# R49 in 2x10 rafters?



## Thunder Chicken

How big of an area is this one section? Was the $3000 estimate for the whole job, or just for this section? Polyurethane foam could get you there with R6 per inch, but if they were quoting you $3000* for batts, then I'd imagine that polyurethane would cost maybe 3-4 times as much. Rigid XPS might work on paper, but wrestling it into the rafters is no fun, and you're not going to get it in if you have nails poking through the sheathing.


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## ryansdiydad

Good call on the foam boards. I found these R13 Iso boards. I could stack 4 of these in the space and get R52. There are a lot of nails poking through the sheathing about 1/8-1/4".. Since I could get R52 with 8" of these boards can I just leave a 1" gap at the top so I don't have to worry about the nails? The soffits will be vented so it will give the usual air flow as if I had batts... 


I have 9 16" on center bays to fill and the distance is pretty much right on 8 feet. So I could fill 3 bays with 4 sheets. So 12 make it 13 sheets total at $27 per sheet doesn't seem too bad... And a hell of a lot cheaper than spray foam..


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## Thunder Chicken

I'd check to see if there were any fire or other code issues for using foam boards in this application. Run it by the building inspector and make sure they're OK with it.

If the nail penetrations are only 1/8-1/4", you can just push the insulation onto the nails. Make sure that the panel is where you want it first. Hopefully those rafters are nice and square and there won't be much gap.


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## ryansdiydad

According to the manufacturer the panels are approved for this use as long as they are not left exposed. They require a 15 minute fire barrier which could be 1/2" drywall or something else. So that leads to my next question to sort out. I don't intend to drywall this ceiling. I intend to use 1" T&G planks. So the question is do 1" T&G planks provide the 15 minute fire barrier. I have seen some references to the IBC that says 19/32" wood panels meet the 15 minute requirement. So on the plus side the T&G is thicker in the middle and hopefuly as thick as the grooves. They will be V match planks.

I suppose if push comes to shove and the building inspector disagrees we will put up the 1/2" and put the T&G over that.. 

I found this reference to usage on top of the roof. Says T&G is good. But its for outside on top of the roof vs. inside the rafters.

6.2 Building Code Fire Performance Requirements
The International Residential Code regulates the use of foam plastic in a roof covering assembly.
6.2.1 Flame spread index of 75 or less determined by ASTM E84 or UL 723 apply to
the foam plastic insulation and foam plastic cores of manufactured assemblies.
Smoke developed index is not limited in roofing applications. (IRC Sec. R316.3)
6.2.2 Thermal barrier is required unless the foam is separated from the interior of the
building by tongue-and-groove wood planks or by wood structural panel
sheathing (at least 15/32” or 11.9 mm thick) (in accordance with Sec. R803) that
is bonded with exterior glue and identified as Exposure 1, with edges supported
by blocking or tongue & groove joints or an equivalent material. (IRC Sec.
R316.5.2)


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## Windows on Wash

Why do you think you need R-49? 

R-38 is plenty when it comes to foam and other air insensitive insulation materials. 

At anything over R-12, the framing is the weak point in that structure.


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## ryansdiydad

R49 is in the building plans. I am in a cold climate zone. I have no clue if he put it as R49 because of a local code requirement.


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## Thunder Chicken

Where exactly are you?

I'd suggest you bring those drawings to the local building inspector and ask specifically what would be permissible fire blocking, and the required R factor. That may clarify things quickly.


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## bowlofkibble

Are you planning on installing baffles to maintain the ventilation?

I am strongly considering doing something very similar but in between ceiling joists in an attached garage. I am thinking of using XPS, formular 150. I've read lots of back and forth on XPS versus polyiso, EPS and so on, but from a performance perspective XPS sounds like the way to go. I've imagined caulking the boards together to reach R50 and then cutting them with a reciprocating saw blade that I will sharpen to a knife like blade using an angle grinder with a flap disc. I imagine making a jig to keep the blade straight, but describing the jig is difficult.

About the polyiso you are looking at at menards, supposedly over time that stuff doesn't hold up it's R value as well as XPS, something about moisture absorption potential I think. But another thing to note is the rebate price holds the requirement "Must be installed by licensed contractor." 

Are you planning to just put it up there with tape or spray foam around the edge?

The thing that worries me most about putting rigid foam between joists and rafters are arguments relating to potential moisture issues. Some folks make it sound like it's a dire possibility, others say otherwise, frankly I doubt I even understand moisture problems much beyond the word mold.


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## Thunder Chicken

If there is any way for warm interior air to get around the insulation and contact the cold sheathing, you have potential for condensation and moisture problems. You either need to seal the rafters up vapor tight or ventilate the cold side to allow cold humid air to escape. You really ought to talk to the building office to get their take on this. I'd hate for you to go through all the expense of getting R-49 and then have moisture damage to your roof or ceiling.


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## ryansdiydad

There will be an air gap between the boards and the roof and vents will be in the soffits. So all is well there. 

The tongue and groove will be painted with any large gaps around the edges/trim caulked before painting. 

We are good on the fire rating of the tongue and groove as well. 

I will probably end up spray foaming gaps on the edges and dont fill well and taping seams between boards thou if I am lucky I will get fill sheets up there with no butt joints.

Where do you see the licensed contractor call out? I already placed the order online and got the sale price.


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## ddsrph

One thing to consider is the board facing. If foil faced you will have multiple vapor barriers
That could trap moisture between layers.


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## JJcarlins

You must find out in the manufacturers specifications if it is allowable to stack boards together. If the boards have vapor barriers on them you may have a future moisture issue. I would also check with the architect to see if that would be a substitute to spray foam before going thru the expense and labor.


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