skinny on insulation...

andrelaplume2January 8, 2008

I have read numerous posts here on ways to properly insulate / finish a poured concrete basement that has not had any know water issues for 20 years. Alas, there appears to be some question as to how to properly insulate.

I am thinking of starting my basement. I will work one wall at a time.....this may be a long project. Anyway, I have one wall with no electrical needs so I figured I'd start there and see how it goes and then decide if I want the hassle of proceding or letting a contractor take over.

My plan was to:

1) Put up 1/2" pink board over poured concrete wall

2) Put a pressure treated 2x4 sill up tight against and along the bottom of the pink board; attaching to concrete floor.

3) Add a 2x4 sill to the ceiling beams, again up tight against the pink board.

4) Stud in vertical 2x4s evey 16 inches

5) Add unfaced fiberglass insulation between studs.

6) Affix drywall to studs and finish.

How does this sound? I have found opposinng views on butting the 1/2" pink board right up agains the poured concrete. I have read that liquid nails and such will leave air pockets for condensation to build up. I have read posts suggesting that a 1/2" gap somehow left between the poured concrete and pink board to prevent moisture build up / mold. I have read similar posts on using craft backed insulation between the studs over the pink board.

I want to get this right so what will work best and be most cost efficient?

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The best single source of the best practices appropriate to any climate is at Building Science Consulting.

An inch of extruded polystyrene (XPS) would be a better bet against the wall to reduce the temp. differential and impede condensation. Adhere it as closely as possible, whether with adhesive or mechanically as with concrete screws. The bottom plate should be on a one-inch thick strip of XPS to elevate it above liquid water and to separate it from concrete, through which water vapour will move into the wood. At the very least, put a strip of 6 mil under the plate to impede vapour flow.

    Bookmark   January 8, 2008 at 10:31AM
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Caution. I have found other reputatble sources that contradict Building Science. Would not jump to that as being the "authority." Also, what they say contradicts many local town requirements. So, just make sure you do research.

    Bookmark   January 8, 2008 at 6:30PM
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Another source of reliable information is the US Department of Energy's Office of Energy Efficiency and Renewable Energy Building Technology Program, of which Building Science and Dr. Lstiburek are major consultants.

Of course, you may have to comply with out-of-date Codes and requirements of local building departments.

Here is a link that might be useful: US Department of Energy--Buildings Technology

    Bookmark   January 8, 2008 at 7:52PM
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Aside from the issues of insulation, the only thing that I'd comment on is your steps 2 and 3. Poured walls are not always plum or straight. If you place the sill tight against the wall, you have no wiggle room when you try to set a plum, straight wall. So, inspect the walls to see if you'll need the sill to come into the room to allow for leaning or crooked walls. I wouldn't bee surprised if the sill needed to be spaced an inch or more from the wall.

    Bookmark   January 9, 2008 at 8:45AM
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Thanks. I have looked at many of these Energy sites etc and indeed they do contradict each other. I figured I might find some real advise here based on folks who learned from finishing their basements.

    Bookmark   January 9, 2008 at 1:23PM
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saxmann1, BSC is the default "expert" for this kind of advise. I would be interested in reading info sources that contradict the work and opinions of BSC.

    Bookmark   January 9, 2008 at 7:59PM
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I am interested in this discussion as well.

I am about to finish my basement and my contractor is recommending a traditional framed wall set a couple inches in from the poured concrete, but no XPS. Of course the BSC article recommends XPS directly attached to the wall. But their pictures don't indicate any kind of gap between the XPS and the framed wall.

It seems to me that since XPS is not a pure vapor barrier, that a gap would be beneficial to provide some air-drying capability for any moisture that passes through the XPS. If there was FG directly touching the XPS, i would think it would eventually soak in moisture and never dry out.

What are others thoughts on this?

What have most people done? Gap or no gap?

    Bookmark   January 13, 2008 at 3:42AM
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..everyone is afraid to say in fear they might be wrong! Ha! Just kidding. I'd love to know the answer.

    Bookmark   January 18, 2008 at 4:45PM
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As I mentioned in my previous post, I expect that gap is required for straight, plum walls.

    Bookmark   January 18, 2008 at 8:44PM
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Sure, if the walls aren't straight, there will be a gap somewhere. But, that doesn't address the issue of what is best for insulation and moisture. Let's assume the wall is perfectly straight... would you leave a gap?

In a similar thread I have received some opinions and so far no one has recommended any kind of gap. The research papers from buildingsciences do not show any kind of gap. I sent an email to buildingsciences asking about a gap. Not surprising, they didn't respond.

    Bookmark   January 18, 2008 at 10:18PM
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Almost time to start to finish the basement. I've been lurking and posted a few times. Ideally, everyone seems to agree, the best way to start is with 1/2" to 2" pink or blue board against the walls and framing over that. Well, the 2" is not affordable where I live, even the half inch (2 X 8 sheet) is $10 a sheet. I saw something else and wondered if it would be acceptable. Its 1/4" folded (acordian style) blueboard. 48" X 50'. Would this be acceptable? I'd glue it up first and then frame over it then add fiberglass insulation.

Would ths be suitable under carpet or flooring?

    Bookmark   July 21, 2008 at 11:12AM
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andre... I am in the same place as you, applying for my permit today. I have read all I can, asked everyone I could. and decided to go this route: 1/2 xps glued to poured walls, seams taped. 2x4 walls, treated floor plate, 24 oc walls, going to put 7/16 osb up horizontally, staggered joints, then a 3/16 primed bead board on top of that. Only thing I haven't decided on is faced or unfaced R-13 bats (probably faced) I will only have a small gap between xps and studs if the walls aren't completely plumb, maybe 1/2". Im in Kansas, basement is dry (so far) lot of $$ spent on gutters and grading. my basement is pretty temperate, but I have two hvac outlets running into new finished area. this is the plan, unless someone says I'm nuts and can back it up with some strong facts, I'm open to suggestions btw. my finished space will be about 750 sq feet, and will be a rec room for the boys for the next few years, until I can re claim it and make it a proper pub!!

