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Insulating PWF basement

The "new plan" is to insulate the outer wall of my PWF basement with strips of EPS between the studs since I've found the material at a cost lower than mineral wool bats.. I'll cut strips of the sheet goods and install them much like bats for a total of 7 1/2"thickness. Inside the foam will be 3 1/2" of mineral wool as an ignition barrier for a cavity r of 45. Since my outer insulation is not air permeable, I should not need an interior vapor/air barrier. The question is is a friction fit of the foam enough of an air seal to prevent condensation on the sheathing, or should I use can foam around each panel? .

Asked by Jerry Liebler
Posted Tue, 06/17/2014 - 12:38

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9 Answers

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Jerry, you definitely cannot approach the theoretical R value that the rigid foam can give you if you cut and friction fit it between studs. Everyone here will tell you the bridging of the studs (heat loss to outflow of BTU's) must be offset by using contiguous (unbroken) insulation on the exterior. And rigid foam cannot be cut neat and clean; the time to cut and fit it is endless; you can't gang cut it as each piece needs to be measured and fit; the seams can only be sealed with tape so there is no R value in the gaps and that increases bridging as well as defeats the overall R value per stud bay and undoes your insulation and construction investment.

By the way, seam and air sealing tapes do not work so well on copper treated, wet and cold PT lumber. If you let PT lumber dry, it’s a lot harder to work.

Ground contact pressure treated ply absorbs water like any other, it just doesn't rot. It will pass vapor and water from outside to inside and vice versa, of a PWF.

A contiguous layer of foam (at least one layer of XPS) will be a great start at a double vapor/water barrier. The remaining layers can be EPS. The XPS has the better perm rating. Use overlapping seams and seam tapes on the foam. Insulate on the sides and bottom, use a tested combined vapor barrier/WRB on all five sides, outside your framing and rigid foam.

By the way, and I know this from my own mistakes. Many builders (like me) learned the hard way that we need to insulate and vapor/water seal the PWF to the top sills. The above grade section is also vulnerable to moisture so it needs to be moisture isolated and conditioned just as the below grade section.

Answered by flitch plate
Posted Tue, 06/17/2014 - 13:09
Edited Tue, 06/17/2014 - 13:11.

2.
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Fitch,
You said "Everyone here will tell you the bridging of the studs (heat loss to outflow of BTU's) must be offset by using contiguous (unbroken) insulation on the exterior.
Well, I don't think so! How "thermal bridging" works and what can be done about it are often miss understood. Wood has a rather poor thermal resistance/ " between r1/" and r1.25/" and it is going to conduct some heat when there is a temperature difference, that is NOT going to change by putting insulation over it! Insulating the spaces between the wood will reduce the heat flowing through the assembly but. not as much as a continuous layer of the same material. covering both the wood and stud spaces. With r45 in the cavities and accounting for the wood at r 1/
.25/" the area between the plates of a wall framed 16" OC will have the same heat loss as a uniform area of r37 or 9 1/4" of EPS to which there is no way of fastening any siding material.
Interior insulation does work! With one of the foam layers perimeter sealed with spray foam the foam and spray foam for an air seal that will keep moist interior air away from the cold sheathing.
The question is is it really needed? Using mineral wool bats in both the inner and outer layer would be less labor to install but requires an air barrier to keep interior air from the sheathing. It may well be simpler to use house wrap, taped and sealed on the inside of the outer wall.

Answered by Jerry Liebler
Posted Tue, 06/17/2014 - 14:37

3.
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Jerry … Why post the question if you have the answer?

Maybe you misunderstood this principle. The majority of heat loss in a well-insulated and airtight assembly is by thermal bridging.

I did not say interior insulation doesn't work (I use it all the time), I said that cutting and friction fitting rigid foam is a waste of the very feature you are paying for so dearly to get when you buy high embodied energy, HCFC processed, petroleum-based products for insulation: you lose the contiguous insulation and the thermal bridge disruption that exterior rigid foam can give.

