Author Topic: Attic insulation - choice of adding fg, blow-in, or foam underneath roof?  (Read 1922 times)

NaN

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In my 1960's house, the attic has most of the original 3.5" thick rock wool insulation. It is double faced (so on top of the ceiling panel and facing outwards towards the attic). The ceiling joists are 2x8's and the roof has 2x6's. Why? It was built for a flat roof but it seems the roof was a modification to the original plans. We get a bit of snow each year, but right now, either because of the lack of good insulation or because of our high altitude sun, the snow melts fairly quickly off the roof (only about 15 deg pitched roof). It is a fairly dry environment here.

We are looking to improve the insulation in the house for comfort and reduced energy use consumption. ROI is not a huge concern though it certainly helps. My choices are:
1) R30 unfaced rolled out on top of the existing double faced original rock wool insulation
2) R38 faced down after pulling up existing rock wool insulation
3) Blow in foam
4) Insulate bottom of roof and close of ventilation

I listed this in my order of preference. Does anyone have their own recommendations? My thoughts/questions:
-Is it okay to roll out unfaced insulation on top of existing upward faced insulation or would I be better off pulling out the old insulation entirely? I am concerned about the double vapor barrier in between the ceiling and the attic.
-Is there any reason why blow in foam is better than rolling out some insulation? It seems blow in insulation requires a lot of prep work for ventilation, etc.
-I assume closing off the attic and insulating underneath the roof is probably not needed. We have no plans to open up the attic for anything.

NaN

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Re: Attic insulation - choice of adding fg, blow-in, or foam underneath roof?
« Reply #1 on: February 21, 2020, 09:20:48 AM »
I am thinking because of the pretty dry environment I should not worry about the vapor barrier. Plus the paper is so old that it crumbles fairly easily. It would be a nasty job to tear out everything.

Instead of R30 unfaced insulation on top of old insulation, I might just put down R19 unfaced Owens Cumming insulation to place on top of the existing insulation. This will bring the insulation to the top of the joists, and then I can lay down R30 unfaced insulation over the top perpendicular to the joists.

lthenderson

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Re: Attic insulation - choice of adding fg, blow-in, or foam underneath roof?
« Reply #2 on: February 21, 2020, 10:17:19 AM »
I have not worked around any double faced insulation so I don't know about it's application but you should be just fine adding unfaced insulation on top of it, especially since you said the exposed facing in the attic is old and crumbly, i.e. not much of a vapor retarding layer anymore. I wouldn't put another layer of faced insulation (in any orientation) on top of the two faced insulation. One vapor retarding barrier is enough and three even if one is crumbling might cause other problems if humidity levels get high enough. I wouldn't remove the old insulation because it is messy, a big mess to dispose of unless you have a pickup and would cost you a lot more money.  The cost of going with a higher R-value faced insulation is a lot more than just putting a layer of unfaced lesser R-value insulation on top.  I would not spray foam the bottom of the roof unless you plan to make the attic a living space. It is a lot more expensive, you still have to remove all the old insulation and dispose of it and if your roof ever leaks, it makes the leaks very hard to detect until they cause a lot more damage. It can be worth it for the added usable conditioned space but isn't worth it if it will remain an unused attic.

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Re: Attic insulation - choice of adding fg, blow-in, or foam underneath roof?
« Reply #3 on: February 21, 2020, 10:18:12 AM »
i put 13 of blown cellulose on top of R19 Fiberglass in my attic about 7 years ago. It made a significant  difference in how long the snow stayed on our roof compared to our neighbors. between that and a few other improvements we cut the heating bill by more than half. It did require adding baffles between all the rafters for good air circulation, but the work was worth it.

Around here option 4 wouldnt be a good idea. Especially if there is a partial vapor barrier under the old insulation. Youd get condensation building up in your attic and then mold and rot...

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Re: Attic insulation - choice of adding fg, blow-in, or foam underneath roof?
« Reply #4 on: February 22, 2020, 07:55:01 AM »
I don't see loose fill, blown in insulation as an option.   This is the most common way I see insulation added to attics around here, and it's pretty economical too.  Just blow however many inches you want right over top of what's already there.

