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Attic Insulation = R19 Fiberglass Rolls vs. 2x8 Rafters

Attic Insulation = R19 Fiberglass Rolls vs. 2x8 Rafters

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  #1  
Old 02-03-20, 01:30 PM
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Attic Insulation = R19 Fiberglass Rolls vs. 2x8 Rafters

I just bought a house built in 1959. The attic consists of trusses spaced 24" on center, each with a 2x8 bottom/rafter. I want to purchase Guaridan R19 spun fiberglass rolls from Menards (and yes, cost is a factor). I intend to run two layers, with the second perpendicular to the first for a total R38. Several questions:

Q1) There's existing fiberglass roll insulation between the rafters, atop the ceiling - but it's very thin and seems compressed. I'm thinking I should just remove it. Am I right?

Q2) The Guardian R19 fiberglass roll insulation is 6.5 inches thick. The 2x8 rafters are 7.5" tall, so there'll be a one-inch space atop the first layer of kraft-faced R19 fiberglass rolls. When I add the second layer of unfaced R19--perpendicular to the first and perpendicular to the rafters--there'll be a 1-inch space/gap between the two layers of insulation. Will that gap cause the second layer to sag? Will it diminish the insulation value in any way, or cause any other problems?

I think it might let air in (between the two insulation layers) around the PERIMETER of the attic space. That might defeat the benefit of that second layer - within a foot or so of the perimeter. What do you guys think?

Q3) Anybody want to warn me away from the Menard's Guardian insulation?

Q4) How to get rid of the old faced spun-fiberglass insulation? Landfill?

Thanks
 
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  #2  
Old 02-03-20, 06:22 PM
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There is no need to remove the old insulation, it's not hurting anything,

The gap is insignificant, the insulation will compress slightly and the batts will sit flush,

I would ask that you consider installing some type of blown in insulation, I have done several homes with cellulose.

Just think of all the work it's going to take dragging up all those rolls of insulation, all the crawling around the attic attempting to lay out insulation into and across all those joists/trusses.

You will spend a fraction of the time installing the blown in material.

Spend some of that extra time air sealing all the elec and plumbing openings, that is big plus to improved thermal efficiency.
 
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Old 02-04-20, 04:04 AM
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I agree that blown insulation will do a better job. Most big box stores will give you a day's free rental of their blower if you buy a minimum number of bags.
 
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Old 02-04-20, 04:51 AM
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Putting down two layers of R-19 will be very labor intensive particularly when moving around the first layer. I'd consider putting a single layer of R-38.

Most important, though, is to spend the extra time on sealing the attic. You can find yourself wasting your money if all you do is lay some fiberglass. Things to look at for sealing include the perimeter, light fixtures and wiring, piping for plumbing, and the attic access points.
 
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Old 02-04-20, 05:34 AM
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Remember if your attic has soffit vents you need an air gap between the roofing and the soffits.
I would remove the existing insulation, seal the ceiling then lay one layer of new , then the other layer of new and then pit the old insulation on top of that.
Cannot see the sense in throwing away the old stuff.
 
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Old 02-04-20, 05:36 AM
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I second the vote for blown insulation. Around here fiberglass is used almost exclusively but it behaves similarly to cellulose. They both will fill every nook and crannie to form an air tight seal and will not form gaps like batt insulation can.
 
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Old 02-04-20, 08:51 AM
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Thanks for the helpful suggestions/advice.

And thanks for the warnings about sealing all openings into the attic. I'll have to yank the existing roll insulation to accomplish that, and since it's faced it kinda complicates whether to put faced or unfaced over it... There's an attached garage with a ceiling and no insulation. I might just move the existing insulation over there (two birds, one stone).

I failed to mention why I've been avoiding the blow-in approach. Maybe my reasoning is not good, so I'd appreciate hearing what you guys think. I'm not locked into anything yet::

1) I may need to do extensive work in the attic after insulating it. All that loose insulation in my way as I work, and bunches of it falling down into the living space through "new ceiling holes" seems like a PIA. I figured I'd cut the roll insulation into 8' batts. If I mark strategic locations I figure it'll be easy to lift the 8' batt out of the way, do the work and then replace the batt. What do you think?

