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How can I best retrofit and insulate an existing cathedral ceiling?

Hi Gentlemen/women:

Thanks for this excellent blog. I have a 22 year old home with a "great room" and cathedral ceiling. From inside it is v-groove pine, probably no taped sheetrock beneath but I wasn't around when it was built, and 12 inch fiberglass with kraft paper. (It has the built up/down rafters you have mentioned). There is a vented area above fiberglass, probably about an inch and a half (soffit and roof vents; there are a couple of dormers). One can see ice and icecles on the roof, and the place takes alot of wood (and propane) to heat. Oh, I'm in northern Vermont, zone 6.

The original roof will need replaced in next few years so a good time to consider improvements. If done from above, I know you (Martin) prefer poly foam. This can easily be blown into the bays once decking is removed. Can that be blown right over kraft paper once fiberglass removed? (if only to keep uncured foam from squeezing through board seems and knots). Can we do a few inches of foam, due to cost, and then reuse the fiberglass batts above? Can those batts be compressed a bit to fill up the remaining cavity and thus reach the underside of the sheathing?

If so, what then? The venting would of course then be closed off, so what about above the decking? You seem more comfortable with a venting area, although it would be simpler if none were needed at that point). But can we go with no further insulation? I can see the benefit of more rigid foam above, but am surprised at how much (and how much expense) is still needed above, not to metion the challenging furring issues above xps to move to wood for roof attachment.

Thanks

Asked by Howard Gentler
Posted Sat, 01/04/2014 - 08:52

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24 Answers

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Howard,
There are lots of ways to fix this roof. You don't have to use spray polyurethane foam, although that is one option.

One of your main problems is that you don't have an air barrier at your ceiling. All you have is tongue-and-groove boards.

There are two main ways to proceed.

1. One approach is to use spray polyurethane foam, installed from above, to create an air barrier at your ceiling. To do this, you would need to remove the fiberglass batts, and to use either the old kraft facing or new cardboard to keep the spray foam from oozing between the cracks. You can either install a small amount of foam -- perhaps 2 inches of closed-cell foam -- as an air barrier, followed by another type of insulation, or you can install a thick layer of spray foam to provide all of your roof's R-value.

If you go this route, you will need to carefully seal the air inlets and outlets of your ventilation channels.

2. The other approach is to gain access to the area near the soffit vents and ridge vents (probably by removing sections of roof sheathing from above) so that you can seal these areas. Then you can install all of the necessary R-value above the roof sheathing by installing several layers of rigid foam, followed by another layer of roof sheathing and new roofing. This system would leave the existing fiberglass batts in place, but wouldn't depend on the batts to provide any R-value.

Answered by Martin Holladay, GBA Advisor
Posted Sun, 01/05/2014 - 08:34

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Thanks, Martin. I really like option # 1 which I can wrap my mind around. I think I can seal the vents well. You must feel confident of this unvented version despite Dr Joe's (BSC) discomfort. With this option, using say 3" of sprayed foam and putting back the fiberglass above would still be decent R value, -perhaps 46, using the 28 claimed for the fiberglass and 18 for spray foam at 6 per inch. Is it important whether the insulation reaches the underside of the roof decking re-installed, or is that irrelevant?

I see that option # 2 is now much favored, but I can't get around how expensive and difficult this would be. Up here in northern Vermont (zone 6), it seems I would need quite a thickness of rigid foam above. One then needs a whole additional decking with the awarkness of screwing the upper decking down thru all those inches of foam. You'd need 10" screws it would seem.

Would there be any sense in condensing the 12" fiberglass batt some (still above some foam), and adding some cellulose above that for additional R value?

Thanks again,

Howard

Answered by Howard Gentler
Posted Sun, 01/05/2014 - 12:57

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Howard,
Q. "Is it important whether the insulation reaches the underside of the roof decking re-installed, or is that irrelevant?"

