R38 with 2x10 Rafters


  #1  
Old 04-20-18, 01:39 PM
S
Member
Thread Starter
Join Date: Apr 2018
Location: USA
Posts: 9
Received 0 Votes on 0 Posts
R38 with 2x10 Rafters

Hey guys, new to the forum. We are kicking off an attic conversion and I need to insulate to R38 on the 2x10 rafters with a 2 inch air gap. By my math the 2x10 is 9 1/2 inches, leaving me 7 1/4 for insulation. Is there a fiberglass product that will get me R38 in that small of a space?

Thank you in advance.

Scott
 
  #2  
Old 04-20-18, 02:22 PM
XSleeper's Avatar
Group Moderator
Join Date: Dec 2004
Location: USA
Posts: 27,337
Received 990 Votes on 901 Posts
I would suggest you use a 1x2 on each side to create your airspace. Then install a layer of 1" Dow Super Tuff R-board (R 6.5) or foil faced Tuff-R, (air seal all edges) followed by Roxul Comfort batts (R30). They ought to give you credit for 5/8" drywall being R 0.5625. And the airspace above should qualify as an "Air gap" that might be worth R-1.

Let's see if Bud chimes in with his opinion.

Foam baffles followed by spray foam would probably give you the best r-value AND air sealing if you can afford it.
 
  #3  
Old 04-21-18, 05:00 AM
S
Member
Thread Starter
Join Date: Apr 2018
Location: USA
Posts: 9
Received 0 Votes on 0 Posts
XSleeper,

Thank you for the quick response. I was curious if I got anything for the drywall. Would the roof sheeting count as well? (Never considered the airspace)

Based on this site https://www.archtoolbox.com/material...n/rvalues.html your numbers are spot on and would get me to the R38.

The only other option I have come up with on my own was to do a 3 inch air space, 2 inch rigid foam R10 and 5 inches of closed cell spray foam. But as you said that looks like it would cost about double what you suggested?

Scott
 
  #4  
Old 04-21-18, 05:44 AM
XSleeper's Avatar
Group Moderator
Join Date: Dec 2004
Location: USA
Posts: 27,337
Received 990 Votes on 901 Posts
The greatest amount of problems with attic conversions is moisture. Frost occurs on the underside of the decking when warm moist air makes its way up there. So yes, your 2" rigid and 5" of closed cell foam would be superior, if you can afford it. The larger ventilation area is helpful, and no interior air or moisture should make it through.

I have seen houses with cathedral ceilings in the spring (when all the frost in the roof melts) that are so bad it looks like someone has a garden hose up in the attic.

Might be worth it for peace of mind.
 
  #5  
Old 04-21-18, 12:07 PM
S
Member
Thread Starter
Join Date: Apr 2018
Location: USA
Posts: 9
Received 0 Votes on 0 Posts
Thank you again, I was re-looking at the comfort batts as well, the R30 is 7 1/4 after the 1 inch rigid I would only have a 1 inch air gap. Looking the the spray foam is the way to go. Any experience with the DIY kits? No issue doing spray over 2 inch rigid correct?

Scott
 
  #6  
Old 04-21-18, 12:14 PM
XSleeper's Avatar
Group Moderator
Join Date: Dec 2004
Location: USA
Posts: 27,337
Received 990 Votes on 901 Posts
I do not. From what I have heard and read, this one of the things that you will be glad you hired out.
 
  #7  
Old 04-21-18, 03:23 PM
B
Member
Join Date: Oct 2008
Location: New England
Posts: 10,524
Received 37 Votes on 34 Posts
Cooking so will be back later with some guidance, thanks X. But the numbers shown don't seem to make it, I need to review the thickness of R-30 Roxul.

As for including the drywall I think that is already assumed and the air gap I have never seen an allowance. My understanding is cavity excluding bridging effects of the 2x? material and just the insulation.

There is another detail concerning the dew point of the inside surface of the rigid. But since I think you will need more than 1" you will probably be ok with 2' or more.

Later
Bud
 
  #8  
Old 04-21-18, 03:51 PM
S
Member
Thread Starter
Join Date: Apr 2018
Location: USA
Posts: 9
Received 0 Votes on 0 Posts
Thanks Bud, appreciate it.