2) I've never done blow-in before (fear).

3) I assumed the blow-in approach would be more expensive.

NEW QUESTIONS

Q5) Would installing a single layer of R38 between roof trusses/rafters really be as effective as two perpendicular layers of R19? A 1-1/2" gap above each rafter would not be helpful.
:
Q6) I'm in Temperature Zone 5 near the Great Lakes. If I blow in, would cellulous of fiberglass be the better choice?
 
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Old 02-04-20, 09:10 AM
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R-values are cumulative so R-38 is equivalent to 2 layers of R-19. Having said that, from a coverage perspective, laying a layer perpendicular is superior.

If you're planning on doing work, it could become difficult to get around if you can't see the rafters.

In your area, R-49 to R-60 is recommended.

 
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Old 02-04-20, 09:16 AM
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I would agree, do your other work first rather than fight the new insulation regardless what it is.

The blown in is pretty cheap, and getting the machine is free as noted.

Not bad at all, it's a dirty job but basically you have someone feed the machine and your up in the attic spraying it around.

Also, the cellulose helps in sealing some, it creates a solid layer of material but should still try to get to all the openings!
 
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Old 02-04-20, 04:51 PM
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Per Marq1, "...basically you have someone feed the machine and you're up in the attic spraying it around."
Yeah, that's another reason blow-in doesn't work for me. I just moved here and have no help. None.

Per Tony P. "
In your area, R-49 to R-60 is recommended."
Not according to the Zone maps I consulted. But it's a moot issue. R30 or maybe R38 breaks my budget, and I think I'd have to raise the roof another two feet to fit all that R60 insulation in there...

 
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Old 02-04-20, 05:20 PM
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I'd say you're best served by retaining the existing insulation over the living area and adding to it.

You'll be best served by focusing on sealing which could be a huge benefit.
 
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Old 02-05-20, 03:32 AM
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Sometimes circumstances determine what you can do so it looks like blown in is not an option that will work for you.

Blown in has some insulation pluses but it is not all pluses.
If you have to work up there in the future fiberglass will spring back relatively quickly many blown in types will not.

Do the sealing as best you can and make sure that soffit vents allow ventilation and you should notice a big improvement.

 
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Old 02-05-20, 03:41 PM
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Thanks for encouraging words, manden.

I think I'll be using two layers of R19 laid perpendicularly. I'll cut the rolls into 8' batts for easy removal when work is done later. And yes, I'll be sealing any openings from below. I think I'll move the existing (compressed/squashed) faced fiberglass roll insul over to the empty garage ceiling. The garage is a much smaller area, so I'll probably remove the facing from layer two (and layer three?) I don't expect to heat the garage, but might on occasion.
 
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Old 02-16-20, 05:37 PM
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See images below.

Image A shows the gaps produced by installing R19 unfaced Layer 2 perpendicularly atop faced R19 Layer 1. A 1-inch gap results between layers. I think this will degrade effectiveness of Layer 2.

Image B shows one possible way to stop air infiltration between the two insulation layers. Cut 1" strips of R19 insulation (6.5 inches thick) and sandwich those filler strips between upper and lower layers. Position the filler strips around the attic perimeter.

Do you guys know for sure whether that's necessary or not?

Does anybody have a quicker, easier solution?

What if I cut 1" deep slits in the bottom surface of the Layer 2 pieces where each rafter is crossed? That way maybe the fiberglass would sort of "straddle" the rafter instead of sitting up on top of it.
 
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Old 02-17-20, 03:53 AM
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What size are the ceiling joists? normally they are 2x4s or 2x6s. R-19 normally fills a 2x6 cavity.
 
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Old 02-17-20, 05:41 AM
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Do you guys know for sure whether that's necessary or not?
Your not installing ridged insulation, it will settle down between the joist and there will be no gap, your over thinking this!
 
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Old 02-17-20, 09:36 AM
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Didn't read the entire thread but did see a concern about the kraft facing.. New science is not very concerned about a vapor barrier layer (don't see your location) and once air sealed the paint will be sufficient. Citation somewhere .