A. The best method -- the code-approved method -- would use only spray foam.Then you will have an unvented roof assembly. If you add fiberglass or cellulose above the rigid foam, you (technically) need a ventilation channel above the insulation. However, if the spray foam operator does a really good job of air sealing at the bottom of the rafter bays, you'll probably be safe without the ventilation channels. You should probably talk to your building inspector before proceeding if your jurisdiction requires a building permit.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 01/06/2014 - 11:29
Edited Mon, 01/06/2014 - 11:30.

4.
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Martin:

There are no building inspectors up here in my rural area (and no building permits needed for anything), -a mixed bag perhaps, but also why your advice is so valuable. The usual advice is from someone selling something, or if not, then with an entrenched way of doing something which may or may not be soundly based.

But how about this? If a few inches of foam were used, then the 12" fiberglass batts could go above. If they are compressed a small amount (there is some 14" of depth here), the already existing ventilation system could be retained, which, while possibley not perfect, should be worth something. The challenge would then be something above the fiberglass batts keeping them from expanding upward and filling the vent space. Any ideas on this? Would something like chicken wire work or is there something better (or is expansion unlikely once compressed)?

Or, if we did a metal roof, we could allow the batts to fill to the undersided of the decking, block up the vents (or divert them for new venting), then use strapping (2x4's I guess) vertically over the rafters and strapping horizontally for the screwing surface, allowing an easy ventilation space above the decking. Is this a plausible method?

Thanks

Answered by Howard Gentler
Posted Mon, 01/06/2014 - 12:50

5.
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If you are re-roofing you can block the vents and dense-pack cellulose over the batts drilling from the exterior side, as long as you install R25 as rigid foam above the roof deck, and under your new roofing roofing.

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_sec006.htm

That would take 6" of EPS (preferred material in your climate), preferably installed in two layers of 3" with the seams staggered and sealed. The purlins/furring would have to be through-screwed to the rafters with pancake head timber screws 24" o.c., but that's still a do-able thickness using those methods.

You can't just fill up the rafter bays with fiber and vent the roof on the exterior in your climate- the plywood or OSB itself is too vapor-retardent, but that approach could plausibly work in a zone 5 climate (with caveats.) The reason under-deck ventilation works is that the deck itself isn't between the interior moisture drive and the vent cavity.

But putting up the 6" of EPS keeps the deck temperature high enough to keep it from adsorbing too much moisture over the winter. Dense-packing the bays will cellulose (nearly) eliminates convective moisture transfer between the kraft facer of the fiberglass and the roof deck, which means the ~0.4 perm permeance of the kraft facer becomes more meaningful, even if it isn't an air-barrier. And the cellulose will "share" the moisture load with the roof deck, protecting it somewhat.

With 11.5" of dense fiber in a 2x12 rafter bay the center-bay R is about R40, and with the R25 of EPS above the roof deck thermally breaking the rafters you would have an appropriately high-R roof for the climate.

Answered by Dana Dorsett
Posted Mon, 01/06/2014 - 19:22

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BTW: At VT type propane and electricity prices you can probably get a high-efficiency ductless mini-split to pay for itself in under three years, if you have a large open-floor plan zone to heat/cool with it. Great Rooms are often high-ceiling large rooms with open archways to other spaces, and it could be a reasonable cost-and-carbon reduction strategy going forward, after you've fixed the deficiencies of the building envelope. (I've seen a ski condo at Smuggler's Notch heated and cooled with ductless heat pump technology, if you're worried maybe it wouldn't work in your neighborhood.)

Answered by Dana Dorsett
Posted Mon, 01/06/2014 - 19:28

7.
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Dana: Thanks for these details. A few questions occur.

--drilling holes and dense packing cellulose eliminates the need to remove decking, no small advantage. Since the bays are already within about 2" of filled with batts, does the dense packing compress the batts, or is this method basically filling the 2" ?

