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Old 12-31-2010, 12:24 PM   #1
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Frame over blanket insulation?


We have a brand new home in the midwest and I want to start finishing the basement. It is a ranch with a walkout basement. Most of the area to be finished is above grade. The builder installed this blanket insulation on all the perimeter walls. Its basically fiberglass right on the poured concrete foundation with a vinyl face and its nailed right into the foundation every few feet.



My question is can I frame right up against that? I've read the building science article and I know the "ideal" solution is to tear it all out, put up XPS, frame, then insulate, but I'd hate to waste all this insulation. Another thing is our neighbor had the same situation, hired a contractor, and he just framed right up against it.

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Old 12-31-2010, 05:40 PM   #2
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Frame over blanket insulation?


In our climate, this would be a big no-no. It simply collects all the condensation during warm months, and then is useless, and creates mold. Can't say what is correct for the mid country, it might be OK.

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Old 12-31-2010, 09:02 PM   #3
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Frame over blanket insulation?


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... but I'd hate to waste all this insulation.
You read the article and know what you should do. Excellent. So why are you thinking of postponing throwing all the insulation away, with more insulation and lumber, at a later date? We all try the quick and easy way out now and then, but I THINK I have learned to bite the bullet and git 'er done right; that will be quicker and easier in the long run, right? IMO, think again and follow what you know will work. Forget that some contractor "fixed" your neighbor (which may work; dunno).
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Old 01-03-2011, 12:46 PM   #4
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Frame over blanket insulation?


Well I started tearing out the blankets and bought 1/2" rigid foam to glue to the wall.

Is it possible to add that blanket insulation in the attic?
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Old 01-03-2011, 01:18 PM   #5
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Frame over blanket insulation?


Typical fiberglass batts are very porous, which is why fiberglass is used for air filters. In an attic, left open to the air, it is the worst insulation you could use. However, if you install it and blow cellulose over it, that "seals" it and it will work better. I would not throw it away, as it will do something just lying around in the attic. It depends on what your time is worth to you. Are you sure 1/2" foam is worth the trouble? R-2.5 or 3 is pretty skinny. I'm a bit of a heat loss freak, so I would not consider anything less than 2". What is the difference is the cost of the various sheets? That, of course, is your call. Good luck on the project; they are only beginning... houses are never finished, I don't think. j
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Old 01-03-2011, 01:38 PM   #6
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Frame over blanket insulation?


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Typical fiberglass batts are very porous, which is why fiberglass is used for air filters. In an attic, left open to the air, it is the worst insulation you could use. However, if you install it and blow cellulose over it, that "seals" it and it will work better. I would not throw it away, as it will do something just lying around in the attic. It depends on what your time is worth to you. Are you sure 1/2" foam is worth the trouble? R-2.5 or 3 is pretty skinny. I'm a bit of a heat loss freak, so I would not consider anything less than 2". What is the difference is the cost of the various sheets? That, of course, is your call. Good luck on the project; they are only beginning... houses are never finished, I don't think. j
Since I am still insulating the frame wall I decided that extra 1/2" wasn't worth the extra cost. The price difference was about a couple bucks per sheet, but I'm trying to walk that fine line doing it as cheap as possible without sacrificing quality. Thanks for the replys.
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Old 01-03-2011, 01:50 PM   #7
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Frame over blanket insulation?


Use the XPS or EPS,(XPS is preferred has a higher R value than EPS per inch of thickness) 1" minimum, 2" is better.

You can use the fiberglass insulation between the studs, but you will have to cut slits in the plastic, so as not to create a second vapor barrier.

Another use for the fiberglass insulation would be between the floor joist in the ceiling of the room you are fininshing. It will help eliminate sound transmission.
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Old 01-03-2011, 02:04 PM   #8
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Typical fiberglass batts are very porous, which is why fiberglass is used for air filters. In an attic, left open to the air, it is the worst insulation you could use. However, if you install it and blow cellulose over it, that "seals" it and it will work better.
Yes fiberglass is used for air filters, but it is also used for car bodies. The density (#'s/ft3) of the material being used will determine is porousity, saying that fiberglass insulation is the worst insulation there is, is just plain BULL.
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Old 01-03-2011, 03:34 PM   #9
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Yes fiberglass is used for air filters, but it is also used for car bodies. •• I've never seen a car body made out of fiberglass BATTS. Both filters and batts are very similar composition and not too far off on the density. Yes, the insulation is more dense than a filter, but it is still a very porous material. Batts are obviously nothing like the matting you use in a fiberglass car body.

