Very Very Confused on Insulating a Basement

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daveswoodhauler

daveswoodhauler

Minister of Fire
May 20, 2008
1,847
Massachusetts
As the post reads, I keep on reading conflicting information on what type of product to use when insulating interior basement walls, and whether or not to use a vapor barrier. (Had a thread up regarding my furnace, but didn't want to confuse things) Seems like its not a cut dry answer, as it depends on your climate)

1.) I have not completed a heat loss calculation yet (thanks for the tip), but my question is mainly on whether to use rigid foam insulation or bat type insulation.
2.) Also, in the spring/summer, I will be taping plastic to the walls and floor of the basement to determine if I have a moisture issue or condensation issue. (Summertime it smells musty, but no signs of water/leaking....never any puddles after heavy rains)

Foundation is completely below grade, and the ceiling joists in the basement are insulated with R19 Bat insulation. We will be finishing off 2/3rds of the basement.

I have been monitoring temps in the basement, and the coldest I have seen is 53F when it was about 13F outside. (Temps are being taken from the far corner of the basement, furthest away from any light/furnace etc..)

Some things I read states that R10 is needed for the basement, other items I see is R19 is needed.

I would like to go with the rigid foam board insulation, but its max is about R8 for 2" think, so I guess I would be putting up additional layers. (The reason I am thinking about rigid vs bat is that even though we don't get water, and I don't think we have a moisture issue, it does get a musty smell downstairs, and have read that the regid is better versus moisture.)

Again, just looking to see what others have done as I have read so many articles my head is starting to spin, and many of them conflict each other.

Thanks.
 
If I remember correctly, vapor barrier first, then vertical stringers (studs)/ insulation then another vapor barrier ( if using ridged ins, fiberglass bats come that way) dry wall ( recommend moisture resistant). You can also add ridged insulation on the exterior of the foundation to a depth equal to or a little greater than the frost line for your area ( couple foot worth at least) from the top of sill plate down or where ever the exterior finish starts.
 
I have a finished basement and I have to use a dehumidifier during the warm months (woodstove seems to take care of it in the winter). If you have a musty smell, I would invest in a good dehumidifier.
 
Thanks both. The part I am really getting the conflicting info is on the vapor barrier issue....just don't want a basement full of mold.
 
The modern recommendation is to use NO vapor barrier or retarder on the inside of basement walls in cold climates. Use unfaced batts, or better, rigid foam/XPS that is vapor permeable, like foamular or Dow's blue board (NOT the SuperTuffR, which has the radiant barrier and is vapor impermeable). The basement walls need to be able to dry to the inside.

Batts will work, but I dislike using them in walls, because over time they usually slip and pull away from the wall. In fact, it is hard to get them tight to the wall without compressing them, which would greatly lower their R value. Any air space between the batt and the wall will greatly lower your insulating value. Rigid foam cut to fit, tight to the wall, is much better, but more than double the money.

R10 is adequate for a basement, assuming all the walls are below ground. 2 inch thick foam generally gives R10.
 
dave11 said:
The modern recommendation is to use NO vapor barrier or retarder on the inside of basement walls in cold climates. Use unfaced batts, or better, rigid foam/XPS that is vapor permeable, like foamular or Dow's blue board (NOT the SuperTuffR, which has the radiant barrier and is vapor impermeable). The basement walls need to be able to dry to the inside.

Batts will work, but I dislike using them in walls, because over time they usually slip and pull away from the wall. In fact, it is hard to get them tight to the wall without compressing them, which would greatly lower their R value. Any air space between the batt and the wall will greatly lower your insulating value. Rigid foam cut to fit, tight to the wall, is much better, but more than double the money.

R10 is adequate for a basement, assuming all the walls are below ground. 2 inch thick foam generally gives R10.
Click to expand...

hi dave
when you say air space between the batt and the wall which wall are you talking about? the basement foundation wall or the new sheetrock wall?
 
fbelec said:
dave11 said:
The modern recommendation is to use NO vapor barrier or retarder on the inside of basement walls in cold climates. Use unfaced batts, or better, rigid foam/XPS that is vapor permeable, like foamular or Dow's blue board (NOT the SuperTuffR, which has the radiant barrier and is vapor impermeable). The basement walls need to be able to dry to the inside.

Batts will work, but I dislike using them in walls, because over time they usually slip and pull away from the wall. In fact, it is hard to get them tight to the wall without compressing them, which would greatly lower their R value. Any air space between the batt and the wall will greatly lower your insulating value. Rigid foam cut to fit, tight to the wall, is much better, but more than double the money.

R10 is adequate for a basement, assuming all the walls are below ground. 2 inch thick foam generally gives R10.
Click to expand...

hi dave
when you say air space between the batt and the wall which wall are you talking about? the basement foundation wall or the new sheetrock wall?
Click to expand...

Well, technically the answer is "Which ever wall you're trying to keep heat from passing through."