--I'd thought xps was preferable to eps, -that they were somewhat interchangeable, but with the higher R value of xps allowing for less thickness, desireable in this case. What is the reason eps is preferred?

--so the above deck venting is not a good idea, I now see, but what do you think of my other thought (although more work and expense than your good idea) of the couple inches of foam, the slightly compressed fiberglass batts above, and accomodation for a below deck vent space (we could use what is already in place)? Just considering options. The added depth about the roof involves some visual issues that might be an issue (for my son, -his place). The ridge sides and the soffit facias are both very wide now and additional trip boards would be needed (which might even look good!),

The splits are new to me and I will look into those further.

Howard

Answered by Howard Gentler
Posted Tue, 01/07/2014 - 10:09

8.
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Dense packing will compress the batts to a higher density, increasing their low-density R3.3-ish per inch to something like R3.6-3.8/inch, and increasing the air retardency of the fiberglass (though the cellulose layer will still be significantly more air-retardent. The cellulose will also be something like R3.5-3.7/inch when blown at 3.5lbs density, bringing the total fiber layer just north of R40, but a highly air-retardent R40.

XPS is higher R/inch than EPS, but only initially, and would have to be replaced at every re-roofing to maintain that R. The reason for it's higher-R at the same thickness and density is due to it's HFC blowing agents, which it loses most over over the first 50 years, at which point it's R value is barely more than EPS. The blowing agents themselves are also very powerful greenhouse gases- the primary component is HFC134a, which has a global warming potential of about 1400x CO2. The R-value of XPS is an "presumed lifecyle average", but that presumed lifecycle is a couple of decades, whereas a quality steel roof should last 50-100 years. By contrast, EPS is blown with pentane (~7x CO2 GWP), almost all of which has dissipated by the time it hits the distributors yard, and it's rated R value is it's fully-depleted R, so it's performance will be the same at year 50 as it was in year 1.

The mini-splits best suited to your climate are the Mitsubishi H2i "Hyper Heating" series (MSZ FExxNA models) , and the Fujitsu Halcyon XLTH (eXtra Low Temperature Heating") series (AOU-xxRLS2-H models.) Both are fairly high efficiency in both heating and cooling mode, and both have a rated output at -25C/-13F or lower. The H2i's turn themselves off at about -18F to self-protect and automatically re-start when it warms up a degree or so. The XLTH series keeps on going forever, and is still putting out some unspecified but real amount of heat at -32C/-26F, according to a blogger in Quebec. It's not clear if the change in mechanical tolerances at those very low temps cause excess wear, but it doesn't quit.

The fact that you have a propane fired heating system and a wood stove to back it up, it doesn't really matter if the mini-split isn't sized for the 99.99th percentile temperature bin- if it's sized for the average daily winter load it's still covering something like 75- 80% of the heat load of the zone at -10F (or whatever your 99% outside design temp is), and over 90% of the total heat for the season, which has a substantial impact on propane use.

The average heating coefficient of performance you'll see out of this class mini-split will be about 3.0, which means it delivers 3x the amount of energy as heat as it consumes in electricity. To normalize mini-split power use to gallons of propane burned in a 95% efficiency condensing boiler or furnace you can estimate using the following:

Source fuel BTU per gallon of propane is about 92,000. The amount of heat delivered to the house in a 95% burner is about 0.95 x 92,000 = 87,400 BTU.

A kwh has the heat equivalent of 3412 BTU. At an average COP of 3 the heat delivered is about 3 x 3412= 10,236 BTU

So the power used by the mini-split to deliver a propane-gallon's worth of heat is 87,400/10,236 = 8.54 kwh.

According to EIA data residential retail electricity in VT runs about 18cents/kwh, ( http://www.eia.gov/electricity/monthly/epm_table_grapher.cfm?t=epmt_5_6_a ).so heating with the mini-split is about the same cost as heating with a condensing propane burner with $0.18 x 8.54kwh = $1.54/gallon propane.

