# Insulation in Attic



## jklingel (Dec 21, 2008)

BigRedFan said:


> ....so please be nice...


 You're kidding, right? :laughing: First, your location and more details of the structure will be helpful. What size boards in the roof? Accessible now? Etc. Fiberglass batts are a poor insulation when left exposed; they are not great to start with. If you must use them, run rigid over the top and tape the seams. On the inside, air seal and use a vapor retarding system. Only use a vapor barrier is code mandates it.


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## AndyGump (Sep 26, 2010)

J, what is so bad about fiberglass-batt insulation?

Andy.


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## jklingel (Dec 21, 2008)

Poor quality; some batts have knots, some compressed. Not dense enough to stop internal air currents. R value decreases as it gets colder. Nasty crap to handle, but bugs and vermin love doing what they do (like voiding their bowels and bladders) in it. Does not fit as tight as more rigid stuff, like Roxul. It is OK, but the worst of the batting materials. Dense packed in walls (blown in) it does well. I have 13" of it in my walls, but regret not knowing in 1980 what I know now; it is doing fine, partly because of how it is installed, but something like dense packed cellulose would be far better. That is my take on the stuff.


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## AndyGump (Sep 26, 2010)

Well, I think fiberglass has its place and uses, in this situation I think he could use both FG and XPS to get the required R value for the Ithaca NY. area (BigRedFan, Cornell University).

The idea about increasing the rafter space from top of knee wall to bottom of ceiling joists is a good one and he could have a lot more insulation in there if he used about 6" of foam with a 1" clearance to the deck for air flow. Then use FG on the knee walls and the ceiling joists.

He can still use R-19 in the walls even though the studs are only 2 x 4s.

Andy.


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## jklingel (Dec 21, 2008)

AndyGump said:


> Well, I think fiberglass has its place and uses...


 Everyone has to make their own call on all this, but I have no use for FG batts unless as a last resort if nothing else is available. We spend too much time doing all this stuff, so why put the worst stuff in? Sometimes you have no choice, though....


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## Windows on Wash (Aug 30, 2011)

Seal off the floor joist bays with rigid foam, spray foam and seal.

Re-align the fiberglass batts on the knee wall so they are properly spaced out and have the proper depth. Cover the kneewall with a rigid foam (preferably a foil faced ISO of at least 1" and 2" is preferable). Seal all the seams.

Make sure you don't obstruct the venting of the roof with the foam.

Cut and access hole to the small attic. Air seal what you can get to and loose fill blow the attic.


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## BigRedFan (Oct 9, 2012)

Ok guys I live in Nebraska outside of Omaha. The knee walls are framed with 2x4's and the cieling is 2x6

I am pretty competent with most DIY stuff but know very little about insulation. I have access to behind each side of the knee wall as well as the vaulted spaced through an old ventilation area. 

I am at a loss of what to do because I do not want to spend that much $$ on foam, but at the same time would like to improve the tempature control in our master bedroom


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## BigRedFan (Oct 9, 2012)

I am looking at Roxul online and it seems like I can get a much higher R value than Fiberglass batts for the RValue. What if I did the Roxul and then did some sort of rigid foam on top of that??

Do you think I could fit the higher RValue Roxul in the 2x4 walls?


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## BigRedFan (Oct 9, 2012)

Ok I think I will tackle the Knee walls before moving onto the Vaulted ceiling area. 

I think the plan that I will do is to use the R15 roxul instead of FG batts. On top of that R15 I will use a 2" rigid foam board that will add an additional R10.

That should give me a combined R25 in the walls, well above what I would need for nebraska


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## Windows on Wash (Aug 30, 2011)

Make sure the foam is rated for exposure in those areas.

If not, it needs to be covered.


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## BigRedFan (Oct 9, 2012)

Windows on Wash said:


> Make sure the foam is rated for exposure in those areas.
> 
> If not, it needs to be covered.


how do i tell if it is rated for attic exposure?

also what do i need to do around the existing electrical boxes

http://www.homedepot.com/Building-M...Id=10053&langId=-1&storeId=10051#.UHYZ_BRGJ5Q

something like that is what i had in mind


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## Windows on Wash (Aug 30, 2011)

The foam will have to have a minimum of a foil facing.

Around the electrical boxes, spray foam.

They shouldn't stick out past the outer edge of the stud wall anyway.


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## Gary in WA (Mar 11, 2009)

Roxul is way better than fiberglass in the wall cavity. Foam board (which stops thermal bridging of the studs and acts as an air barrier) or house wrap on the attic side is good. Don't forget to air block under the knee wall (stop air movement in the rooms floor/ceiling space) and over it, leaving an air space to vent it yet protect the slope insulation from air. ADA the drywall on the conditioned side: http://www.buildingscience.com/documents/information-sheets/air-barriers-airtight-drywall-approach/

Use baffles or foil-faced polyiso board at the soffit inlet vents to prevent wind-washing of the knee-wall attic insulation, air seal the wiring/plumbing holes and can lights, weatherstrip the access door and insulate it: http://www.buildingscience.com/docu...aluation-two-advanced-weatherization-packages

Caulk/canned foam the drywall/plate joints before insulating floor, "A", "D" (Air leakage paths): http://www.conservationtechnology.com/building_design.html#AirLeakagePathways

Caulk the bottom plate/floor joint against air leakage, as per code minimum, #6: http://publicecodes.cyberregs.com/icod/irc/2009/icod_irc_2009_11_sec002_par021.htm

Use DIY f.b. baffles rather than plastic or foam manufactured ones because the floor cavity insulation could touch the roof sheathing (on both sides of the baffle) for thermal conduction and "night sky radiation" over the exterior wall, compromising the R-value of the insulation; http://www.buildingscience.com/documents/published-articles/pa-crash-course-in-roof-venting

How are you planning the sloped ceiling insulation?

Gary


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## BigRedFan (Oct 9, 2012)

GBR in WA said:


> Roxul is way better than fiberglass in the wall cavity. Foam board (which stops thermal bridging of the studs and acts as an air barrier) or house wrap on the attic side is good. Don't forget to air block under the knee wall (stop air movement in the rooms floor/ceiling space) and over it, leaving an air space to vent it yet protect the slope insulation from air. ADA the drywall on the conditioned side: http://www.buildingscience.com/documents/information-sheets/air-barriers-airtight-drywall-approach/
> 
> Use baffles or foil-faced polyiso board at the soffit inlet vents to prevent wind-washing of the knee-wall attic insulation, air seal the wiring/plumbing holes and can lights, weatherstrip the access door and insulate it: http://www.buildingscience.com/docu...aluation-two-advanced-weatherization-packages
> 
> ...


Slopped ceiling I was going to blow in some loose fill cellulose


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## Gary in WA (Mar 11, 2009)

Be sure the cellulose has air movement (not just behind a baffle) above it to dissipate any moisture it gains; http://www.buildingscience.com/docu...archterm=attic+fiberglass+batts+in+flat+attic
Just add f.b. in the tight areas to get a higher R-value per inch, then Roxul as cellulose will slide down (as you can't dense-pack it) leaving weak areas. ADA the ceiling drywall also. 

Gary


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