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Old 01-10-2017, 12:14 PM   #1
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Insulation and vapor barrier help


I have a couple of questions:

New house build, Zone 5

1). On above grade exterior walls (Zone 5) I am planning on using the flash and batt method. 1" spray foam and fill the rest with fiberglass batting (2"x6" walls). Do I need a vapor barrier on the interior wall (between studs and drywall)?

2). Exterior 8" concrete foundation, (basement) below grade. I will be putting 2" XPS rigid foam on the exterior of the concrete. When I finish the interior of this floor with 2"x4" framing, what do I need for insulation and vapor barrier?

Thanks

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Old 01-10-2017, 01:41 PM   #2
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Re: Insulation and vapor barrier help


Here's a link that discusses a similar situation involving exterior foam in a wall assembly and whether to use or not a vapor barrier. Not is the answer but here is the link:
http://www.greenbuildingadvisor.com/...od-thing-right

As for the exterior rigid on the foundation my concerns would be how well it can be encapsulated and protected from physical damage and invasion from insects, like ants. Ants love rigid foam to nest in and they only need the smallest opening to do so. As for the interior the stud wall cannot be in contact with the foundation and I don't like using treated wood inside the living space. Plus, the wall shouldn't be set an inch away from the foundation because that allows air to circulate.

Zone 5 is not exceedingly cold so I would keep all of my rigid on the inside, detail the rim joist, and set my wall directly against the rigid. Then fill the 2x4 wall with Roxul (no vapor barrier) and cover with drywall.

BUT, before you consider finishing the basement you need to be sure water and moisture vapor issues have been addressed on the outside all the way down to and under the footings. A dry basement requires a major effort.

Bud

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Old 01-10-2017, 01:48 PM   #3
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Re: Insulation and vapor barrier help


Thanks for your reply,

Actually, code here is 2" rigid foam on the exterior of the foundation, so I don't have a choice on that. Another thing, whether it's right or not, all basements here frame with 2x4's right against the concrete. That's all I've ever seen.

With 2" foam on the outside, I'm wondering what is needed on the inside when finishing the basement.

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Old 01-10-2017, 02:32 PM   #4
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Re: Insulation and vapor barrier help


Not to be argumentative, but here is a link to the 09 IECC, the most frequently referenced insulation requirements and NONE of the zones specify exterior only. They will say interior or exterior. Determine what code your local authority is following and ask why if they insist on exterior only.
https://energycode.pnl.gov/EnergyCodeReqs/

If you must install it on the outside for some reason (local codes rule), consider an overhang at the rim to allow the sheathing to be flush or extend outside of the rigid or whatever you use to cover it.

As for non-treated wood against the foundation, I believe that is a code issue. The homes that have done so may have installed a plastic vapor barrier between the wood and the foundation, but that becomes a problem as to how the concrete will dry.

"That's all I've ever seen" not a good way to approach a new home with all of the modern energy improvements available. Things have changed a lot and much of it is very good. Building new gives you an opportunity to easily and inexpensively incorporate many energy efficient upgrades.
https://buildingscience.com/document...ts?full_view=1

Bud
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Old 01-10-2017, 03:36 PM   #5
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Re: Insulation and vapor barrier help


I'm guessing I was told foam on the exterior because I told him the basement will be unfinished at this time.

Thanks for the info.
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Old 01-10-2017, 03:51 PM   #6
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Re: Insulation and vapor barrier help


Bud9051-

You sound fairly knowledgeable on this subject, just for the heck of it, if this was your house, how would you insulate the foundation on an unfinished basement, 8" poured walls with a walkout.

I did misspeak, I am in zone 6 up here in Minnesota.

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Old 01-10-2017, 05:33 PM   #7
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Re: Insulation and vapor barrier help


You may be correct that the requirement boils down to conditioned (finished) vs unconditioned space and being a new home you would need to follow the newest codes they have adopted. The 09 codes I linked above already show zone 6 as requiring r-15 so that would need to be clarified.

Since rigid insulation is so expensive, my choice would be one or two inches of rigid on the inside, then my stud wall, then fill those cavities with the Roxul. The rigid would protect my framing and give me a continuous insulation layer. Then the r-15 in the cavities should bring my total above the code requirement. Of course you would need to complete the wall with electrical and drywall but that might be no more expensive (maybe less) than the full exterior rigid plus whatever is needed to cover and protect it. Besides it would be done instead of being on your future project list.

Bud
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Old 01-11-2017, 09:08 AM   #8
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Re: Insulation and vapor barrier help


Thanks Bud9051,

I'm trying to figure out why less rigid foam would be used if I did it on the inside vs. the outside? I can see a touch difference due to the linear footage of outside is slightly larger than the in side due to the concrete thickness. I may be missing something, please correct me if I am.

Not that I don't agree with your suggestion, but this would mean for me to do almost the complete finishing of side of the exterior walls. The least I could is rigid foam around the interior and stud wall. Leave the electrical and batting for later.

Currently my budget cannot afford any finishing of the basement, but I do want to do it the best possible yet affordable way.