Residential propane pricing in VT is averaging about $3.76/gallon as of last week:

http://www.eia.gov/dnav/pet/pet_pri_wfr_dcus_svt_w.htm

Assuming your mean winter temp is something like 15F (it's probably warmer- Burlington's mean temp over the three winter months is about 20F see: http://weatherspark.com/#!dashboard;a=USA/VT/Burlington ), a 1-ton mini-split like the -FE12NA or -12RLS2-H is delivering over 15,000 BTU/hr at that temp, and the -FE18NA or -15RLS2-H models put out more than 20,000BTU/hr, and may be close to your whole house heat load at that temp if it's built fairly tightly.

http://smartgreenbuild.com/pdf/Fujitsu-RLS2H.pdf

http://www.mehvac.com/UploadedFiles/Resource/Slim-M-Series.pdf

www.rmi.org/cms/Download.aspx?id=10410&file=2013-05_HeatPumps.pdf&title=...

Note, VT will give you a $750 kick-back on the first unit, and another $500 for a second of you are heating with oil or propane:

http://www.efficiencyvermont.com/For-My-Home/ways-to-save-and-rebates/Au...

http://www.efficiencyvermont.com/docs/for_my_home/general_info/CC_HeatPu...

The bigger units run between $4-4.5K, installed, all-in costs (pre-rebate). If it's displacing 800 gallons of propane a year at a net savings of about ($3.76- $1.54=) $2.22 +/gallon , it's paid off in under three heating seasons. The one-tons run about $500 less. A mostly DIY installation will run about $2500 for the big units, allowing for $300 in tech-time for the final system charging and test, using all the tools you never wanted to own, and experience you don't have (unless you are a refrigeration tech who has installed and commissioned dozens of mini-splits.) A DIY installation for a 3/4-ton or 1-ton sometimes pays for itself in the first heating season.

Key to getting the sizing right is to figure out the peak and average heat loads of the zone that can realistically be heated with a mini-split using Manual-J or I=B=R methods. Size it for the average load to get the best value, peak load for highest efficiency, or split the difference, use the output at +5F.

Mitsubishi Hyper Heating output at +5F:

BTU out___model number

10900 MSZ/MUZ- FE09NA

12500 MSZ/MUZ- FE12NA

21600 MSZ/MUZ- FE18NA

Fujitsu Halcyon XLTH output at +5F"

BTU out___model number

15000 AOU/ASU 9RLS2-H

17000 AOU/ASU 12RLS2-H

18000 AOU/ASU 15RLS2-H

Answered by Dana Dorsett
Posted Tue, 01/07/2014 - 12:59
Edited Tue, 01/07/2014 - 13:18.

9.
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Thanks Dana for all this technical information about the splits; definitely food for thought.

I like your idea for improving this roof, -it sounds effective, and although it will be expensive it sounds less so than most other options, especially any using spray foam. With your model there is no decking removal, and if metal roofing is used there would not be the need to purchase an entire other decking, and this is a large roof area. Most of the lumber needed over the rigid foam for roof attachment (ie 2x4's) would still be needed for new decking if that route were pursued.

Any other ideas appreciated.

Howard

Answered by Howard Gentler
Posted Tue, 01/07/2014 - 15:23

10.
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A foam-over job on the roof isn't cheap, but re-roofing time is an opportunity moment- once you have the new roof up it'll be 50 years before you do it again, unless the roof deck rots out on you and needs repair. At the present and future price of propane the dense-packing and foam has a reasonably cost effective internal rate of return (IRR) just on fuel savings, not even counting the insurance it offers against future roof deck replacement.

At mini-split heating costs the IRR on the insulation isn't nearly as good, but you have to count on replacing the mini-split every 20-25 years too.

