# Can you over insulate the attic ?



## Mach1320 (Oct 19, 2018)

Hey all : looking to insulate an uninsulated attic space above an addition. I was planning on two layers of R38 giving R76. I notice most charts say up to R60 maximum, any reason for this ? My studs are 24” so I can get 24” R38 or 23” R30 but aside from cost any reason not to double down on the R38? Soffits are not vented but the roof is as is the far wall of the attic ( old school large hole in wall vent ) the price between the 30 and 38 are pretty insignificant so if not detrimental I don’t see why not just put down the R38


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## Nealtw (Jun 22, 2017)

Mach1320 said:


> Hey all : looking to insulate an uninsulated attic space above an addition. I was planning on two layers of R38 giving R76. I notice most charts say up to R60 maximum, any reason for this ? My studs are 24” so I can get 24” R38 or 23” R30 but aside from cost any reason not to double down on the R38? Soffits are not vented but the roof is as is the far wall of the attic ( old school large hole in wall vent ) the price between the 30 and 38 are pretty insignificant so if not detrimental I don’t see why not just put down the R38


 Studs????????
In an attic you might talk about rafters holding up the sheeting of the roof. 
Joists holding up the ceiling below. Studs would indicate a room up stairs which would be a different story. 
You need soffit vents and a completely sealed ceiling first. Bud will be along to talk about R values.


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## Marson (Jan 26, 2018)

Probably not, except you hit a point of diminishing returns. Fiberglass is kind of a crappy way to insulate. No matter what it says on the wrapper, the actual amount of insulation value is going to depend a lot on how well you can get it to fit without voids. Also, air sealing is critically important. No matter what R value you have,if warm air is leaking into the attic, that heat is gone.


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## Gregsoldtruck79 (Dec 21, 2017)

I agree with Marson here about the fitting issue. I used to have my own small insulation business back in the 70's and anytime I could blow insulation in an attic, I would do so. If a person does not do a great job of fitting those batts in your 24" O.C. ceiling joists, there will be gaps. 

And then add up all the cracks, they can add up to a lot of leakage. This is why the pros doing new houses (and old) use blown insulation as its faster to do and better than batts IMO. 

The insulation blowers can be rented now from Lowes and HD with their blown insulation products. They are not even close to quality of the pro unit like I had, but with patience and a good mask for all the dust, they will insulate an attic with blown in pink or cellulose insulation.

BUT, some people like me now with bad knees, can not crawl around in an attic. So batts in this case installed from below by the homeowner on a stepladder would be their choice, am sure. JMO


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## Bud9051 (Nov 11, 2015)

As mentioned it involves diminishing returns. It also involves weight but fiberglass should not be an issue. 

From the r-60 you mentioned I'm assuming far north and cold. But why no soffit venting, usually a good idea in a cold climate. The baffles that go with soffit vents also keep the insulation away from the bottom of the roof which can cause condensation and ice issues.

Give me your climate region and square footage of attic floor and I can estimate the savings between your two options. I know you said cost was not an issue but sometimes the savings are so minor it doesn't rationally justify the effort, especially when there are better and easier places to spend those dollars.

Bud


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## Mach1320 (Oct 19, 2018)

It’s about 255 sq ft area, I like blown in but given what I’m working with I think the batts would actually be easier and with a layer perpendicular I would think I’m good on gaps. As for the soffits this house was built in the late 40s and this addition in the 70s in guessing but I’ve remodeled so wall surfaces and ceiling are new. This is in Michigan. I was surprised no soffits but I figure between the large hole in the far wall and the roof vents it’ll get cool air inside. With no insulation it Ice dams terribly


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## Mach1320 (Oct 19, 2018)

Also to clarify, my main issue with blow in right now is that this addition hardly has any pitch so the sides are not very high between the actual truss and the bottom joists so I fear I won’t get much R value as I don’t tons of space to blow the stuff in high. This is a rather unusual situation compared to most attics I’ve seen. Not even sure the machines have enough hose to go where I need


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## SPS-1 (Oct 21, 2008)

No, you can not over insulate your attic. (Except if insulation blocks soffit vent air flow).


Doubt your chart says R60 is the maximum you should insulate your attic. More likely it is trying to imply that its not worth the cost to insulate above R60. I have R70 - R75 in my attic, two layers of rockwool batts overtop of the original blown in fiberglass..


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## Bud9051 (Nov 11, 2015)

With a low slope you will be challenged to get a lot of insulation in there and avoid having that insulation up against the bottom of the roof. Foam insulation would give you the combination of best r-value and great air sealing.

Not sure about your building codes but some do not allow fiber insulation in direct contact with the roof deck. But getting ventilation in there from soffit to upper vents is a high priority, it helps to remove both moisture and that unwanted heat that is creating those ice dams.

Now, for your heating costs for that room, ceiling only. I used 6,000 heating degree days and the traditional equation: Q= u x a x HDD x 24 where "u" is 1/r and a = area. For a rough guide I divide the results by 100,000 to bring it down to gallons of oil at $2.50 per gallon.

If you installed R-30 the yearly heating cost would be $30
For R-60 it drops in half to $15
and for R-76 down to $12.

Note, that is just the 255 ft² of attic, not walls windows or air leakage, but it puts the added insulation into perspective. Bumping it from R-60 up to R-76 saves about $3 per year.

Corrections welcome if others can crank out some different numbers.

Bud


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## Mach1320 (Oct 19, 2018)

I think the soffit isn’t vented by design, there isn’t much of an overhang. The previous owners insulated the roof but not the floor space and it rotted the hell out of the roofing plywood so I had all that replaced and added vents. In the original attic of the house they did the same and it didn’t seem to trap moisture but I’m leaving the roof bare. I suppose I could pull the bits off where it touches the roof. I figured I won’t run the perpendicular later against the roof so that air can move around still.


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## joed (Mar 13, 2005)

As long as you maintain proper ventilation, too much insulation won't hurt anything. At some point it becomes a waste of money for any heat savings vs. cost of insulation.


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## Mach1320 (Oct 19, 2018)

Actually leaning towards blown in the more I think about it. 18-19” of cellulose vs 24” in batts and half the price


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## Bud9051 (Nov 11, 2015)

You should check the mfgs guidelines on the weight of cellulose, especially with ceiling joists 24" on center. If you have 1/2" drywall and use the newer/lighter cellulose they show 10.25" as a max. The reference I saw was *here*.

Bud


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## Mach1320 (Oct 19, 2018)

I actually used 5/8” board for the ceiling in that room, it was some heavy stuff


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## Mach1320 (Oct 19, 2018)

Also,

Can I pack this stuff against the roof sheeting ? I just got in there and it’s way lower than I thought with the peak about 42” from the drywall so obviously to get any decent insulation packed in I’m gonna have to get it up on the roof and there’s no way I can physically get to the soffit area to block anything off the pitch is nothing here are a couple pics 

https://imgur.com/a/3oAW677


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## joed (Mar 13, 2005)

You need ventilation from the eaves into the attic. If you install baffles then you pach the insulation against the baffle.


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## Mach1320 (Oct 19, 2018)

My eaves are not vented though ?


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