# How to get R38 insulation in a 2:12 pitch with 2x6 rafters.



## HotRodx10 (Aug 24, 2017)

Unless you have a vaulted ceiling, where the underside of the rafters is the ceiling, usually you would insulate between and over the ceiling joists (on the 'floor' of the attic), so it wouldn't matter what size the rafters are.


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## LuapYllier (Jan 2, 2022)

HotRodx10 said:


> Unless you have a vaulted ceiling, where the underside of the rafters is the ceiling, usually you would insulate between and over the ceiling joists (on the 'floor' of the attic), so it wouldn't matter what size the rafters are.


Have you been in the attic of a 2:12 pitch roof? You have to come several feet from the outside wall the rafters rest on before you get to a 12" space between the bottom of the joist and the bottom of the vent baffle. Since you are not supposed to compress the fiberglass I am just asking how this is normally achieved.


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## Thom Paine (Nov 24, 2021)

Sprayed Closed Cell Foam is rated approx R-7 per inch..
Save your stress for later...
After you see the prints.
The prints will call out specifically what must be done.


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## HotRodx10 (Aug 24, 2017)

LuapYllier said:


> Have you been in the attic of a 2:12 pitch roof? You have to come several feet from the outside wall the rafters rest on before you get to a 12" space between the bottom of the joist and the bottom of the vent baffle. Since you are not supposed to compress the fiberglass I am just asking how this is normally achieved.


Ah, I misunderstood your dilemma. I wouldn't think an inspector would require something that can't be achieved. Anyway, that said, fiberglass can be compressed, but it doesn't have the stated R-value when it is. R-38 batts compressed into a 6" space will have a higher R-value than a 6" (R-19) batt, but not the full R-38.


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## carpdad (Oct 11, 2010)

It's low slope roof. You may need better than even 2x8 for the span.
If mine, I would leave 2" space (or usual 1.5" baffles) between roof deck and insulation. As such, I would frame the ceiling joists so I have at least 14" space from eave.


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## chandler48 (Jun 5, 2017)

In post #3 you allude to an "attic". If that is the case, you would not insulate the rafters anyway. You would insulate the ceiling joist area and poke the insulation to the low edge, just short of the soffit vent area with a long stick so as not to impede air movement over the insulation.


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## LuapYllier (Jan 2, 2022)

Thom Paine said:


> Sprayed Closed Cell Foam is rated approx R-7 per inch..
> Save your stress for later...
> After you see the prints.
> The prints will call out specifically what must be done.


I am aware of the spray foam option but the only thing to spray it on in this situation would be the underside of the baffles. Is that where it should go?
My other thought was to use 1" spacers along the rafters at the joint with the roof decking and then put in a couple layers of 2" rigid foam board until it gets high enough to put in the 12" batts.

As far as the prints go, I have already seen the architectural portion and the sections just say "Batt or spray insulation to be determined by builder" for the ceiling and "faced fiberglass batt insulation - coordinate with energy calculations" for the walls. The E-calcs say R38 ceiling and R13 for the walls...which is the code here. So unless the structural portion gives me more detail, I have to figure this out on my own. The only thing I do not have is the dimensions of the framing, but I can make a good guess.


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## LuapYllier (Jan 2, 2022)

carpdad said:


> It's low slope roof. You may need better than even 2x8 for the span.
> If mine, I would leave 2" space (or usual 1.5" baffles) between roof deck and insulation. As such, I would frame the ceiling joists so I have at least 14" space from eave.


I plan to do the usual 1" or 1.5" space above the insulation by way of either spacers or baffles. Can you explain in any more detail what you mean about the framing? What I am used to is that the rafters sit on the wall with a birds-mouth and the ceiling joists also rest on the wall right next to the rafters. How would you go about lifting the rafters without lifting the joists?


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## LuapYllier (Jan 2, 2022)

chandler48 said:


> In post #3 you allude to an "attic". If that is the case, you would not insulate the rafters anyway. You would insulate the ceiling joist area and poke the insulation to the low edge, just short of the soffit vent area with a long stick so as not to impede air movement over the insulation.