FWIW: The dense-pack + exterior rigid foam approach is exactly what will happen on my own central MA home when the current shingles are toast (I'm figuring it will be less than 5 years.) Between inhibiting the mice currently nesting in the crappy R38 fiberglass of the cathedral ceilings in the addition, and the ice-damming on the 1920s 2x6 raftered cathedral ceiling portions it'll be worth it, even if natural gas stays at it's current price forever. The dense pack & foam-over may only take ~3000 BTU/hr off my design condition heat load, but not having to pull snow off sections of the roof after every nor'easter to avoid ice dam back-ups is definitely worth something going forward. And if energy prices rise across the board, the 3000 BTU/hr load reduction will also worth something in raw financial terms, even if at current gas prices (near the all-time record low, on an inflation-adjusted basis) the pay-out on fuel savings alone would be pretty long.

Answered by Dana Dorsett
Posted Tue, 01/07/2014 - 15:54

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Yeah, it's kind of a shame that fiberglass is Rolls Royce mice nesting material. Even great cats can't get into that cathedral ceiling. Did you plan on leaving the R-38 batts alone beneath the deck? I don't know the R value of mice, but the fiberglass still has some!

I know this will not be cheap; -reroofing alone would not be, but this method is cheaper than some others, and as DIY'ers we save much that way. I don't have a world of expertise, and accept there are some things best hired out, but seek out the correct methods and have found over the years that the concientiousness applied to one's own place and future headaches (or better yet, lack thereof) can make up for alot.

If we went with metal roofing above rigid foam, I guess just horizontal 2x4 strapping would be needed, doubling in purpose for securing the foam layers and attaching the roofing. Does that seem right?

Thanks again.

Howard

Answered by Howard Gentler
Posted Tue, 01/07/2014 - 19:03

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The plan at my place is to leave the batts in place and dense pack over them drilling from the exterior, sealing the drill holes with closed cell polyurethane to be planed flush with the roof decking. I have local access to multiple vendors of reclaimed roofing foam from commercial demolition/re-roofing at ~25-30% the cost of virgin stock, which takes some of the sting out of it. Some will ship decent sized lots within New England (insulationdepot.com even ships nationally for a price, and a warehouse in Framingham MA.) When you're REALLY close to pulling the trigger on the project, if you can't find a local source for 3" EPS, give them a buzz- they almost always have 3" EPS in stock (sometimes even unused surplus EPS.) If you have your own truck and are willing to make the drive, most of these vendors are happy to sell even small lots at a huge discount, f.o.b. their storage lot, but make sure they'll vouch that they have the goods you want in the quantities that you want, in good-to-excellent shape before making the drive. R/T from northern VT is an all-day affair, and it needs to be worth your while. (Broken down beat up EPS isn't worth the trip at any price- you want the primo-stuff.)

Yes, using 2x purlins 24" o.c. through-screwed to the rafters for mounting the standing seam roofing gives you ample hold-down on the foam. Tape the seams on both layers with an EPS-specfic tape if you can find it. Try 475 Building Supply as a source for that sort of stuff- the box store folks and most contractor suppliers have never heard of it. see http://www.foursevenfive.com/ Second best would be to paint the seams with an acrylic latex primer and seal it with housewrap tape.

Answered by Dana Dorsett
Posted Tue, 01/07/2014 - 19:28

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Appreciate the link to the recycled foam board. That is a reasonable trip, I have a truck, and could borrow a trailer if needed.

As for sealing the foam board seems on the roof, would spray foam work? I have a foam gun which works well and can provide a small controlled bead. The foam should have at least the perm resistance as the board. Seems faster. Would it be better to intentionally leave say 1/4" space at seems in order to really get foam in, rather than on the seem? Little but not minor details.

BTW, I was wondering why you have to fill the drill holes on your roof decking. Unless there are many more and larger than I think, they would be permanently sealed by the foam board above. Sounds like a bit of work so I wonder why it's inmportant. I'm pretty sure it's not to foil mice travel!