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Re: Attic insulation - choice of adding fg, blow-in, or foam underneath roof?
« Reply #5 on: February 22, 2020, 08:31:39 AM »
Taking into account your order of preference (when I upgrade my attic I plan on going a route closer to #4), I would suggest going with route #2 with an added step.

The added step is air sealing; in a 1960s house there is going to be air leakage everywhere. The top plates are not going to be seal, the plumbing/electrical/hvac/can lighting/ect penetrations will not be sealed, drywall could be missing in spots. Personally I would inclined to use a 2-part foam insulation kit as a spray air sealer over these areas, though you could use the can foam to seal it up. These air leakage issues (and stack effect) will not be addressed by just adding more insulation and you likely will not find all of them without removing the old insulation.

I plan to move the insulation to the underside of the roof deck AND maintain venting when I upgrade our attic. This will allow me an air barrier layer without any penetrations, make the attic semi-conditioned (for storage and better access), and bring two short runs of HVAC ducting full into the heated envelope.

I suggest you watch this video: https://www.youtube.com/watch?v=1BcqeQJqp08 to get some ideas.

TomTX

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Re: Attic insulation - choice of adding fg, blow-in, or foam underneath roof?
« Reply #6 on: February 22, 2020, 08:55:57 AM »
1) Get a can of foam at your local Home Improvement Store and seal every penetration. Then get duct mastic and seal every leak in your ducts (presuming they are in the attic)

2) Loose fill blown cellulose

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Re: Attic insulation - choice of adding fg, blow-in, or foam underneath roof?
« Reply #7 on: February 22, 2020, 09:14:40 AM »
1) Get a can of foam at your local Home Improvement Store and seal every penetration. Then get duct mastic and seal every leak in your ducts (presuming they are in the attic)

2) Loose fill blown cellulose

I'll second the idea of loose fill, though I don't have a strong preference between cellulose and fiberglass; I have a slight preference toward cellulose because I understand the convection currents within it will be less than fiberglass. Plus it doesn't itch.

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Re: Attic insulation - choice of adding fg, blow-in, or foam underneath roof?
« Reply #8 on: February 22, 2020, 10:19:18 AM »
Another vote here for air sealing before adding insulation. I'm hoping this year to finish air sealing our attic, after which I'll add baffles and blow in a bunch of cellulose

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Re: Attic insulation - choice of adding fg, blow-in, or foam underneath roof?
« Reply #9 on: February 22, 2020, 10:49:00 AM »
Another vote here for air sealing before adding insulation.

I will also add another vote for this and for using blown-in insulation. We were pretty close to making this a DIY project for our home, but once we compared the cost to using a contractor, it made more sense to go with a contractor because they're able to get better bulk rates on materials, they'd complete the job quicker, we wouldn't have to worry about prep work and cleanup and we wouldn't have qualified for an energy rebate unless we had a professional do the work.

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Re: Attic insulation - choice of adding fg, blow-in, or foam underneath roof?
« Reply #10 on: February 22, 2020, 12:58:42 PM »
1) Get a can of foam at your local Home Improvement Store and seal every penetration. Then get duct mastic and seal every leak in your ducts (presuming they are in the attic)

2) Loose fill blown cellulose

I'll second the idea of loose fill, though I don't have a strong preference between cellulose and fiberglass; I have a slight preference toward cellulose because I understand the convection currents within it will be less than fiberglass. Plus it doesn't itch.

It's easier for blown fiberglass to be "over lofted" - basically fluffy enough that air can flow through it fairly well, reducing insulation value.

NaN

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Re: Attic insulation - choice of adding fg, blow-in, or foam underneath roof?
« Reply #11 on: February 23, 2020, 07:12:45 AM »
The joys of a 1960s house. The pitch of our roof is such that I realistically can't pull out all the insulation and put in R38 on top in areas. So #2 is not an option. The other options are #1 and #3.

We did get a quote from a contractor for blown-in insulation and it was $3k, with a $500 rebate. The DIY approach for laying down fiberglass batts would be less than $800, so way less. In my area, working with contractors is terrible at the moment. It is a combination of everyone being busy (quoting high) and also the quality of the workers.

