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Frame over blanket insulation?

27K views 26 replies 9 participants last post by  Do It Right 
#1 ·
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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#3 ·
... 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).
 
#5 ·
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
 
#6 ·
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.
 
#7 ·
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.
 
#10 ·
... 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.
 
#13 ·
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-buy-choose-fiberglass-insulation-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/building_america/pdfs/db/35017.pdf

Gary
 
#14 ·
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.
 
#15 ·
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
 
#16 ·
Not a problem! I know what it tastes like too.....
I was shocked when doing the research on low density f.g. I didn't know about the testing until a few years ago. Not many people know about the convective loops as most sites state it but the gravity of it all doesn't jump at you. I think I may need to change my "How to" title as only a few have read it.

I'm starting another different one on sheathing that is just as interesting......

p.s. Been there, done that, will again.

Gary
 
#26 ·
Lets get down to some further details here. Yes, rip it all out. When you place your foam board (assume 2" thick here), you will glue it to the walls. Use the right glue and do a good job of applying the glue. You may have to fasten them to the walls as well but that will have to be your call depending on wall configuration. In between the panels, you will want to caulk or spray foam or tape joints. Now if the joints from each side of the foam panels fit together tight, you might be bale to leave out the caulk/spray foam but you should still tape ALL seams. You are looking to make it airtight as possible. If you need to caulk, do it. 2" foam gives you a vapor barrier. No need for any more poly.

As for the floor, if you have the head space, putting down a 1" of XPS foam and then 5/8" or 3/4" T-n-G plywood is one method. You lose a little less than 2" of headspace this way. If you have the space, great. If you are going to do this, do the floor first then the walls. You will end up screwing the plywood-XPS sandwich to the floor with tapcons or similar. Don't need to glue XPS to the floor. If you do the floor first, then the walls, when you frame in the walls, the bottom plate will then go on top of the floor plywood directly. With a fastener, you will need to go through the bottom plate>plywood>foam>concrete.

Overall you are making a 5 sided pink or blue foam box. You need to make it tight as possible. It is not a cheap method. I would also recommend going over to youtube (if you can watch videos w/o a problem) and search on "holmes on homes basment" or something like that . He demonstrates the method and seeing it done is good.

New construction may put XPS under the concrete. Unless you are willing to rip the floor up, put it on top.

If you add more insulation between studs, others said to use roxul. That is good.
 
#23 ·
A question about the building science article

what does it mean when it says to install a capillary break on top of the footing between the footing and perimeter foundation? When it is not new construction but simply finishing a basement, what can you do to prevent that rising damp or the effects of it?

also, their diagram shows optimal insulation in new construction having rigid foam below the concrete slab floor. Again, with finishing, do you all advise rigid foam on top of the slab (even one that has a good finish that needs no leveling, etc.)?

Great article.
 
#25 ·
what does it mean when it says to install a capillary break on top of the footing between the footing and perimeter foundation? When it is not new construction but simply finishing a basement, what can you do to prevent that rising damp or the effects of it?

also, their diagram shows optimal insulation in new construction having rigid foam below the concrete slab floor. Again, with finishing, do you all advise rigid foam on top of the slab (even one that has a good finish that needs no leveling, etc.)?

Great article.
A capillary break is an EPDM gasket, or such, or DryLok paint. On an old slab that does not have a vapor barrier underneath it, I have not heard of a great solution. Whatever you do on top to stop the wicking will leave the slab wet. All I can think of is to take the whole thing out and re-do it; not a great choice, either. As for insulating now, I think you'd be OK w/ rigid foam and all-weather wood framing, (a couple options there) but it better be vapor open on top (no vinyl flooring, etc) so it can dry. That is assuming that you have a relatively dry basement. But, this is only a guess on my part.
 
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