Bud
 
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Old 02-17-20, 10:13 AM
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marksr: What size are the ceiling joists? normally they are 2x4s or 2x6s. R-19 normally fills a 2x6 cavity.
The rafter bottom of each truss is a 2x8. The nominal height of that wood member is 7-1/2" which is one inch taller than the insulation thickness.
 
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Old 02-17-20, 10:20 AM
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Ya, I saw that in your title a while after I posted Why not use R-25 for between the joists ??
 
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Old 02-17-20, 10:22 AM
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Marq1 - Your not installing ridged insulation, it will settle down between the joist and there will be no gap, your over thinking this!
I agree with you in theory, but sInce no one here has tried what I'm planning, I won't know for sure how much 'settle' will occur until I experiment after buying all that insulation. I'd rather have my ducks in a row beforehand.

I'm seeking advice here because I want to avoid unnecessary hassle and expense. I'm not overthinking. It's just THINKING.
 
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Old 02-17-20, 10:26 AM
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Bud9051 - New science is not very concerned about a vapor barrier layer (don't see your location) and once air sealed the paint will be sufficient.
Gee, an industry opinion that does not require buying more tools or materials. How'd that happen? Makes sense to me in light of new paint formulations and current techniques of sealing all openings from below. Thanks.
 
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Old 02-17-20, 10:29 AM
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marksr - Why not use R-25 for between the joists ??
Apparently (as in, maybe) failing to insulate above the joists lets a lot of temperature exchange in the gaps in insulation caused by those rafters/joists. So (maybe) the second layer is important. .
 
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Old 02-17-20, 12:15 PM
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When filling joist bays as full as possible out over the exterior walls it is important to have those baffles that were mentioned. Some come with a tail or flap that installs down over the end of the insulation to reduce what is called "wind washing". Tests have shown the effects of incoming air can go completely across the attic floor. (no reference) Such is the nature of fiberglass insulation.

As for the joists conducting heat they do have an r-value, r-1.25 per inch. The bigger problem would be any air gaps which would be subject to air circulation. Ideally the cavity fill should end up flush to the top of the joists, maybe plus a little, and then the second layer perpendicular so seams do not line up.

Since you are pre-planning the batt install so it can be shifted out of the way for future work, installing sections of batts in the same direction as the first is fine, just keep them shifted so seams are covered.

As for how much, while you are up there more is better than less, certainly better than coming back later to do it again.

Bud
 
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Old 02-18-20, 12:16 AM
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but sInce no one here has tried what I'm planning
You think this is the first time that anybody has ever installed multiple rows of insulation in an attic?

There was the perfect solution offered, blown in insulation, but that isnt going to work!

This is not an issue!
 
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Old 02-18-20, 10:51 AM
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You think this is the first time that anybody has ever installed multiple rows of insulation in an attic?
There was the perfect solution offered, blown in insulation, but that isnt going to work!
This is not an issue!
Dude, you should try to relax. Maybe go outside and get some fresh air.

Some guys think that when a new forum member appears he must be new to the topics discussed in that forum. Some of those guys are impressed with themselves and get a little heavy-handed. I'm not new to renovating houses, so if you can't carry on a reasonable discussion, I'm sure there are other threads in need of your expertise.
 
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Old 02-18-20, 11:12 AM
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Bud9051 wrote:
..."wind washing"....incoming air can go completely across the attic floor. (no reference).
Thanks for the information, Bud. I looked into it a little based on your comments. The examples I found showed the wind moving the blow-in flakes around, so the attic floor was exposed near the soffit vents. I think (maybe) Wind Washing is mostly a problem for blow-in insulation. Looks to me like fiberglass bats will remain in place. The existing batts (20+ years) have not been rearranged by wind washing and don't show any signs of having lifted due to wind under them. Thanks for the suggestion.

...joists do have an r-value, r-1.25 per inch. The bigger problem would be any air gaps which would be subject to air circulation.
Ha! Good to know! So a 2x8 has an R-value of just over R9. Better than I thought.