    Bookmark   July 21, 2008 at 12:50PM
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Using that 1/4" acordian stuff will save me a fortune over the 1/2" 2X8 sheets. So many people think I am nuts for not just leaving a half inch of 'space' between the poured walls and the way homes were built for decades until recently...still if this blue board stuff will prevent mold etc I want to try to afford it....the question is, will the 1/4" stuff suffice for preventing mold etc. I can then use the fiberglass for my insulation.

It seldom gets above 65 or below 55 down there so insulation is not even my main concern. Mold and moisture is....our basement has no leaks, alway dry so far as I can tell.

    Bookmark   July 21, 2008 at 3:23PM
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I understand that you're looking for insulation help but I have a question/comment on the original plan. I'm curious about the wall with "no electrical needs". What do you mean? Any wall that has >2 linear feet of space needs a receptacle. That's the code requirement.

    Bookmark   July 22, 2008 at 7:39AM
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yes, electrical would be fished through the framework.

    Bookmark   July 22, 2008 at 1:35PM
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If you do not want a dehumidifier in the room I would put in at least one return and one air vent into the room. I don't care where you put it.. if you had a 750sq ft. room anywhere with no ventilation it would eventually become stagnant.

    Bookmark   August 9, 2008 at 10:52AM
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i am in the beginning stages of finishing my basement on my 5 yr old home. i, like many of you, have read the numerous "authoritative" documents on how to best perfectly prevent moisture and mold from becoming a problem in the basement. i currently have no moisture or water infiltration. it seems to me that gap between the concrete wall and new framed drywall sounds reasonable whether the gap is 1/2, 1, or 2 inches. the question to me is what is the most efficient/economical way to this in the upper midwest climate where i live. i am leaning towards the xps but now don't know if i should put it against the concrete or on/or between the studs of the new drywall. maybe it does not even matter if there is a very little difference in temperature and humidity between the two walls. maybe (as i have heard somewhere) a vent cut into the drywall just for air exchange between the two walls placed every few meters might help in letting everything balance out and "breathe". any thoughts?

    Bookmark   September 1, 2008 at 7:30PM
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From my experience, finishing several basements and remodeling several others. Here's what I've come up with...

-Don't put a vapor barrier directly against the concrete wall
-Don't fasten drywall directly to a concrete wall
-Take care of any moisture problems before hand(If necessary use a concrete sealant, and/or put in drainage around the exterior footing.

-Check local building codes
-If using foam against the concrete, use something that will allow the concrete to breath. The best product seams to change often, so ask around.
-Insulate against the outside plate, above the foundation, between floor joists. This will alleviate cold floors in the winter.
-Use treated lumber along all concrete surfaces, with a
vapor barrier between the concrete and framing.
-Frame out from the wall far enough to plumb the wall(Use a minimum of 2x4 framing)
-Insulate with bare face mineral (fiberglass insulation)
-Staple a 6 Mil vapor barrier to the studs
-Use paperless mold resistant drywall if you have any fears of flooding or other mold inducing problems.

I have had some people tell me that the 6 Mil vapor barrier isn't a good idea in their area, due to the humidity etc... Here in the northwest, I have never had a problem with it.

    Bookmark   September 3, 2008 at 3:25AM
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lets say I skip the XPS and I leave 2" between the concrete wall and the framing. I frame. Now, I can get r13 insulation for between the studs. Do I use the kraft backed stuff? If so, which way does the paper go, towards the concrete to towards the inside of the room. If I do not use paper backed insulation, ie non backed fiberglass...what holds it in place?

    Bookmark   September 4, 2008 at 10:38AM
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The paper goes towards the room. You staple the ears of the paper to either side of the stud, leaving the stud face exposed. If you're going to skip the XPS, I'd just check the plumb of the concrete. Then get the walls as close to the foundation as possible, while keeping them plumb. I don't think I would use the paper in a basement though. I've taken quite a bit of old insulation out with mold in the paper.

    Bookmark   September 5, 2008 at 11:33AM
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I'm also determining the method of insulation for my basement, and after reading through dozens of forums and threads, have yet to see the subject of insulating when you have braced walls. My current plan is to use 2" XPS against the concrete, tuck tape the seams, and then use spray foam to caulk the edge of the XPS where it meets the beam. I will then frame the walls next to the beams, which will create an airspace of 5" or so between the XPS and the drywall. I'm thinking of adding R14 Roxul to the wall to increase the R value to 24 to qualify for a tax credit. My basement is pretty dry, only minor weep on one wall due to an issue I'm addressing with an eaves trough. But I figure that any moisture that does get in, will have plenty of airspace to dry. Planning on using DriCore for the floor as well. I like the idea of a nearly isolated space within a space.

    Bookmark   September 6, 2008 at 6:55PM
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I've done quite a few old buildings, and sometimes you cannot get around leaving a little space behind the framing. I have never had a problem with it where it had been fairly dry. If your floors are not insulated, be sure to insulate the gap between joists out at least as far as the wall. I usually spray foam along the top and bottom of the outside plate, then fill the rest up with fiberglass flush with the inside of the framing. I'm sure there are probably a hundred other ways to do it, but it has always worked fine for me.

    Bookmark   September 7, 2008 at 12:17AM
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