I am working on solutions for a cathedral type ceiling rehab project in a century home in Northern Ohio. The challenge is that there is limited space for insulation and limited ceiling height. There is no viable ventilation in the application (timber frame construction). I am considering a layered application (see attached illustration) -- from the exterior sheathing down of 2" polyiso foam (r11), 3.5" of rock wool (r15) and then using either Prodex Total (r-16, 5mm Reflective Insulation) or Double Bubble as an insulating vapor barrier over the joists that will then be covered with drywall.

My questions:

1) I could incorporate a 1/2"air channel between the polyiso and the roof deck, however, there is no means of venting it (I intend to seal the polyiso in place with spray foam). Is there any value in adding an airspace between the foam and the roof deck?

2) Is there any problem with using this combination of materials in this manner? Prodex's specifications show a R16 value of the product (with an airspace). This application would eliminate the airspace. What is a viable expectation of the r-value of such a product in this application?  

3) Would another combination in this application be advisable?

Thanks for your input!

Tags: insulation, polyiso, reflective, roxul

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Excuse me as I try to stop laughing... Ok, now please repeat after me, there is no such thing as an R16 reflective product. The highest R-Value one can get is basically a .5 to 1 and that is caused by a 1" airspace & not the product itself.

As for the questions - the airspace is not required for a hot-roof system which should either be the polyiso applied directly to the sheathing or closed cell foam sprayed directly to it.

The Roxul is a nice choice for foam board, but if you go with a sprayed you would be better with cellulose sprayed in.

As for vapor issues - if you follow the codes & install enough foam board or CC foam the vapor barrier is not required

 

Hi Danial and Sean, I went to their web site and it is sad they are allowed to post such claims about their products. 

Two by six rafters are simply limited as to how much insulation you can put between them and then limited by conduction through the rafters themselves as to total r-value.  Filling the cavities solid with foam and then at least an inch or rigid over the interior surface would be about as goo as you are going to get.

Do you have a code mandated r-value you are trying to achieve?  If so, we can give you some configurations to comply.  I don't think any code would accept the claims of the foil product in question.

Bud

I would suggest 3" of foam and an R-13 fiberglass batt. The interior application of rigid foam is the best addition to that system.

The 3 inch foam application without stuffing but with the rigid over the rafters is a good application your client will likely be very happy with, but the code guy will complain.

Roxul is quite dense, and it holds its form very well. If you use rigid foam and seal joints with spray or single component 'canned' foam or you just spray closed cell foam against the roof deck , the interior surface of the foam is not likely to be as flat as your illustration indicates. (Roofing nails, framing irregularities, etc.) Air movement in the space between batt and foam will reduce performance somewhat, but I don't know if significant testing has been done to identify how much. I would expect this problem to be more significant as the foam gets thinner because the temperature at that interface will be lower.

Fiberglass has a better chance of fitting the inconsistent surface of the foam. I assume you have a 5 1/2 inch cavity and you are thinking a 3 1/2 inc batt and 3 inches of foam will not fit. True, but the compressed R-13 batt will yield about R-11 and I want it to conform to the foam surface so I need the compression. Also, the foam at 3 inches will do 90% of the insulating duties anyway.

Thank you, Pat, Bud & Sean.  Yes, the reflective insulation's claims seemed to be based on a sealed airspace rather than the product's performance. I'm going to try to get approval for your suggestion of 1" rigid over the rafters. 

Hi Danial,

I ran into this link which deals with rigid foam plus compressed fiberglass insulation.  The pdf is covering how fiberglass increases in r-value per inch when compressed.  But their teat involves a 5.5" cavity and illustrated the most you can expect as an example.  http://www.flashandfoil.com/pdf/Foam-with-compressed-batt.pdf

With insulation, there are numbers and codes and then there is performance.  The difference often relates to the quality of the install.  Air paths, even small ones can greatly reduce the effectiveness of your design.  A quality air barrier is important.

As for the reflective product, I wish it were true.  That level of performance would be welcomes with open arms as so many are looking so hard to find ways to reduce energy costs.  I could go into great detail on the uses of a radiant barrier, but your application in a cold climate produces poor results.  If they have proper testing that shows otherwise and they are listening, they need to speak up.

Determine what is required and the recommended options and let us know.

Bud

One more question -- is there any difference in performance if I reverse the layers of insulation -- that is, if I placed the batt insulation against the roof deck, then installed the polyiso toward the lower surface of the joists, then finally, used 1" rigid over the joists?  Thus, rather than sandwiching the fiberglass between foam products, it becomes the most exterior insulating mass.   Thank you all for your help.

Don't do that. Keep your first plan. Permeable materials interior of impermeable materials in cold climates.

To echo Pat's thoughts on "don't do that" - you need to check out the codes on Hot Roofs or as the IRC calls them "unvented attic assemblies"

For 2009 it is section 806 & there are other areas to that do apply - 2 big ones to consider

5.1. Air-impermeable insulation only. Insulation shall be applied in direct contact with the underside of the structural roof sheathing. (Closed cell product)
5.2. Air-permeable insulation only. In addition to the air-permeable installed directly below the structural sheathing, rigid board or sheet insulation shall be installed directly above the structural roof sheathing as specified in Table R806.4 for condensation control. (FG, cellulose, open cell, mineral wool, etc...)

5.3 simply states that using either of these methods require that the amount of foam used must also meet a certain R-Value which is found on their chart & depends on your climate zone thus no flash & batt or splash & dash is allowed there. Along those lines you still have an overall U-Value for the assembly that needs to be met & if one was crazy enough to compress FG into an area they would not be granted an R-11 more like an R6 or 7

For more on Hot roofs & some of the applicable codes - http://blog.sls-construction.com/2011/what-is-a-hot-roof

Why not just fill up the cavity with 19 layers of the Prodex for a total of R-300+??

That would put the condensible surface - - -??

And with that many shiny surfaces the heat would not be able to escape.

Ed Minch

:-)

Excellent help and great link -- thank you, Sean!

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