Yes exactly. However, when I say Attic I just mean that there are both rafters and joists...at its highest point (the peak of the roof) there might be 18" to 24" of space. So on the lower half of the roof slope the space is smaller than the required 12"...I know I can stuff it down the eaves between the baffles and the drywall, but you aren't supposed to compress it so I am trying to get some experienced opinions on the right way to do this. 
I also know that no one builds homes with this kind of shallow roof anymore so people with experience with this situation may be few.


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## Thom Paine (Nov 24, 2021)

You might probably wait until you have prints in hand; then get someone , who knows, to explain them to you. 

Among professionals, there is nothing unusual or unknown about the roof pitch and rake.

You are trying to make a decision with inadequate information.


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## LuapYllier (Jan 2, 2022)

Thom Paine said:


> You might probably wait until you have prints in hand; then get someone , who knows, to explain them to you.
> 
> Among professionals, there is nothing unusual or unknown about the roof pitch and rake.
> 
> You are trying to make a decision with inadequate information.


I understand what you are saying...I will obviously be looking at the plans when I get them, but I am a planner at heart and like to be able to see in my mind what I am going to be doing. I can already see that there will be an issue with this. I can see the space in my head. I post on a forum hoping that some helpful professionals, as you say, who would find nothing unusual about this...might describe how they usually deal with it. Thought that was what this place was for.


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## Thom Paine (Nov 24, 2021)

Assumptions often get us in trouble.
But, damn the torpedoes, here I go:

From your writings, I assume:

What you are planning has great importance.
You have minimal understanding of the building process means and methods.

Someone is furnishing, to you, the plans for your project.

SO, 
The concerns you have should, could, and most easily be addressed by the plan drawer.
If not, find another person for that duty.

I have seen emotional breakdowns and divorces caused by such misunderstandings.

Relax a bit, learn and enjoy the process.

FYI. assuming again, you reside in an area subject to a building authority, know that once your plans are submitted and approved no structural alterations can be made without new engineering approval.. 

For peace of mind, You should take your concerns to the party drawing the plans. 

Remember: relax, learn, and enjoy !


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## chandler48 (Jun 5, 2017)

We don't know where you are located, but in general terms, a 2 :12 pitch won't handle much of a snow load, nor will it drain water well.


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## LuapYllier (Jan 2, 2022)

Thom Paine said:


> Assumptions often get us in trouble.
> But, damn the torpedoes, here I go:
> 
> From your writings, I assume:
> ...


Doing the best work I can, not assuming I know already how best to do something, and meeting the code requirements are all very important to me, yes.
I have framed entire houses on a crew years ago but we moved on to the next home after dry-in so during those builds I never installed insulation.
I have built several small structures that were unconditioned and did not require insulation.
I have remodeled several rooms including bathrooms from bare studs so I am familiar with most general processes, however none of those rooms included new ceilings. I just added drywall over the existing.
I am quite familiar with the building process. I 100% felt that I could have built this addition to code and to structurally sound practices without having any plans drawn up, but you can't do that on a permit and because I am adding square footage to the home I wanted to do this on the permit. I have been waiting just over 4 months now for the drawings to be completed so I am getting a little antsy...I probably would have been mostly done by now had I not needed the plans.

I don't know why my question seems to have struck a nerve with you or something, a forum is for discussing things like this. I can provide any information you might like to hear but I don't know how the importance of the plan, where the plans come from or my experience has any relevance on how a professional who has done this time and again might address the situation I describe.


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## LuapYllier (Jan 2, 2022)

chandler48 said:


> We don't know where you are located, but in general terms, a 2 :12 pitch won't handle much of a snow load, nor will it drain water well.


I am located in Florida, there is zero snow load. I would prefer a steeper pitch but the rest of the house is 2:12 so I am matching. It drains water fine.
I appreciate your concern though.


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## Bud9051 (Nov 11, 2015)

I stopped reading but will post my thoughts, sorry if I duplicate:
Look up "raised heel truss". The designer of your structure should be familiar with them and should use them or similar to provide the needed space for code level insulation, especially over the exterior walls.