Howard

Answered by Howard Gentler
Posted Wed, 01/08/2014 - 08:11

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Can foam is better than nothing for sealing the seams, but it's not the best solution. Rigid foam insulation does expand and contract with seasonal temperature changes- it's hard to keep them sealed forever with any goop-solution. Tapes will continue to work as air barriers even if larger gaps open up between adjacent sheets of foam over time, and as long as that gap is thermally broken by the other layer of foam the performance hit is pretty small.

I fill the holes on the roof decking since that's what I've defined as the primary air barrier, and that's where the roofing felt will go too. In my case I'll be mounting a vented OSB nailbase on furring. Standing seam would be complicated on this hipped roof with multiple dormers (this 1920s bungalow is cute, but it's not how I'd design an affordable high-performance house. :-) )

Answered by Dana Dorsett
Posted Wed, 01/08/2014 - 18:44

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Is there any reroofing option in my scenario that can avoid the foam over approach? If I understood one of Martin's comments correctly, using all spray foam in the bays would permit this. But, foam is outrageously expensive and I'm thinking that it would take 8" (which there is room for) to approach R-50, and I haven't researched cost. Martin mentioned that 2-3 inches of foam, installed carefully for sealing, with other insulation above would probably be safe, but sounded uncertain. And I'm not sure if he was speaking of something specific above the foam, like packed cellulose, rather than fiberglass batts, but I have some interst in re-using the 12" batts as they are already there.

Trying to weigh all otions and their accompanying pros and cons, including cost of course.

Answered by Howard Gentler
Posted Fri, 01/10/2014 - 11:48

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Howard,
The options are outlined in this article: How to Build an Insulated Cathedral Ceiling.

You seem to be going around in circles, to some extent. I know that it is discouraging to have a cathedral ceiling with bad details. Good solutions aren't cheap.

I suggest that you re-read the article and call up a few insulation contractors and roofers. Choose a contractor whose advice seems to match what you know about the building science issues involved. Get some bids; if necessary, get a bank loan.

Good luck.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 01/10/2014 - 12:04

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But you don't really need R50 in foam, only R24. If you have R24 foam over the roof deck you can fill up the rafter bays with fiber. At 15% framing fraction a 2x12 raftered roof full of 3.5 lbs density cellulose (with 6" or so of compressed fiberglass batt comes in at about R35 after factoring in the thermal bridging of the rafters (allowing R2 for the heavy wood decking).

With R24 foam over that R35 packed fiber joist-bay you're running about R60 as the whole-assembly all-in R value of the roof which is WAY better than a code-min R49 batts-between joists attic.

Answered by Dana Dorsett
Posted Fri, 01/10/2014 - 19:33

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I may be going in circles, -it wouldn't be the first time. I re-read your referenced article, Martin, along with all the included links, and have been reminded of some principles I had forgotten in asking questions. We are DIY'ers to a great extent, though sometimes mix and match with hired labor and pros. Nothing against contractors, but their use multiplies the cost so we do whatever we can. Even if/when using a contractor I'd want to be as informed as possible since not all keep up with the science and mechanics adequately, as you well know from readers issues.

The job is still a little ways off, so I am thinking ahead. A big issue is whether or not we remove deck sheathing. If we decide to, or find that we NEED to due to moisture damage, that dictates how we proceed. With sheathing removed I think a couple/few inches of foam sprayed from above makes sense to give us the air barrier we don't now have, some good R value, and permits re-use of the 12" batts above (unless I misunderstand that). This would fill the rafter bays and we would seal off current vents. I think Martin said this was not technically code, but should work. We would then reuse decking, and although not an official vent space, the purlins (run horizontally) used above decking for metal roof provide a cold air layer. Or, going an extra mile might be vertical purlins and horizontal nailers with actual air in/outlest, but I'm not thinking this is necessary.