The density (#'s/ft3) of the material being used will determine is porousity, •• That is very true. But, we are talking about fiberglass BATTS here, not mats. Just picking up a roll of mat material demonstrates the difference in density. I believe that very tightly woven fiberglass matting is actually a pretty good insulator, but I am not sure on that.

saying that fiberglass insulation is the worst insulation there is, is just plain BULL. •• Actually, it is very accurate. FG BATTS are not typically dense enough to stop internal convective loops in a wall, which is one reason they are not a great wall insulator. And, if left exposed, they are very poor at stopping air movement; air movement means btu's flying south. Pick one up and look at it or blow through it. You can read about FG and other insulation materials on buildingscience.com, for one site. Greenbuildingadvisor.com is another. There you will read about fiberglass being blown DENSE PACKED into walls and doing very well, ie, about the same as cellulose. It's called Spider Something. You will also read about BATTS being a very poor choice for most house insulation needs. You are welcome.
Pls see after the bullets.

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Old 01-03-2011, 03:42 PM   #10
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Frame over blanket insulation?


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... but you will have to cut slits in the plastic, so as not to create a second vapor barrier.... Actually, Jack, that is not quite right. It sounds good, but it just does not work. The slits account for a very small percent of the vapor barrier's area, so they are not effective unless you have a great deal of wind causing the wall to "pump" (generally not a good scenario). You can read about that on the two sites I mentioned, too. In a wall, we hope, the VB is not moving, so there is not driving force to take advantage of the slits.

It will help eliminate sound transmission. Now THAT it does.
Pls see after the bullets.
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Old 01-03-2011, 04:29 PM   #11
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Frame over blanket insulation?


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Pls see after the bullets.

All I need ask Jklingel is have you really ever looked at the R-values of the materials

FG Bat - 3.1-4.3
HDFG Bat - 3.6 - 5
Cellulose - 3.1-3.8/in

Bout time you quit spreading incorrect information about the qualities of FG.

And the last time I used insulation, I believe it was surrounded by other structure, if you were to apply the same statement "And, if left exposed, they are very poor at stopping air movement;" any insulation, would either blow away or collapse, so your comparison is not realistic.

Fiberglass is actually more stable in an attic than celleous as it will absorb less moisture 1% compared to up to 20% with cellulose, resulting in a more stable R-value. (R-value is reduced as moisture content is increased)

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Old 01-03-2011, 05:22 PM   #12
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Frame over blanket insulation?


you best do some serious studying. partial information is dangerous. j
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Old 01-03-2011, 07:54 PM   #13
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Frame over blanket insulation?


Basement Insulation Systems NewB, use the builder's grade (low density) fiberglass in the ceiling without the plastic, slit or not. It would still be a vapor barrier, 99.9% effective even with slits. Read this on low density (think convective loops) f.g. in walls or attic; http://www.diychatroom.com/f98/how-b...ulation-90438/ It collects the dirt well, though.
So you probably read this on page #9;

However, walls with 0.75 inches of extruded polystyrene and 3.5 inches of fiberglass batt insulation in the cavity would perform well as long as interior humidity was controlled below 50 percent during the summer. Increasing the extruded polystyrene to 1.0 or 1.5 inches would improve performance even with higher interior relative humidity during the summer months. This part of the analysis assumed that the concrete wall had a relative humidity of 100 percent at the exterior temperature. Since these studies were for a climate location similar to Minnesota, the thickness of rigid insulation (R-value) could be proportionately reduced in milder climates.
http://www.eere.energy.gov/buildings...s/db/35017.pdf

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Old 01-03-2011, 08:48 PM   #14
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Frame over blanket insulation?


Well I don't often eat crow, but on further investigation, thanks to links posted by GBR in WA I would say that crow is what I have to eat.

Thanks GBR in WA and to Jklingel I have to say I was wrong or stuck in the 80's.

As far as FG being a total outcast I would never say that, and if you have used cellulose it is extreemly dusty, and the chem's it is treated with for fire and mold resistance are a health hazard, that being said.

A job done well with cellulose as compared to a job done equally well with FG, the cellulose will indeed perform better.
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Old 01-04-2011, 12:19 AM   #15
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Frame over blanket insulation?


Thank you, Jack. You have balls, for sure, and I compliment you on that. Next time I may be the one eating crow, or worse, but I've done that enough that I try to be conservative on my responses; doesn't always happen as planned, though. And, FWIW, I have 13" of FG batts in my walls, and they are doing OK (I have mentioned this on other posts, too, so as to not look like I am a cellulose industry troll or whatever). I also installed 5.5" in the lid, and covered it with a foot or more of cellulose. In 1980 I did not know much about cellulose (hell, I didn't know much about ANYTHING), or dense packing cellulose wasn't around then; dunno. If I may gingerly disagree w/ the borates in cellulose being a health hazard, I will. They are hazardous to bugs and rodents, but not to humans. That is why installers don't need haz mat-type suits, like w/ some foams. Google Robert Riversong, who is back in VT (I think) and has been dense packing cellulose for decades. He has the system pretty well figured out, and is extremely knowledgeable of a variety of building science issues. He teaches it, and is a regular on greenbuildingadvisor.com. I've learned a lot from reading over there and buildingscience.com in the last year or so; pretty educated folks, and a lot of those guys/gals also build. Yep. Dirty fingernails and all. BTW: there is also cotton and rock wool, which I personally would use before FG; I just hate that itchy stuff, besides everything else. Y'all take care. john

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