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Old 11-06-2008, 08:43 AM   #1
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Guidance for Insulating Slanted Ceilings - Cathedral Ceilings


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? http://www.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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Last edited by mdlorenz; 11-06-2008 at 09:16 AM.
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Old 11-09-2008, 01:00 AM   #2
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Guidance for Insulating Slanted Ceilings - Cathedral Ceilings


Sorry, this is not an answer. I am also up against the same situation. Under Project Showcase, German House rebuild. I too have a cathedral ceiling. The locals here say to go with 160mm-200mm, 8"-10" of hard foam. I really think that is overkill, really expensive. I am going to use 100mm, 5" of hard foam and then a perferated radiant foil on the outside with air gap between the actual roof. I will super seal the radiant barrier on all the edges. I believe that there is a point of deminishing return on the thicker you go with the hard foam. I have no hard figures but my gut feeling. If anybody has a recomendation please chime in on this. Dorf Dude

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Old 11-09-2008, 08:16 AM   #3
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Guidance for Insulating Slanted Ceilings - Cathedral Ceilings


I can't give any advice on the actual method for insulation,
but I know very well the importance of air flow in the dead space of your home,
no matter how you insulate, heat will make it's way from your living space into the dead space in the structual/rafter areas and must have a clear path to escape through the exhaust vents located at or near the roof ridge.


ESP Energy, some reading material on insulation.
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Old 11-10-2008, 03:02 AM   #4
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Guidance for Insulating Slanted Ceilings - Cathedral Ceilings


make sure you leave airspace for ventilation between the decking,and insulation(foam air channels),then put as much as possible,I would prefer someone install closed cellular foam(not individual pieces),as it has a very high R-value (6.5 per"),without gaps for the air to get by
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Old 11-11-2008, 11:07 PM   #5
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Guidance for Insulating Slanted Ceilings - Cathedral Ceilings


A "Minimum" of 1 1/2" air space on top of the insulation will properly allow air flowage, if the eaves have 100% continuous Intake Ventilation and you have a continuous Ridge Ventilation system for the proper balanced exhaust.

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Old 11-25-2008, 01:48 PM   #6
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Guidance for Insulating Slanted Ceilings - Cathedral Ceilings


A respected contractor came by the other day & he wants to take the sheetrock down & put up rigid foam over the rafters, & then blow in dense pack cellulose, & seal up the space. I do understand that with some applications venting is important, but w/ 2x6 rafters it seems like I don't have enough room for proper venting w/ adequate insulation...

Feedback much appreciated... There are merits to both arguments...right? & technically neither is wrong, but just different schools of thought?

Also if the space is sealed up properly & no warm air seeps in through the foam board, & then through the dense pack cellulose... then I'm fine... no warm spaces will touch cold spaces...

I'm getting many different opinions & I just dont' know which to o with.
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Old 11-26-2008, 04:57 AM   #7
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Guidance for Insulating Slanted Ceilings - Cathedral Ceilings


NO GOOD ,WITHOUT AN AIRSPACE YOU WILL BE CHANGING ALL THE WOOD because it will rot out due to trapped condensation,the only way to do that is with closed cell foam,because the r-value for that would be r-38.75(actual),and cellulose doesn`t come close to that,plus the cellulose will hold the water from condensation-----you can also add furring strips 2" to the bottom of the rafters to allow for more conventional insulation if you prefer
this site is regarding the closed cell foam http://soythane.com/EZFoamulator
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Old 12-08-2008, 09:09 AM   #8
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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 (at least unvented in slanted ceiling space)... 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. Any opinions?

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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Old 12-08-2008, 11:12 AM   #9
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Guidance for Insulating Slanted Ceilings - Cathedral Ceilings


You can spray foam those angles ceiling cavities from the outside by removing and replcing that section of shingles and roof decking.

You can also leave a gap for airflow under the roof deck. You can insulate the backside of the walls from the cubbies, and do the flooring in the cubbies if you take up the boards where you have them. It has been my experience that these types of homes have some flooring in the cubbies, but not the entire area is covered.

If you have access to the top, then you can do the ceiling too.

If you need a new roof too, then even better to do it all at once.

Just be sure to use a 2# or better closed cell foam. 2# is actually best for this application because any higher density of foam for this purpose will diminish your return from the start. Higher density foams yield less but are more sturdy.

Just be sure that your roofer does spray foam as well as residential roofing, as I do.
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Old 12-08-2008, 11:23 AM   #10
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The roof was done 4 years ago, so that's not an option.

I was thinking of doing it from the inside. Rip down the sheetrock on the slanted ceilings, remove old insulation & have it sprayed full of foam, put sheetrock back up (possibly even rip a 2x4 in half & extend the rafters by 1 3/4" to get up near R-40).

There is no flooring in the kneewall space... just sheetrock from the ceiling below. It is a cold space presently. I'm thinking that I'll leave it as such & spray enough to air seal the kneewall floor (back side of the 1st floor (kitchen) ceiling) & then blow in Cellulose to get up near r-30 or r-40. Rather then spraying the underside of the roof decking, & in essence making the kneewall a warm space.

As for a gap for air flow... I'm not positive it's necessary if I spray foam the whole thing. The space above & below will be vented. W/ the foam no air or moisture should penetrate the slanted ceilings... correct? And if I foam the floor in the kneewall, then technically if done well, the result would be that no air / moisture would penetrate that area either, leaving that envelope sealed up tight excapt for the proper "attic " space, which has a ridge vent & 2 gable vents & I would assume gets plenty of ventilation.
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Old 12-08-2008, 02:27 PM   #11
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Guidance for Insulating Slanted Ceilings - Cathedral Ceilings


As Long As The R-value Allows Nothing Past,it Shouldn`t Be An Issue According To Some,haven`t Done It Personally,but Aaron Is One Of The Best Guys To Talk To About That
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Old 12-09-2008, 12:10 AM   #12
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WHen you spray foam to the underside of the roofdeck, you create a heat sink with the insulation. This will lead to premature deterioration of the shingles in those areas. This is why we still want the air space.

Also, it would not be a large job to remove a section of that roof and decking, spray the small areas, then close it back up like new. There might be a slight shade variation, but you wouldnt have to rip up the living space. This is my opinion, and the way I would do it if it were my house.

You will get R35 at five inches of closed cell foam, and this includes the venting airspace.

At R3.5 per inch of unsettled cellulose, you cannot get that much and it can rot if you do get a roof leak.

I see no benefit to cellulose or fiberglass insulation with the initial payback period of foam being so short.
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Old 12-09-2008, 08:30 AM   #13
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Thanks Aaron. With me doing the demo, & sheetrock work, & prep work, takign the roof off & spraying in from the outside would be considerably more expensive, as I couldn't do that. I do hear you about the heat sink aspect of spraying the underside of the decking.

Considering all this information, I think that I may try to extend the rafters (tack on half a 2x4) then create an air channel w/ furring strips (should I go more then 1.5"?) , & a 1" piece of foam board, then have them fill the rest of the cavity w/ foam. It's a bit more work, but seems like it will be better for the roof.
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Old 12-10-2008, 09:14 AM   #14
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Guidance for Insulating Slanted Ceilings - Cathedral Ceilings


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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Old 12-10-2008, 11:01 AM   #15
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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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