Do the heat loss calc’s on your assembly. Someone here can give you the URL for the formula. Divide it into the cost of the foam and compare it to a contiguous assembly. With the poor fit in the stud bay perimeters, which you just can’t avoid, your heat loss rates will be 20% greater. That could be a summer advantage. You might want to do the calc’s with high moisture content PT lumber (sheeting and studs) vs dry PT lumber and sheathing.

http://www.buildingscience.com/documents/reports/rr-0901-thermal-metrics...

http://www.greenbuildingadvisor.com/blogs/dept/musings/how-install-rigid...

http://www.greenbuildingadvisor.com/blogs/dept/building-science/thermal-...

Are you suggesting mineral wool batts on the PWF exterior, underground? And Tyvek? How can you possibly maintain an R value in your wall with wet insulation and framing?

If you want good foam cutting tools, check Wind Lock:

http://www.wind-lock.com/66/ecatalog.htm

Answered by flitch plate
Posted Tue, 06/17/2014 - 18:42
Edited Tue, 06/17/2014 - 18:45.

4.
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"Maybe you misunderstood this principle. The majority of heat loss in a well-insulated and airtight assembly is by thermal bridging. "

To you it may be a principle but that doesn't make it trrue! The example I've been talking about with studs at r 13.5 and cavity at r45 with studs 1.5" wide and cavity 14.5" wide looses over 3 times as much heat through the insulation as it does through the studs. Adding insulation thickness between studs reduces the heat flow through the studs as well as through the insulation because the thermal path through the studs is lengthened.

"Are you suggesting mineral wool batts on the PWF exterior, underground? And Tyvek? How can you possibly maintain an R value in your wall with wet insulation and framing? "

NO! INTERIOR.

This question is motivated by the previous discussion in which Martin insisted that only a non permeable insulation should be used on the INSIDE of a PWF to avoid condensation, I grudgingly, am willing to accept his answer IF I can find a way to do it practically. The alternative is to follow the PWF design guide and install poly, or Tyve on the INSIDE of mineral wool bats.

Answered by Jerry Liebler
Posted Tue, 06/17/2014 - 21:07
Edited Tue, 06/17/2014 - 21:24.

5.
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Jerry,
You wrote, "This question is motivated by the previous discussion in which Martin insisted that only a non permeable insulation should be used on the INSIDE of a PWF to avoid condensation."

I want to clarify my position, so that there is no misunderstanding.

If you want to insulate your below-grade wall on the interior, you should use either rigid foam or closed-cell spray foam on the interior side of the pressure-treated sheathing. The reason for this recommendation is that you want to prevent warm, humid interior air from contacting the sheathing.

It is also possible to insulate your below-grade wall on the exterior, using rigid foam (XPS or EPS), mineral wool, or closed-cell spray polyurethane foam. If you decide to insulate the foundation on the exterior, you need to protect the above-grade portion of the insulation from UV rays (sunlight) and physical abuse.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 06/18/2014 - 04:44

6.
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The question is is a friction fit of the foam enough of an air seal to prevent condensation on the sheathing, or MUST I use can foam around each panel? .I think the answer is a friction fit is fine because, while some condensation may occur, the condensation itself will act to complete the air seal..

Answered by Jerry Liebler
Posted Wed, 06/18/2014 - 08:07
Edited Wed, 06/18/2014 - 08:17.

7.
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"I think the answer is a friction fit is fine because, while some condensation may occur, the condensation itself will act to complete the air seal.."

Whaaaat?

Answered by Malcolm Taylor
Posted Wed, 06/18/2014 - 11:30

8.
Helpful? 0

Jerry,
If you want to install rigid foam against the interior side of the sheathing, you have to do it in an airtight manner. "Friction fit" won't work.

The method you propose is called "cut and cobble." Most cut-and-cobble devotees seal the perimeter of each piece of rigid foam with caulk or canned spray foam. More details can be found here: Cut-and-Cobble Insulation.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 06/18/2014 - 14:25

9.
Helpful? 0

Martin,
Thank you for the link! I do believe that a PWF is about the lowest risk place to use "cut and cobble" There are no significant consequences of a less than perfect air seal as the heavily pressure treated wood can be very wet for a very long time and will never support mold growth or loose strength. I suspect that the amount of moisture accumulated in a PWF, with cut and cobbled foam, over a season will be less than a mineral wool insulated PWF with an interior poly air barrier..

Answered by Jerry Liebler
Posted Wed, 06/18/2014 - 15:35

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