I am thinking of laying down R19 unfaced fiberglass batts right now and then possible considering blown-in insulation on top at a later time. With the R19 batts down I can still access a lot of the top plates and seal off holes.


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Re: Attic insulation - choice of adding fg, blow-in, or foam underneath roof?
« Reply #12 on: February 23, 2020, 08:11:56 AM »
1) Get a can of foam at your local Home Improvement Store and seal every penetration. Then get duct mastic and seal every leak in your ducts (presuming they are in the attic)

2) Loose fill blown cellulose

I'll second the idea of loose fill, though I don't have a strong preference between cellulose and fiberglass; I have a slight preference toward cellulose because I understand the convection currents within it will be less than fiberglass. Plus it doesn't itch.

It's easier for blown fiberglass to be "over lofted" - basically fluffy enough that air can flow through it fairly well, reducing insulation value.

Blown cellulose, all the way.  It's more flame retardant than fiberglass, it's better for the environment, it's less of an irritant, can't be 'over lofted'.

TomTX

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Re: Attic insulation - choice of adding fg, blow-in, or foam underneath roof?
« Reply #13 on: February 23, 2020, 10:53:03 AM »

I am thinking of laying down R19 unfaced fiberglass batts right now and then possible considering blown-in insulation on top at a later time. With the R19 batts down I can still access a lot of the top plates and seal off holes.

You can seal better right now before you put down the batts. Spend $20 on some cans of spray foam and a few hours. Done.

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Re: Attic insulation - choice of adding fg, blow-in, or foam underneath roof?
« Reply #14 on: February 23, 2020, 03:27:44 PM »
The joys of a 1960s house. The pitch of our roof is such that I realistically can't pull out all the insulation and put in R38 on top in areas. So #2 is not an option. The other options are #1 and #3.

We did get a quote from a contractor for blown-in insulation and it was $3k, with a $500 rebate. The DIY approach for laying down fiberglass batts would be less than $800, so way less. In my area, working with contractors is terrible at the moment. It is a combination of everyone being busy (quoting high) and also the quality of the workers.

I am thinking of laying down R19 unfaced fiberglass batts right now and then possible considering blown-in insulation on top at a later time. With the R19 batts down I can still access a lot of the top plates and seal off holes.

Around here the big box home stores will loan the equipment for blown cellulose for free with the purchase of enough insulation. 

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Re: Attic insulation - choice of adding fg, blow-in, or foam underneath roof?
« Reply #15 on: February 23, 2020, 04:30:29 PM »
I don't know about Lowe's, but last time I checked, Home Depot only does the free rental if you're blowing in fiberglass, and Menard's doesn't do a free rental at all.  Either way the rental cost isn't going to break the bank.

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Re: Attic insulation - choice of adding fg, blow-in, or foam underneath roof?
« Reply #16 on: February 23, 2020, 04:37:06 PM »
Go to the big box store, for blown in, they typically give you a free rental if you buy a minimum number of bags.  Buy that, return the rest.  Easy DIY job


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Re: Attic insulation - choice of adding fg, blow-in, or foam underneath roof?
« Reply #17 on: February 23, 2020, 04:57:33 PM »
This probably goes without saying, but I'll say it anyway just in case it's not obviously to those who read this thread in the future looking for DIY blown-in insulation advice; if you don't already have some, make sure to pickup masks, eye protection and gloves while you're at the Big Box store.

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Re: Attic insulation - choice of adding fg, blow-in, or foam underneath roof?
« Reply #18 on: February 23, 2020, 05:10:50 PM »
This probably goes without saying, but I'll say it anyway just in case it's not obviously to those who read this thread in the future looking for DIY blown-in insulation advice; if you don't already have some, make sure to pickup masks, eye protection and gloves while you're at the Big Box store.
...and Id just go ahead and buy a disposable Tyvek suit for another $9.  You are basically going to be working inside a dust-cloud or several hours.  Think of what PigPen from Peanuts must experience on a daily basis :-P

If its going to be hot outside (as in above 80F/17C) do yourself a favor and start as early in the morning as possible before the attic heats up.. 