Thanks again, I do value any of your suggestions.
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Old 01-11-2017, 10:10 AM   #9
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Re: Insulation and vapor barrier help


First you would need to confirm whether your basement will need r-10 or r-15, interior or exterior. back in 09 it was r-15 so not sure why you were told r-10.

but in any case on the inside you can combine the rigid r-value with the cavity fill and exceed your requirement starting with only 1" of rigid. That saves a little if you are meeting r-10 and a lot if you are meeting the r-15.

Basically, by nor finishing it now they seem to be requiring you to meed the full code and thus on the outside. If you put it on the inside you have to cover it, but that just means finish the walls.

I have encountered a number of homes with exterior rigid and unfortunately time has not been kind to the foam. One covered the foam with a parge coating of cement which when I was there was broken and falling off in spots. Another was left exposed and was in poor shape from the sun and physical damage. Other than that their performance was as expected, but not really better than on the inside.

Personally, I have had large and small ants attack my foam boards while in my storage shed. Over one winter some very small ants turned 2 4x8 sheets of 2" blue into swiss cheese. Since my house is covered with 3.5" of blue for exterior insulation I will be super sealing the bottom area and applying some chemicals to be sure they don't get in. 3 puppy dogs limit where and how much insecticide i can use.

The other saving is no longer needing to protect the foam board. Just gluing it to the exterior without protection above and below grade would be a mistake. Plus there is the aesthetic issue of the rigid extending 2" or 3" out past the foundation, plus a covering. Sorry, I'm rambling

Bud
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Old 01-11-2017, 10:46 AM   #10
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Re: Insulation and vapor barrier help


Thanks or the clarification.

I was told 2" rigid which is R-10, but I am not sure why that was said. I'll look into this further. When talking about this to my builder etc., there is a concern on (as you said) how to cover it up and keep it protected.

I'm glad you gave me your suggestions as this will most likely change the way I do it. Interior application may be the way to go.

As far as the outside of the basement walls, do you agree that I would just put the water proof spray (or whatever is appropriate) and just back fill it? With the appropriate drainage at the footings of course.

Thansk
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Old 01-11-2017, 12:50 PM   #11
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Re: Insulation and vapor barrier help


<"As far as the outside of the basement walls, do you agree that I would just put the water proof spray (or whatever is appropriate) and just back fill it?" The term waterproof spray can vary from something not much better than paint to some of the newer (newer to me) materials which cure to a continuous membrane and will last for years. The latter would be better but only your builder would know.

The process of creating a truly waterproof basement starts below the footings and below the slab. The link I usually reference is no longer there and a quick search did not turn up another source but I'm sure they are out there.

Part of the decision making process deals with your local soil make up and the nature of your building site. Example, having perimeter drains around the footings does little good if there is no place for that water to drain to. And wet footings can feed that water up into the entire foundation wall.

I know you are familiar with that phrase "that basement smell", it is an example of how difficult it is to make a dry basement. Do some searching to see if you can find an article that starts with a moisture barrier below the slab and footings, then you will know they are serious.

Bud
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Old 01-11-2017, 01:23 PM   #12
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Re: Insulation and vapor barrier help


Late to the party.... Zone 5 requires R-7.5 (ccSPF), and Zone 6 = R-11.25 in your 2x6 wall to limit condensation on the inside face of the SPF where your HVAC system needs to remove it; https://buildingscience.com/document...r-requirements

Then you would be good (in Zone 5) up to 35% Relative Humidity.... which- IMHO- is a little risky with the SPF (0.82 Perms) bonded to the OSB (1 Perm) for total of around 1/2 Perm as you are doubling them- your vapor retarder in the middle of wall. So if the OSB does get wet, say with Hardie or wood lap siding and no rain-screen, it would only dry to the outside (because of VB placement location) with no pressure/temperature available to drive from the inside to help- hence- much longer dry time= longer time to rot OSB; http://www.ntcinsulation.com/the-ins...-cold-climates

Could you describe the wall make-up....

Gary
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Old 01-11-2017, 02:38 PM   #13
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Re: Insulation and vapor barrier help


Gary,

It is an 8" poured concrete foundation wall mostly below grade (basement). Zone 6.
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Old 01-11-2017, 04:42 PM   #14
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Re: Insulation and vapor barrier help


I was asking on the above-grade wall, where you need 2-1/4" of SPF (cc) as the "flash" against the OSB. IF set on using only 1" SPF, then you also need an interior vapor retarder (Type 2- asphalt paper face, etc.) per first link I gave.

Gary
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Old 01-11-2017, 04:50 PM   #15
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Re: Insulation and vapor barrier help


Quote:
Originally Posted by Gary in WA View Post
I was asking on the above-grade wall, where you need 2-1/4" of SPF (cc) as the "flash" against the OSB. IF set on using only 1" SPF, then you also need an interior vapor retarder (Type 2- asphalt paper face, etc.) per first link I gave.

Gary
I am doing spray foam on the main floor 2x6 walls. I was looking for info regarding my basement insulation options.

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