Just saw this product at the depot, 2 inch Thermasheath rigid at R13.1 Could I do three sheets of this? (Not sure price wise how it would compare to spray)

https://www.homedepot.com/p/Thermash...3010/100573703

Scott
 
  #9  
Old 04-21-18, 04:21 PM
S
Member
Join Date: May 2015
Location: United States
Posts: 469
Received 26 Votes on 18 Posts
I can't comment on the other insulation plans, but with regard to the DIY spray foam kits, I used one. I did walls, not overhead, and my biggest complaint about the actual application was that it was messy. I had lots of drips from the end of my nozzle that landed on the floor. Prep work to protect things is a must.

I had one of the component tanks run low and my mix ratio wound up off. It didn't expand, just left slime on top of the previous layer of foam, so I had to cut the bad area out and respray. And then disposal of the tanks and residual material was a pain. I had to bleed off pressure from the tanks, and I was supposed to be able to knock out a hole in the tank to add some stuff to neutralize the chemical. Easier said than done. And proper protective gear is an absolute must from application til the end of cleanup. I got complacent during the cleanup, took my goggles off, and what do you know, that's when I got a splatter in my eye. That was unpleasant.

In summary, spraying it was easy, and actually kind of fun. The prep work, the cleanup, the risk of bodily harm, all not so fun. Your mileage may vary, but I certainly wish I would have found the extra money to hire it out.
 
  #10  
Old 04-21-18, 04:32 PM
B
Member
Join Date: Oct 2008
Location: New England
Posts: 10,524
Received 37 Votes on 34 Posts
Prepared this before I saw your reply so will post as is.

By the numbers.
2x10 is only 9 ¼" deep
Less a 2" air gap leaves 7 ¼"
Less 1" for right Polyisocyanurate leaves 6 ¼" (r-6.5)
Install Roxul at a R-21 for a total of R-27.5

Options are to either increase the 2x10 with the addition of a 2x4 or slightly less or switch to more foam products with a higher r-value per inch.
*************************************

Do you have enough space to pad out the current 2x10s with another 3"?

R-38 sounds like a warmer climate than where I'm located, try R-49, or are you taking the reduced value based upon area ratio?

Have to mention, converting an attic to living space has egress, ceiling height, and maybe other code requirements. Are you pulling permits?

Bud
 
  #11  
Old 04-21-18, 06:18 PM
S
Member
Thread Starter
Join Date: Apr 2018
Location: USA
Posts: 9
Received 0 Votes on 0 Posts
Bud thanks, and yes on permits and egress, all those goodies. I would rather not sister out the 2x10s anymore as to your point it basically reduces the ceiling height another 3 inches. R-49 must be a bear to meet. We are in NJ. Right now it looks like the plan is the original

3 inch air gap
2 inch rigid R10
4 inches spray foam R28

@SuperSqirrel thank you for the feedback on the DIY foam. Spraying looked kind of fun, and I hate clean up lol. But I think its my best option at this point in time.
 
  #12  
Old 04-21-18, 06:31 PM
B
Member
Join Date: Oct 2008
Location: New England
Posts: 10,524
Received 37 Votes on 34 Posts
@SH " Spraying looked kind of fun, and I hate clean up lol. But I think its my best option at this point in time."

Are you saying you are going ahead with doing the spray foam your self?
In addition to Squirrel's description there are other issues. Bad installations usually associated with poorly trained installer have been blamed for resulting fires. At 4" you are pushing the limit on single pass application. Too thick and it fails to cure on the inside and produces a lot of heat. I'm not a pro but the articles were readily available a few years ago.

In addition the results are rather lumpy meaning a complete fill will be overfilled in many areas requiring trim work.

In any case get some quotes for having the work done by pros.

My rant may be mute as I'm not sure if you do intend to DIY the foam.