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Re: Attic insulation - choice of adding fg, blow-in, or foam underneath roof?
« Reply #19 on: February 24, 2020, 06:44:56 PM »
I don't know about Lowe's, but last time I checked, Home Depot only does the free rental if you're blowing in fiberglass, and Menard's doesn't do a free rental at all.  Either way the rental cost isn't going to break the bank.

We don't have Menard's up here, I think it was home depot that we got the cellulose blower from, but it could have just as easily been Lowe'ss.

I did the math and figured out how much insulation we'd need to meet modern standards and bought that. Then as we were blowing it in (I loaded the hopper) i did some more math and realized that it would only be a couple hundred dollars more to max out what would fit under the vents and we might as well do it while we were there. Now the house has ~R60 in the attic which is probably what the modern standard should be up here anyway.

NaN

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Re: Attic insulation - choice of adding fg, blow-in, or foam underneath roof?
« Reply #20 on: February 25, 2020, 05:54:56 AM »
The unique part about our house and climate is that during the summer we do not need air conditioning (though many here have it). I am curious how a modern home with R60 on the top would fare without AC during a hot 95 degree day outside? Most new homes are built close together and on a placement city so I understand why.

Granted there are at least 3-4 months of quite a bit of heat use, so improving insulation saves on natural gas. Just trying to figure out how much I want to bring it to what a "modern" house has when we will not have AC.

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Re: Attic insulation - choice of adding fg, blow-in, or foam underneath roof?
« Reply #21 on: February 25, 2020, 06:05:29 AM »
I did the math and figured out how much insulation we'd need to meet modern standards and bought that. Then as we were blowing it in (I loaded the hopper) i did some more math and realized that it would only be a couple hundred dollars more to max out what would fit under the vents and we might as well do it while we were there. Now the house has ~R60 in the attic which is probably what the modern standard should be up here anyway.

Glad you did some quick math and realized you could/should take your attic up to R-60.
FWIW, unless you live in Zone 1 (which is the gulf-coast/Florida/Hawai'i inside the US) the current recommendation is to insulate attics up to R-60.  Even in southern Florida there's reason to go up to R-49.

With blown cellulose, the actual material is ridiculously cheap.  To go from R-49 to R-60 (an increase of R-11) it's roughly $12 per 100sqft.  That works out to $240 more for a very large 2,000 square foot attic.  You're only adding an 90 minutes or so of additional work to blow another 20 bags.  Payback period is just a few years.

tl;dr - if you're going through the trouble of adding insulation, go the full R-60.  Make sure you aren't blocking any of your vents and mark your depth beore you start.


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Re: Attic insulation - choice of adding fg, blow-in, or foam underneath roof?
« Reply #22 on: February 25, 2020, 12:08:19 PM »
I did the math and figured out how much insulation we'd need to meet modern standards and bought that. Then as we were blowing it in (I loaded the hopper) i did some more math and realized that it would only be a couple hundred dollars more to max out what would fit under the vents and we might as well do it while we were there. Now the house has ~R60 in the attic which is probably what the modern standard should be up here anyway.

Glad you did some quick math and realized you could/should take your attic up to R-60.
FWIW, unless you live in Zone 1 (which is the gulf-coast/Florida/Hawai'i inside the US) the current recommendation is to insulate attics up to R-60.  Even in southern Florida there's reason to go up to R-49.

With blown cellulose, the actual material is ridiculously cheap.  To go from R-49 to R-60 (an increase of R-11) it's roughly $12 per 100sqft.  That works out to $240 more for a very large 2,000 square foot attic.  You're only adding an 90 minutes or so of additional work to blow another 20 bags.  Payback period is just a few years.

tl;dr - if you're going through the trouble of adding insulation, go the full R-60.  Make sure you aren't blocking any of your vents and mark your depth beore you start.


That was almost my exact logic... We're already here covered in this stuff and it will never be any easier to add insulation. The roof on that house has a very small pitch so even though we used the longest baffles they sold we were limited to R60. When I get around to building my own house (sometime soon after FIRE) I will probably go for complete overkill. A passive house is possible, but not really practical this far north. I'd be happy with burning a couple cords of a wood a year and stable temperatures. Good insulation, south facing windows with a nice view and lots of thermal mass.