If we don't remove decking, a version of what Dana suggests seems best, but I do have the following need for clarity (if I've missed it elsewhere, sorry). Can the current bays be used as is, with the closing of the vents? It is already R-38 in name, so adds much R value. The air barrier would then be in mid-system, so to speak, in the form of rigid foam, and I'm not sure if that is a problem. Closing off the vents should seal the whole package, maybe, and any air reaching the underside of the decking could not reach dew point. If this is a no-no, I guess that means the dense packing from above is a critical piece.

Thanks gents for indulging my circles!

Howard

Answered by Howard Gentler
Posted Sat, 01/11/2014 - 11:00

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You need to eliminate the air space above the glass.

Why? Buoyancy IMO. Read the open cell blog.

Adding the Dana cellulose is worth doing when you reroof. Low cost and most likely very worth doing.

Answered by aj builder, Upstate NY Zone 6a
Posted Sat, 01/11/2014 - 14:06

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Howard,
If you decide to go the "upside-down flash-and-batt" route -- with spray foam on the interior side of the rafter bays, and air-permeable insulation above the spray foam -- I would feel a lot better if you have strapping above the roof sheathing (for metal roofing), so that there can be some drying to the exterior. I think that plan can work (although, as you know, it is a little unconventional and may not meet code requirements).

Answered by Martin Holladay, GBA Advisor
Posted Sat, 01/11/2014 - 16:35

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The space above the glass should be eliminated since the 2-3 inches of foam and the 12" of fiberglass fills the bays to the underside of the sheathing. We might still go with Dana's suggestion, although not sure how much cellulose there would be room for. Dana had said the spray pressue would compact the fiberglass some. Taking the glass out entirely would seem too much trouble and not necessary (removing deck or cutting strips).

The strapping would be easy enough to do if it provides some insurance. And we really aren't burdened by code, but I do like to know it's wisdom (at least when it is based in wisdom!).

Howard

Answered by Howard Gentler
Posted Sat, 01/11/2014 - 17:55

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I'm dubious about the resilience of a "reverse flash'n'batt" approach using an exterior side vented roof deck in that climate. Even minor air leaks though the foam layer could end up making wet spots in the cold roof deck, but maybe.

It's pretty easy to snake a dense-packing hose into the exterior side if they used chutes for maintaining the gap, or even if they didn't it can usually be done by running "air-only" on the blower while inserting the hose, and using a skinnier hose (a 1"er, not an 1-1/4"). With low-density R38s it'll pack out the fiberglass to about 5-6" leaving you 5-6" of cellulose. With high density R38s (which is hopefully what's in there) it'll pack out considerably less, giving you 2-3" of cellulose. If fpr some reason you can't get the hose in, drilling every 4' and just blowing from the exterior can get you about 3lb density, which should be good enough.

Answered by Dana Dorsett
Posted Mon, 01/13/2014 - 18:47

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Hi Dana: Say we do the dense pack cellulose via drilled holes in the decking (pretty sure the batts are not dense, just standard), as you suggest. I assume you are not then including spray foam at rafter bottoms, since that would require deck removal not otherwise needed. I'm assuming too that you suggest this along with the R-25 deck foam over, correct? If so, I know the dense pack fiber provides a pretty good air barrier, but thinking the decking and rigid foam becomes the "real" air barrier, internal to the overall system. So that is no problem, correct? It sounds, Dana, that you're a bit uncomfortable with a system excluding the exterior foam (at least on cathedral roofs).

I hope I'm not still circling, but want to isolate deck removal (and not) options that work. I want a good solution, but am reminded that this oringinal non-effective cathedral roof was done by a reputable local contractor (which I understand was ill informed, standard practice of the time).

Thanks again,
Howard

Answered by Howard Gentler
Posted Tue, 01/14/2014 - 17:48

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Another consideration: quite often with V groove ceiling, the V's (on the interior of the ceiling) lead directly outdoors, or at least directly to the sheathing, bypassing insulation and air sealing methods. this may not show up specifically except during a blower door - or fog test.

Answered by Bob Irving
Posted Sun, 01/19/2014 - 14:34

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