Bud


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## Thom Paine (Nov 24, 2021)

LuapYllier said:


> Doing the best work I can, not assuming I know already how best to do something, and meeting the code requirements are all very important to me, yes.
> I have framed entire houses on a crew years ago but we moved on to the next home after dry-in so during those builds I never installed insulation.
> I have built several small structures that were unconditioned and did not require insulation.
> I have remodeled several rooms including bathrooms from bare studs so I am familiar with most general processes, however none of those rooms included new ceilings. I just added drywall over the existing.
> ...



Trying to ease your angst.... point you in a direction... must not be working... -shrug- 

good fortune to you.


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## LuapYllier (Jan 2, 2022)

Bud9051 said:


> I stopped reading but will post my thoughts, sorry if I duplicate:
> Look up "raised heel truss". The designer of your structure should be familiar with them and should use them or similar to provide the needed space for code level insulation, especially over the exterior walls.
> 
> Bud


Thank you Bud. I will not be using trusses, it is conventional framing, however I can already envision a couple ways to frame a short cripple wall to raise the rafters. This is a good bit of info. Thanks.

EDIT: Yup, looked into it a bit and found a detail that pretty much matched what I was thinking...Thanks again Bud...I think this issue for me is solved with this.



http://imgur.com/y7V4rbn


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## HotRodx10 (Aug 24, 2017)

You're probably way more concerned about this than the AHJ or the inspector will be. Even if the letter of the code requires R-38 all the way to the exterior walls, I seriously doubt an inspector is going to attempt to crawl out there to check. Like Thom Paine, I'm trying to encourage you to relax and not worry about stuff that will get worked out. Having R-19 insulation around the edges of the attic will also be barely noticeable on your utility bill.


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## oggy bleacher (May 3, 2011)

Consider poly iso foam in the ceiling, behind drywall...then cathedral ceiling fiberglass. 1+9 + 30 will get you 40. I furred my ceiling _down_ to give me more room for insulation added purlins for the poly iso to fit in. Low ceiling but i got sections of r80 with 2 layers of cathedral fiberglass and some r13 and radiant barrier and poly iso. This is AZ where 105 is normal. Insulation and shade are the keys to climate control. Do everything possible to get r50 or r60 in florida. If you leave it to the contractor you will get r 13 and an excuse.


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## House Designer (Oct 4, 2019)

LuapYllier said:


> Thank you Bud. I will not be using trusses, it is conventional framing, however I can already envision a couple ways to frame a short cripple wall to raise the rafters. This is a good bit of info. Thanks.
> 
> EDIT: Yup, looked into it a bit and found a detail that pretty much matched what I was thinking...Thanks again Bud...I think this issue for me is solved with this.
> 
> ...


That detail doesn't pass muster structurally. Wall ties (ceiling joists) are normally required to be face nailed to the rafters. This is so that the wall ties can resist the thrust load imparted on the external walls by the dead loads of the roof, and by any live loads such as rain or workers or bundles of shingles above.
Solution: Extend your ceiling joists out to the fascia board so that you can nail the CJ to the rafters.


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## Nealtw (Jun 22, 2017)

I believe the problem is the needed height for insulation.


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## House Designer (Oct 4, 2019)

Yes, Neal, this is my suggestion to nail the continuous CJ to the rafter:
(Blocking and a raised top plate can be used between the ceiling joists for stability, rather than the rim joist sitting above the wall doubled top plate.)


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## HotRodx10 (Aug 24, 2017)

House Designer said:


> That detail doesn't pass muster structurally. Wall ties (ceiling joists) are normally required to be face nailed to the rafters. This is so that the wall ties can resist the thrust load imparted on the external walls by the dead loads of the roof, and by any live loads such as rain or workers or bundles of shingles above.
> Solution: Extend your ceiling joists out to the fascia board so that you can nail the CJ to the rafters.


That's a good point. The detail, as shown would not restrain lateral thrust of the rafters, which would be very high because of the low pitch. It would either push the walls out, or the rafters would slide out. Either way, the roof would probably flatten and collapse.


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