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Re: Attic insulation - choice of adding fg, blow-in, or foam underneath roof?
« Reply #23 on: February 26, 2020, 05:49:38 AM »
The unique part about our house and climate is that during the summer we do not need air conditioning (though many here have it). I am curious how a modern home with R60 on the top would fare without AC during a hot 95 degree day outside? Most new homes are built close together and on a placement city so I understand why.

Granted there are at least 3-4 months of quite a bit of heat use, so improving insulation saves on natural gas. Just trying to figure out how much I want to bring it to what a "modern" house has when we will not have AC.

I am not sure exactly what you are asking.

Insulation slows the movement of heat. In the winter it will slow heat from flowing from the house into that attic (and out the vents). In the summer it will keep heat from driving from the attic into the house (when the house is cooler than the attic), but prevent heat from leaving when the attic is cooler than the house (think overnight lows). Any down side, if there is one, in the summer is easily addressed by opening windows overnight or other air exchange means.

One small thing to be aware of when air sealing: The stack allows hot air to escape through the top of your house and draws in air somewhere below. There are instances where this air flow has prevented a moisture issue from becoming rot, and when the stack effect was reduced the materials were no longer able to dry and rot set in. So, if you think you might have moisture issues down below (such as along the drip edge), not might be a good time to address them.

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Re: Attic insulation - choice of adding fg, blow-in, or foam underneath roof?
« Reply #24 on: February 26, 2020, 06:18:39 AM »
The unique part about our house and climate is that during the summer we do not need air conditioning (though many here have it). I am curious how a modern home with R60 on the top would fare without AC during a hot 95 degree day outside? Most new homes are built close together and on a placement city so I understand why.

Granted there are at least 3-4 months of quite a bit of heat use, so improving insulation saves on natural gas. Just trying to figure out how much I want to bring it to what a "modern" house has when we will not have AC.

I am not sure exactly what you are asking.

Insulation slows the movement of heat. In the winter it will slow heat from flowing from the house into that attic (and out the vents). In the summer it will keep heat from driving from the attic into the house (when the house is cooler than the attic), but prevent heat from leaving when the attic is cooler than the house (think overnight lows). Any down side, if there is one, in the summer is easily addressed by opening windows overnight or other air exchange means.

One small thing to be aware of when air sealing: The stack allows hot air to escape through the top of your house and draws in air somewhere below. There are instances where this air flow has prevented a moisture issue from becoming rot, and when the stack effect was reduced the materials were no longer able to dry and rot set in. So, if you think you might have moisture issues down below (such as along the drip edge), not might be a good time to address them.

Good explanation.  Attic insulation slows how fast your house heats up during the summer from the sun baking your roof and heating up your attic. It will result in lower electricity bills from running your a/c less.  In the winter it slows heat loss from your living spaces into the attic.  The attic space itself will fluctuate with the seasons... it will be much hotter in the summer, much colder in the winter.  In contrast most people try to keep their living spaces somewhere in the 60-80F range all year long.  An attic space might easily go from 110F in the summer to below freezing in the winter for a climate that gets snow but has hot summers.
 
Ill add that its important to ensure your attic space is well ventilated - dont block the rafter vents when insulating!! THe attic needs to breathe to ensure there isnt  moisture build up or a very large delta (temp diff between the surface of the roof and the interior.

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Re: Attic insulation - choice of adding fg, blow-in, or foam underneath roof?
« Reply #25 on: February 26, 2020, 06:34:05 AM »
I did the math and figured out how much insulation we'd need to meet modern standards and bought that. Then as we were blowing it in (I loaded the hopper) i did some more math and realized that it would only be a couple hundred dollars more to max out what would fit under the vents and we might as well do it while we were there. Now the house has ~R60 in the attic which is probably what the modern standard should be up here anyway.

Glad you did some quick math and realized you could/should take your attic up to R-60.
FWIW, unless you live in Zone 1 (which is the gulf-coast/Florida/Hawai'i inside the US) the current recommendation is to insulate attics up to R-60.  Even in southern Florida there's reason to go up to R-49.

