trash the batts that are in the kneewall floor now, & spray that, & the back of the kneewall too. Hopefully eliminating any infiltration from the kitchen & rest of the 1st floor... which is where I think I'm gettign most of my moisture/warm air infiltration.

Then maybe do the same thing to the other side of the house which is all cathedral ceilings.

I mean if there's no air/moisture infiltration, then I don't need to worry about venting... & I can just seal that baby up tight... right?

 
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12-09-08, 06:31 PM   #10  
In my opinion, you need to baffle the rafter area to keep continuous venting to the ridge. Then spray Icynene insulation 4" thick. Icynene has a R value equivalent of R 3.6 per inch but it far outperforms fiberglass of much higher R value.

Good luck.

 
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12-10-08, 06:14 AM   #11  
Update #2

Had another insulation contractor come by last night. Did an energy audit.. blower door..etc. Many recommendations. As for the slanted ceiling/kneewall insulation issues, they recommended that I put rigid foam over the rafters in the kneewall, & continuing that up through the slanted ceilings up to the attic space. Then they'd blow in dense pack cellulose into the rafters all the way down to the soffit. Changing the building envelope so that the insulation is on the underside of the roof, making the kneewall warm space, rather then cold space...

Why can't there just be a RIGHT way to do this. I swear, I'm torn with what to do here... many differing opinions.

 
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12-10-08, 08:02 AM   #12  
I had a thought...

What if I stapled up those baffles the whole run of the roof, from the soffit up to the open attic space, & then foamed it all... That'd get me the most r-value & still maintain the air space....

 
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