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deepstuff 04-25-2009 05:17 AM

Basement insulation / vapor barrier questions

I'm installing batt insulation and then vapor barrier in 2x4 framed walls:
- Should wiring go behind the batt insulation or should the insulation be tucked behind the wiring? Or should the insulation be split with 1/2 in front and 1/2 behind the wiring?
- When installing the vapor barrier do I install small pieces behind the electrical boxes first? How do I seal where the wires penetrate it? tape? caulk?
- I run a bead of acoustical caulk on the top and bottom plates and end or cornor studs. Stick and staple the barrier in place and cut out for electrical boxes.. Seal it, tape or caulk to pieces already installed behind electrical boxes.
- Its in a basement so I also have to do inbetween the floor joists. Is it OK to seal the barrier completely to my top plate leaving 2-feet at the top. Then go back after and use this 2-feet and to seal it inbetween the joists? Will this work with the barrier sealed alonl the topplate and then all along the floor joists above?

Just Bill 04-25-2009 06:21 AM

Batt insualtion is not the best thing to use in a basement. Rigid EPS foam(min 2"), glued or secured directly to the walls will insulate and also act as a moisture barrier. Available at home stores and lumber yards, pink or blue. Owens-Corning has the pink stuff (2x8 sheets)at HD, it has notches for installing furring strips, use tapcons. You can install drywall right over this system.

Bob Mariani 04-25-2009 07:02 AM

But this is not a good idea. You should maintain an air space between the framed wall and the exterior wall. Foam insulation is needed against the concrete wall to provide a capillary break. The air space is needed to allow the wall to dry. A vapor barrier is not to be used except in very extremely cold areas. (Alaska). Run the wires behind the studs. Do not seal the joist above the wall with batt insulation. This will collect moisture and fail and grow mildew. Seal the rim joists with spray foam insulation.

Read more on this, you are planning on doing it all wrong. almost 80% of basements fail due to what you are doing now. Appraisers are now applying no value to these projects since so many are not done to last more than a few years.

deepstuff 04-25-2009 08:23 AM

I'm in a cold area. I have 1.5" styrofoam on the concrete followed by 2x 4 studs and batt insulation.

Bob Mariani 04-25-2009 09:35 AM


Originally Posted by deepstuff (Post 265294)
I'm in a cold area. I have 1.5" styrofoam on the concrete followed by 2x 4 studs and batt insulation.

still need to maintain at least 1/2" - 1" air space between the foam board and the 2 X 4 walls. Especially if you use a vapor barrier, which you can do for your area.

jaros bros. 04-25-2009 10:25 AM

I would recommend you look at the website You'll find a lot of good information and studies. Too much disinformation out there.

jaros bros. 04-25-2009 10:30 AM

Tons of good info. This is just something I quickly copied and pasted.

jomama45 04-25-2009 02:04 PM

I agree with Bob. I just got back from a 7 hour new code seminar for my state, & fiberglass batts & vapor barrrier are no longer allowed at foundation wall level. If you have any exposure though, the wood framing cavity has to be insulated as normal with a VB & be terminated where it meets the bottom plate/top of foundation. Only rigid foam allowed against foundation walls here, & we have generally the same climate as Ohio.

n0c7 04-25-2009 03:11 PM

deepstuff, I'm in Canada as well and just went through the same debate a couple of years ago. I never did find any solid answers, but the general consensus was to split the batts down the middle for wiring, cut and tuck some insulation behind the electrical boxes or other similar areas, and I don't believe its necessary to insulate and vapor barrier the ceiling for moisture, more for noise reduction but you do have to insulate and vapor barrier the exterior perimeter of the ceiling/foundation where you see the wood and concrete meet. Use red tuck tape to seal around the electrical boxes, any cutouts, splits, accidental cuts, etc. Leave space from the foundation wall to your 2x4 wall, I believe mine was 1.5" inches. Using acoustical caulk on the tops, bottoms and sides was also highly debatable. I ended up skipping it. Not saying I'm an expert here, but I spent much time researching and asking people just to get a hundred different answers and this is what worked for me to get the job done.

One thing with this site is that we are outnumbered by Americans and with their wide variety of climates, mainly warm, you're going to get quite the variety of opinions and hear of materials to use that you didn't even know existed. ;)

Chemist1961 04-26-2009 07:44 AM

CDN Feedback
NOC7 makes a good point. You already ahve comments her from several very knowledgeable contractors, but we have different codes here and different regional climates across the country. The building science site originated here in Waterloo and explains the difference in those areas across North America. I often have posted that link for attic insulation and ventilatiion projects.. You need to factor in both materials and climate.