With blown cellulose, the actual material is ridiculously cheap.  To go from R-49 to R-60 (an increase of R-11) it's roughly $12 per 100sqft.  That works out to $240 more for a very large 2,000 square foot attic.  You're only adding an 90 minutes or so of additional work to blow another 20 bags.  Payback period is just a few years.

tl;dr - if you're going through the trouble of adding insulation, go the full R-60.  Make sure you aren't blocking any of your vents and mark your depth beore you start.


That was almost my exact logic... We're already here covered in this stuff and it will never be any easier to add insulation. The roof on that house has a very small pitch so even though we used the longest baffles they sold we were limited to R60. When I get around to building my own house (sometime soon after FIRE) I will probably go for complete overkill. A passive house is possible, but not really practical this far north. I'd be happy with burning a couple cords of a wood a year and stable temperatures. Good insulation, south facing windows with a nice view and lots of thermal mass.

Do you remember how long you baffles were?  I am gearing up for the same thing and my roof isn't a 12/4 I think.

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Re: Attic insulation - choice of adding fg, blow-in, or foam underneath roof?
« Reply #26 on: February 26, 2020, 11:18:06 AM »
Do you remember how long you baffles were?  I am gearing up for the same thing and my roof isn't a 12/4 I think.

I'm pretty sure they were 48 inches. There were made of a thin foam and just stapled in between each rafter. Looking at the roof of that house I always thought it looked less than 4/12, but doing the math now I realize it must have been a 4/12 or we couldn't have fit 13" of cellulose under the baffles. Now I need to go measure it...

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Re: Attic insulation - choice of adding fg, blow-in, or foam underneath roof?
« Reply #27 on: February 26, 2020, 03:53:41 PM »
On a 4/12 48" baffles should allow for ~ 15" of insulation if you take it to the very top?

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Re: Attic insulation - choice of adding fg, blow-in, or foam underneath roof?
« Reply #28 on: February 26, 2020, 04:30:35 PM »
On a 4/12 48" baffles should allow for ~ 15" of insulation if you take it to the very top?

Yeah, about that. I stuck mine down just below the existing fiberglass bats to ensure the cellulose didn't blow out down there and plug the vents. We also stayed a little below the top of the baffle. My intention was to help prevent the wind blowing through the baffles from stirring up the cellulose and depositing it inside another baffle eventually leading to problems.

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Re: Attic insulation - choice of adding fg, blow-in, or foam underneath roof?
« Reply #29 on: February 26, 2020, 05:20:57 PM »
FWIW, some types of rafter baffles can be stacked to create a longer baffle (and you can cut them in half to get two for the price of one!)

Given how little they cost (~$1.75 each), you might consider adding a 24 baffle and overlapping by 6-8 to achieve the desired depth without fear of overtopping your vents.


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Re: Attic insulation - choice of adding fg, blow-in, or foam underneath roof?
« Reply #30 on: February 26, 2020, 05:59:56 PM »
That's a good idea for roofs with a really small pitch Nereo.

NaN

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Re: Attic insulation - choice of adding fg, blow-in, or foam underneath roof?
« Reply #31 on: February 29, 2020, 06:09:05 PM »
The problem with the low pitch roof is getting the baffles secure back at the vent. I still may give it a shot at some point.

Last week I put down a bunch of R19 batts, only cost me $600 for 1300 sf. The lateral temperature gradient still seems to be about the same, but I can sort of notice the furnace going on less (just 2x4 walls with same insulation that was in the attic). From what we had before R11/13 this is 2/3rd less heat loss through the ceiling. If I went up to R60, then it would be another half reduction of an already low number (assuming I have R30 now), or saving just 1/6th of our original on our original heat loss through the ceiling. I am not sure if adding more is worth it at this point. Ah, the interesting fact of diminishing returns.


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The problem with the low pitch roof is getting the baffles secure back at the vent. I still may give it a shot at some point.

Helps to have a skinny brother in law who is used to the crawl spaces on airplanes. He kept saying how roomy the attic was...

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Helps to have a skinny brother in law who is used to the crawl spaces on airplanes. He kept saying how roomy the attic was...

He would have loved my attic. There was a section where I could "fit" by shuffling perpendicular to the joists for about 20 ft. Problem was, once my arms were extended forward there was not enough space to move them back by my side, or roll over. That was a big nope.