Since you are pre-planning the batt install so it can be shifted out of the way for future work, installing sections of batts in the same direction as the first is fine, just keep them shifted so seams are covered.
That's a great idea! I talked with the insulation mfr yesterday and he said the 2nd layer will sag some, but there will be air gaps between the layers caused by the 2x8 rafters standing taller than the Layer-1 insulation. He didn't think it would be a problem but he wasn't entirely sure. It's funny. You'd think he would have encounted the question before...

I think I should use a box knife to score the surface about an inch deep into the fiberglass. It'll be a PIA, but I can do that at each place the Layer-2 batt crosses a rafter top. The score line should help the Layer-2 batts straddle and settle down on the rafter tops. If I go perpendicular, that'll mean a LOT of score cuts. If I go in the same direction but offset (as you mentioned) it'll mean one LONG cut per batt. In either case, the cuts will have to be accurately located so the Layer-2 batts can be accurately positioned.

I'm thinking the easiest and quickest method is to cut 1" fillers and sandwich them between layers, around the attic perimeter (see image 2 above). That would avoid scoring the Layer-2 batts.

Thanks for thinking about my project and making good, creative, helpful suggestions.
 

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Old 02-18-20, 01:11 PM
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I talked with the insulation mfr yesterday and he said the 2nd layer will sag some, but there will be air gaps between the layers caused by the 2x8 rafters standing taller than the Layer-1 insulation. He didn't think it would be a problem but he wasn't entirely sure.
And now you have it from two separate sources that this is a non issue!
 
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Old 02-18-20, 04:40 PM
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As mentioned earlier, I don't know why you are using 6.25" R19 insulation in a cavity that is 7 1/2" deep. Use insulation meant for 2x8s for your first layer and there will be no gap to be concerned about on your 2nd layer.
 
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Old 02-18-20, 04:57 PM
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Menards' R25 unfaced is 8.5" and is what you should be using for that first layer. It is special order however, so you will need to plan accordingly.
 
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Old 02-18-20, 08:24 PM
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Shoestring Budget

I don't know why you are using 6.25" R19 insulation in a cavity that is 7 1/2" deep. Use insulation meant for 2x8s for your first layer and there will be no gap to be concerned about on your 2nd layer.
Menards' R25 unfaced is 8.5" and is what you should be using for that first layer. It is special order however, so you will need to plan accordingly.
The reason is: a very limited budget. I bought the house at auction "As is" and didn't realize the attic was so poorly insulated. No budget for that. It's my residence now, and I'm finding a lot of surprises of which I was unaware. So I'm buying Guardian R19 because it's on sale.

The R25 isn't the right thickness for 2x8s either. As far as I can see, there is no roll insulation intended specifically for 2x8 rafters. Whether you go 1" low or 1" high there'll be gaps. And I should be using Kraft-faced for Layer-1, not unfaced as you suggest.

It seems R38 is a good fit for where I'm located (Temp Zone 5) and 2 x 19 = 38. They say, "Do the math" so I did the math.

 
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Old 02-18-20, 08:38 PM
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Ok, well you seem to have beat all the good answers out of us and you already have decided what you are going to do. Do you still have any questions that are unanswered?

You are wrong about the Kraft facing. And R25 would work just fine as far as the depth is concerned. You would have to fluff it up to get the full 8.5"... and when you lay more on top it compresses the first layer slightly. There would be no gap if you didnt fluff it up, and even if there was, it wouldn't matter. You could stuff any gaps if it concerns you. Traditionally faced insulation isn't used in attics when it's getting laid as a blanket. Unless you think I'm wrong about that too.

And energystar.gov recommends R-49 total. If you are leaving your existing, adding R38 is sufficient. https://www.energystar.gov/index.cfm...sulation_table
 

Last edited by XSleeper; 02-18-20 at 08:58 PM.
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Old 02-18-20, 09:20 PM
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And before you say I'm wrong about the type of vapor retarder you need, consult the map at the end of this article.... and notice what climate Kraft facing is recommended for. In your climate, (zone 5) the paint on your ceiling is the only vapor retarder you need.

https://www.certainteed.com/insulati...re-management/
 
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