In my experience as an installer of vac pipe working in basements in new homes, the current method is to leave the air gap Bob mentions for breathing space, and the fibre insul is split around wiring as per above. The accoustical caulking is the norm in Ontario with 6 mil poly as code for the upper levels of new homes, but I find most basements are not sealed quite as tightly. Different climate zones may recommend different vapour barrier techniques in the basement to promote drying behind the drywall.
Yes you can wrap poly around the boxes and yes you can seal to the top plate and later to the joist bays. Before any of that, I would enure a good seal all along the top of the concrete wall where the rim joist meets the concrete. If your home is older, this was not sealed during construction whereas a new home will have the rim joist wrapped with tyvek on the outside to create a seal along the rim joist. I like expansion foam for this project as it will creep into the gaps. Do your basement windows the same way but use low expansion window foam, in fact I used window and door foam for the entire rim joist.

If you are upgrading an older home, as the air loss is reduced you may develope a need for an air exchanger depending on the changes you make. How old is your home? Mine had a huge air loss equal to a 2 square foot window being open year round according to an energy audit before I started sealing and insulating. This was well built custom home but 35 years old when gas was cheap.

The HVAC guys on this site often caution on these changes affecting ventilation.Where most older homes lose their basement air and a lot of heat ( +- 15%) is in a poor seal along the rim joist, so be diligent in that area and seal the top and bottom outer skirt (rim joist) to your floor above and joist bays. You will want to fill the bays with at least R28.
Consider Roxul insulation batts for your project. Fire and moisture resistant and lots less itchy. Another Canadian PLUS, made here in Milton and in BC as well.
Also check into an energy audit before you proceed. There is a lot of provincial rebate money for Energy efficient upgrades on top of the the NEW Federal tax credit :thumbsup:

ccarlisle 04-26-2009 07:52 AM

Good post, Chemist1961...:yes:

Chemist1961 04-26-2009 07:56 AM

Insulating above the wall
re Bob's concern about the seal above the wall along the rim joist this info is constantly evolving. Roxul should eliminate the moisture concern, but read up on it. Foam methods provide a great barrier as well as insulating however most foam also requires a fire proof barrier be applied to cover it to resist potentially toxic fumes... so I'm sticking with Roxul for now as my basement walls and dropped ceiling will not be finished for some time.

jweiner 04-26-2009 08:46 AM

When we finished our basement, we applied spray-foam insulation directly to the foundation wall. The studs and roughing were already in place. The studs were about 1-1.5" from the foundation wall so the spray foam was able to get behind the studs to completely insulate the walls. This is a great alternative to batt insulation once the studs are already up.

whirly 05-01-2009 08:21 AM

As per Chemist1961 and from the Roxul site ...
I think the following applies to pink bat as well ....

Score the batt with a long serrated knife at the height of the wire. Then cut it to approximately half the thickness. Fold the batt back and fit the wire into the opening. Push the batt into place.
For large drainpipes, make two cuts at 45 angles to form a "V" groove. Then slip the insulation into place. Treat small diameter piping as wiring.
Electrical Boxes
Measure the electrical box and cut the section of the batt out. Slide the batt into place behind the box and push it snugly against the sides of the box.

mrpaulrenaud 02-08-2012 03:02 PM

This is all very interesting. To add my 2 cents, I read most of the new research for below grade insulation.

As I am finishing my basement, I used two 1" layers of double-faced, dura-foam reflective EPS staggered at the joints by about 8 inches, taped with tuck and then over-taped with reflective. In front of that, there is a 1.5" space for the foil to be of any use (minimum is 1" as I recall or the reflective properties are almost negligible). Then, framed 24 o/c in Bluwood (only) with roxul 6" bats.

The rim joist is sealed with 2" XPS, foamed around and another layer of dura EPS over that from the top to the bottom of the plate, also foamed. In front of that, r-14 Roxul.

This entire job was not very expensive really and the temperature is now consistent throughout the basement from bottom to top, with very few exceptions and it went up from an average 53-55 degrees to 69-71. The important thing is, "consistent". It's not 60 in one place and 95 in the other, which would be the sign that the insulation is not working correctly.

The only issue I know I will run into is that many (if not all) inspectors are not up to date and will likely require a VB in front of my bat insulation, which I will do and promptly remove once I get the ok.

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