In a basement, that would be the block wall or foundation wall, so you'd want to have your insulation as tight to it as possible. Leaving even tiny air pockets between the insulation and the wall leaves room for air to migrate, or at east to loop up and down, thus bypassing your insulation. This is why batts don't perform as expected in walls, even if properly installed.
 
I would tend to say that http://www.buildingscience.com/ is probably your best reference source on the latest and greatest "how to" advice on insulating a basement, etc...

You would need to read their pages, but my recollection is they suggest vapor permeable wall construction so that the walls can dry to the inside, and possibly putting a layer of drainage mat against the walls as well...

Gooserider
 
I went with 2 in of Soya base Urethane (http://www.heatlok-soya.com/). I think it'S the bast insulation / vapor barrier / sealant that can be put in a basement. But it is not as cheap $$ as foam board.
 
Thanks Folks. I am trying to get R13, but 2 inches of the foam board only works out to R10.
I looked at the building science docs, and it would seem that its ok to place a combination of rigid panels and batt insulation, correct?
I guess I am thinking of covering the walls with 1" of rigid panels, then framing the walls with 2 X 4's and then using batt insulation.
Any concerns with this approach? (Or just use 2 inch rigin on the walls, frame in, and then use 1" rigid in between the studs....but then that would leave an airgap)
Back to the drawing board.
 
You're good with the 1" panels with studs and batts as long as you are sure to tape the joints in the rigid panels. The panels have to be fastened to the walls with screws and furring strips. We used tapcons. Here's info on how to do it: http://www.buildingscience.com/documents/reports/rr-0308-renovating-your-basment

You don't have to worry about the air gap when you have continuous taped panels.
 
ilikewood said:
Thanks Folks. I am trying to get R13, but 2 inches of the foam board only works out to R10.
I looked at the building science docs, and it would seem that its ok to place a combination of rigid panels and batt insulation, correct?
I guess I am thinking of covering the walls with 1" of rigid panels, then framing the walls with 2 X 4's and then using batt insulation.
Any concerns with this approach? (Or just use 2 inch rigin on the walls, frame in, and then use 1" rigid in between the studs....but then that would leave an airgap)
Back to the drawing board.
Click to expand...

You can get rigid foam panels up to 4 inches thick, or R20, if you really want to go beyond R10. The difference in heat loss rates though gets smaller and smaller as R value goes up.

In my basement, I put up 2 inch deep pressure-treated furring strips over the block walls, then 2 inch foamular between those, cut on a table saw. Counting the R0.5 of the drywall covering, it gives me R10.5, is airtight, and only took off 2.5 inches from each side of the room perimeter.
 
tutu_sue said:
You're good with the 1" panels with studs and batts as long as you are sure to tape the joints in the rigid panels. The panels have to be fastened to the walls with screws and furring strips. We used tapcons. Here's info on how to do it: http://www.buildingscience.com/documents/reports/rr-0308-renovating-your-basment

You don't have to worry about the air gap when you have continuous taped panels.
Click to expand...

I don't deny there are those who do it this way, but if you leave an air gap between the rigid panels fastened to the wall, and the FG batts, then you are lowering the R value of the assembly. Air will circulate in that gap, even if just convection loops, and transfer heat from any surface it touches into the surrounding, uninsulated colder surfaces. Not to mention that any open but secluded space, particularly in a basement, tends to allow rodents, carpenter ants, termites etc. to take up residence without being seen. I've seen enough of it to know to never, ever leave an open space in any type of construction, if it can be helped.
 
dave
how much r value will be lost if there is a air space between the solid and batt insulation? i ask because windows get some of their r value from the air space between each layer of glass. and gas.
 
fbelec said:
dave
how much r value will be lost if there is a air space between the solid and batt insulation? i ask because windows get some of their r value from the air space between each layer of glass. and gas.
Click to expand...

Well, the air space between window panes is generally filled with some inert gas, or at one time I think was a vacuum, which is why these windows are so prone to failure, at least regarding their insulating value, if their seal is disrupted and the gas leaks out, and air comes in.

The air space behind a wall is very different. Even if you could seal it airtight, which would be a big job, at some point in the near future the airtightness would disappear, as wood shrank and expanded with the seasons, etc. If you leave an airspace between two layers of insulation, or between your insulation and the colder surface, and there is any contact between that air and the room, then heat will be able to bypass your inner layer of insulation via conduction through the air. Worse yet, by convection, if any sort of draft is established.

Now of course if the air passages between the room and the air behind your insulating layer are very small, you might not have much of a reduction in R value. But you will have some, plus the likelihood of convection loops forming in the long vertical spaces. This also will reduce your effective R value, but by how much, probably can't be calculated outside a laboratory.

Best, in my opinion, to avoid it altogether, by not designing any insulating assembly with free air inside it.
 