Bud
 
  #13  
Old 04-21-18, 08:39 PM
M
Member
Join Date: Jun 2016
Location: USA
Posts: 179
Received 3 Votes on 3 Posts
I'm not sure how much of an air gap you want, however, your 2x10 is 9.25 inches deep. With a 1.25-inch air gap, 8 inches of R-30 denim insulation will bring you flush with the edge of the 2x10. If you add 2 inches of Thermasheath rigid at R13.1 across the top of the 2x10s, you'll have R43+ of insulation. Plus you can add drywall on top of the Thermasheath with longer screws. Just a thought.


http://oi67.************/2w7im2o.jpg
 
  #14  
Old 04-24-18, 04:54 AM
S
Member
Thread Starter
Join Date: Apr 2018
Location: USA
Posts: 9
Received 0 Votes on 0 Posts
Bud thank you. I am still open to options other than the spray foam. I will for sure get some quotes on it.

@Mad Scientist thank you, I need a 2 inch air gap so I am down to 7 1/4 to work with. I'm already at a 7ft ceiling height on half the room, so I would love to keep the insulation within that 2x10 if at all possible.

Scott
 
  #15  
Old 04-24-18, 03:10 PM
S
Member
Thread Starter
Join Date: Apr 2018
Location: USA
Posts: 9
Received 0 Votes on 0 Posts
Just an update, wondering if it makes sense for me to use rigid foam instead?

Can I layer foil backed rigid? https://www.homedepot.com/p/Thermash...3010/100573703

If not can I do one sheet of the foil backed and two of this product https://www.homedepot.com/p/Super-TU...0464/300528092

6 inches total would give an R39.1

Thank you again

Scott
 
  #16  
Old 04-24-18, 03:30 PM
B
Member
Join Date: Oct 2008
Location: New England
Posts: 10,524
Received 37 Votes on 34 Posts
There are several articles on using layers of rigid foam pieced to fill in as needed. Someone coined the phrase "cut and cobble" so search that and Green Building Advisors.

Here is one that gives a how-to and some caution.
Cut-and-Cobble Insulation | GreenBuildingAdvisor.com

Another option is a hot roof, eliminate the ventilation. Many have done it, especially in the south and research has shown it does not cook the shingles, less than 10° rise at peak of day in the south during summer. I can explain why if you need.

Bud
 
  #17  
Old 04-27-18, 03:39 AM
S
Member
Thread Starter
Join Date: Apr 2018
Location: USA
Posts: 9
Received 0 Votes on 0 Posts
Bud, thank you again for the input.

I started to re-think XSleepers original post.

Does R30 Rockwoll at 7 1/4 inches make sense with 1 inch R6.5 rigid between the studs and drywall?

With the .56 RF for the drywall and RF of 1 for the air gap it gets me to the required R38?

Scott
 
  #18  
Old 04-27-18, 04:19 AM
B
Member
Join Date: Oct 2008
Location: New England
Posts: 10,524
Received 37 Votes on 34 Posts
As I said the requirements are for the cavity insulation and you do not add for drywall and air gap nor subtract for thermal bridging. Only way you would get an exception would approval from your local code official, they rule. Remember, code requirements are minimum. better to have more and to stay with the 2" air gap.

If there is a way to work around the r-38 your code official will be the one who knows how. But get it in writing so it can stay with the house.

Bud
 
  #19  
Old 04-27-18, 07:16 AM
XSleeper's Avatar
Group Moderator
Join Date: Dec 2004
Location: USA
Posts: 27,337
Received 990 Votes on 901 Posts
Does R30 Rockwoll at 7 1/4 inches make sense with 1 inch R6.5 rigid between the studs and drywall?
Go with the 2" gap, 1 1/2" rigid (air seal all edges) then furr the rafters down 1 1/2" for your R-30 and you would have it made.
 
  #20  
Old 04-28-18, 07:29 PM
S
Member
Thread Starter
Join Date: Apr 2018
Location: USA
Posts: 9
Received 0 Votes on 0 Posts
THanks again guys. XSleeper is there a reason to do the rigid first and ferring out the 2x10s over doing the 7 1/4 inch batt with the 1 1/2 inch foam over the studs and longer drywall screws?

Scott
 
  #21  
Old 04-28-18, 07:38 PM
XSleeper's Avatar
Group Moderator
Join Date: Dec 2004
Location: USA
Posts: 27,337
Received 990 Votes on 901 Posts
Air sealing. Insulation is more effective in a dead airspace. If you do the foam on the interior side you will have air moving through the insulation.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: