# two layers of fiberglass batts to achieve R51 attic insulation?



## SPS-1 (Oct 21, 2008)

solarroofingpro said:


> and then TPO roof membrane on very top.


Why ?


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## solarroofingpro (Sep 5, 2021)

SPS-1 said:


> Why ?


low 12:1 roof slope


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## quatsch (Feb 4, 2021)

How much do you estimate your NG & elec. bill will change from B4 
& then after, this change?


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## Domo (Nov 9, 2018)

To answer OP's question; Yes, you can layer unfaced batt insulation to achieve higher R values. Faced insulation can lead to vapor entrapment - so you'd want to avoid that on the "internal" layers...

Looks as if you're making a "tight" house - make sure you have some air exchange and method to monitor/alter humidity. Good for you!


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## solarroofingpro (Sep 5, 2021)

quatsch said:


> How much do you estimate your NG & elec. bill will change from B4
> & then after, this change?


It's a new addition to my house therefore there is no before and after change.


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## quatsch (Feb 4, 2021)

With constant weather, a room elec. heater attached to a kilowatcher device may give you a B4, if you do it on a windless night.
It's a baseline data point.

Unfortunately, I didn't keep the B4 bills, B4 we put in better windows 10 years ago.


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## solarroofingpro (Sep 5, 2021)

quatsch said:


> With constant weather, a room elec. heater attached to a kilowatcher device may give you a B4, if you do it on a windless night.
> It's a baseline data point.
> 
> Unfortunately, I didn't keep the B4 bills, B4 we put in better windows 10 years ago.


sorry putting a heater outside and plugging in a kw meter will do nothing but waste energy. there is no room there now so it is impossible to get a before measurement


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## SW Dweller (Jan 6, 2021)

Hate to tell you the air will never travel like that.
You only have 12 inches of room for insulation. No room for air moving.


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## chandler48 (Jun 5, 2017)

I"m not understanding why you want to loft it to R51. R38 is common and will reside perfectly in your 2x12 framing. You know you can OVER build this thing, right? Have you actually checked into the cost of having it spray foamed? People are put off by what their friends and others say about the cost, when in reality, the long term savings could help pay for it.


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## solarroofingpro (Sep 5, 2021)

chandler48 said:


> I"m not understanding why you want to loft it to R51. R38 is common and will reside perfectly in your 2x12 framing. You know you can OVER build this thing, right? Have you actually checked into the cost of having it spray foamed? People are put off by what their friends and others say about the cost, when in reality, the long term savings could help pay for it.


R38 is the MINIMUM required by code in Charlotte, NC. why would you not want to add more insulation if it makes you save money on utility bills and the house more comfortable? A R13 layer is like 25 cents a sq ft. or $128. hardly a big expense. Labor and 2x4's will add to that, but I'm doing it so no big deal. And I can get as much scrap 2x4's as I want from all the houses that are being build in my neighbor hood. 

Anything less than R60 is not overbuildt IMHO. 

spray foam is about 4x the cost of normal fiberglass or blown insulation. plus I've yet to find a spray foam insulator that wants to price a small project fairly. half of them wont' quote until the building is up. had one dumb company come out and I told them what I wanted and they said they wont' quote until the frame is done. seriously? not even ball park it. lame. 

the reason I want 2 layers of fiberglass is the same reason people put 4 layers of foam board on the roof overlapped vs a single 4" thick board the overlaping stops air movement and heat/cold loss. 

the thermal bridging between the 2x12 rafters and air spaces between the rafters seriously degrades the actual R38 value to more like R20. Covering it with a 2nd layer drastically improves performance.


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## solarroofingpro (Sep 5, 2021)

SW Dweller said:


> Hate to tell you the air will never travel like that.
> You only have 12 inches of room for insulation. No room for air moving.


then tack on a 2x4' on top. this is really not as complicated as your dismissive post makes it sound.


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## chandler48 (Jun 5, 2017)

I believe if you re read SW Dweller's comment, he is showing you that the loft of R38 insulation is 12". You will have an 11 1/2" board and no space for air to move over it. If you add R13, which lofts at 3 1/2", you will need to add a 2x6 ON EDGE to accommodate both the R38, R13 and baffles to move air. I don't think it will work, but I understand your wanting to do it your way. Tacking a 2x4 flat on top of the 2x12's will only give you an additional 1 1/2" to cram in 3 1/2" of insulation. Sort of like a 10 lb butt in a 5 lb pair of pants.


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## solarroofingpro (Sep 5, 2021)

chandler48 said:


> I believe if you re read SW Dweller's comment, he is showing you that the loft of R38 insulation is 12". You will have an 11 1/2" board and no space for air to move over it. If you add R13, which lofts at 3 1/2", you will need to add a 2x6 ON EDGE to accommodate both the R38, R13 and baffles to move air. I don't think it will work, but I understand your wanting to do it your way. Tacking a 2x4 flat on top of the 2x12's will only give you an additional 1 1/2" to cram in 3 1/2" of insulation. Sort of like a 10 lb butt in a 5 lb pair of pants.


look at the original diagram. R38 in 2x12's then on top is R13 in a 2x4's then 2x4 on top of that, 3 layers, shoudl work fine.


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## chandler48 (Jun 5, 2017)

I missed that on the original picture, thinking I was looking at studs. It'll work. Lotta weight, so compensate for it.


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## solarroofingpro (Sep 5, 2021)

chandler48 said:


> Lotta weight, so compensate for it.


2x4x8 is 11 lbs x 36 of them is about 400 extra lbs on a 512sqft roof. about 0.8 lbs extra dead load per sq ft. probabaly less weight than a 2nd layer of shingles. you did make me think about that, but turns out not too bad.


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## 3onthetree (Dec 7, 2018)

Think about how you are going to put in the insulation. After framing, long batts of R13 will have to be snaked over the 2x12s. 
You have mapped out materials to the penny. Yet are spending on 5" lags screws and may need lots of brackets/straps depending on the 2x4 purlins overturning with wind design/uplift.
Think about having nearly an 18" fascia. If you birdsmouth the 2x12 rafter tails, then probably closer to 12".


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## solarroofingpro (Sep 5, 2021)

3onthetree said:


> Think about how you are going to put in the insulation. After framing, long batts of R13 will have to be snaked over the 2x12s.
> You have mapped out materials to the penny. Yet are spending on 5" lags screws and may need lots of brackets/straps depending on the 2x4 purlins overturning with wind design/uplift.
> Think about having nearly an 18" fascia. If you birdsmouth the 2x12 rafter tails, then probably closer to 12".


I dont' care if the facia is 18" because it's in the back yard.insulation installed one layer at a time to avoid snaking it throuogh. I'll need 8 lag screws per rafter x 18 rafters is about 160 screws a tub of 250 is $125.
not trying to argue, just trying to get R50 to R60 insulation on a reasonable budget. basically NOT using sprayfoam which is 4x the cost of normal insulation. 
I found some eps styrafoam insulation for cheap woudl using 3" of that be better than fiberglass batts. I'm guessing I cant just lag a 2x4 on top of it to the 2x12 under neath.


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