If your going to finish the basement with studs and drywall i would spray foam, closed cell only!!!! at least 2" is needed to be called a vapor barrier. you can spray more all depends on how much money you have or money you want to save down the road. Spray foam 2" typically takes about 12-15 years to pay for itself over choosing batt insulation keep that in mind. Just make sure to cover the foam with sheetrock or fire barrier. Also when the guys comes to spray leave the house for a day most manufacturer dont want homeowners around for 24hrs. 30% of your heating energy is lost to uninsulated basements food for thought.
 
NEDLAX said:
30% of your heating energy is lost to uninsulated basements food for thought.
Click to expand...

There's no basis for saying that without seeing a heat-loss analysis for his house.
 
Air spaces in a wall can be a mixed bag, it all depends on how big they are... If they are the right dimension they can actually be insulating, but that is difficult to achieve... To a first approximation the bigger the space the worse it will be for convection losses as you can get bigger and bigger convection loops... Another big factor is the amount of temperature difference there is between the inner and outer surfaces - if the insulation is thick enough that there is little or no difference between the insulation surface temp and the sheetrock temp, there won't be much convection once the temperatures have stabilized...

I agree that it is best to minimize air voids in a wall structure, but I don't see them as causes for panic either...

Gooserider
 
Thank for all the help folks....some good advice here.
Just found out a bigger problem I have as I found a crack in the exterior of the basement, and it goes the entire way from top to bottom, and the crack is also on the inside of the wall as well and looks like the prior homeowners tried to repair, and then the put up sheet rock along the whole side of the wall. Frosts me that they did not indicate this issue on the disclosure form, but now I know why they framed in one wall and ran some rough electrical, insulation, but never finished it. I can probababy dig down the front and repair from the outside, but its right next to the concrete steps, and if I dig the steps are going to collapse in.....might be leaving this one to the pro's
 
ilikewood said:
Thank for all the help folks....some good advice here.
Just found out a bigger problem I have as I found a crack in the exterior of the basement, and it goes the entire way from top to bottom, and the crack is also on the inside of the wall as well and looks like the prior homeowners tried to repair, and then the put up sheet rock along the whole side of the wall. Frosts me that they did not indicate this issue on the disclosure form, but now I know why they framed in one wall and ran some rough electrical, insulation, but never finished it. I can probababy dig down the front and repair from the outside, but its right next to the concrete steps, and if I dig the steps are going to collapse in.....might be leaving this one to the pro's
Click to expand...

Sorry to hear that. If you used a buyer's agent when you bought the house, he/she might be able to advise if you have any legal grounds to pursue the sellers. Depends on state laws and how the sales agreement was worded, and how long ago was the sale.
 
How long have you been there?
Have you seen some wet Springs?
Sometime things are better left alone.
 
We have been there since the fall of 2005.
I pulled off the drywall and insulation last night where the crack appears on the outside, and found the same on the inside.
Crack isn't really that large...appx 1-2 mm and runs from the top to the bottom of the interior wall.
The good news is that I really don't see much of a water stain on the wall or bottom plate...some discoloring on the bottom plate, but seems to be ok. No mold, but I can feel the cold air moving from the exterior to inside where the crack is above grade. I put in gutters all around the house when moving in, so I think that has helped.
I guess my next project will be using the sealing compound in the exterior, and then the outside as much as I can when the spring is here.
At least that way when I can get all the framing/drywall and insulation down, I can have a good view of any water penetration in the springtime.
Just afraid that even if I get the crack filled in, the constant freeze/thaw cycles over time is going to have the crack reappear, and I am now debating if I want to have that on my mind all the time.
 
ilikewood said:
We have been there since the fall of 2005.
I pulled off the drywall and insulation last night where the crack appears on the outside, and found the same on the inside.
Crack isn't really that large...appx 1-2 mm and runs from the top to the bottom of the interior wall.
The good news is that I really don't see much of a water stain on the wall or bottom plate...some discoloring on the bottom plate, but seems to be ok. No mold, but I can feel the cold air moving from the exterior to inside where the crack is above grade. I put in gutters all around the house when moving in, so I think that has helped.
I guess my next project will be using the sealing compound in the exterior, and then the outside as much as I can when the spring is here.
At least that way when I can get all the framing/drywall and insulation down, I can have a good view of any water penetration in the springtime.
Just afraid that even if I get the crack filled in, the constant freeze/thaw cycles over time is going to have the crack reappear, and I am now debating if I want to have that on my mind all the time.
Click to expand...

Is this a concrete block wall?

Is the crack the same width along it's whole length, or does it widen at one end?

A full-thickness crack in a foundation wall, especially if any movement has occurred, probably should be looked at by a reputable company for an opinion, even more so because you're going to cover it up. The crack formed for a reason. Masonry doesn't move much from temperature changes, but the surrounding soil does, especially if kept wet.

Are you sure there are no other cracks? There's often a "partner" crack on an opposing wall.
 
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