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Dew point in wall calculation/calculator?

JohnWalker's picture

Is there a simplified means of calculating the dew point in a wall? I am considering various insulation strategies and want to assure myself I haven't created a significant sheathing condensation risk.

 

Thanks

 

John 

Short answer:  No. You need (post #196031, reply #1 of 15)

Short answer:  No.

You need to figure the temperature and dewpoint profiles and then see if the temperature and dewpoint lines cross.

The temperature profile is relatively easy, at least in theory.  But accurate moisture transmission numbers for the various materials would be hard (make that impossible) to come by, so dewpoint is a thumb suck.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Calculating the temperature (post #196031, reply #2 of 15)

Calculating the temperature at each layer of a wall is relatively easy to do. Most would assume that the RH inside would tend to drift outside ... increasing RH as it does. Making that assumption, the dewpoint would be easy to determine. But as Dan says ... the vapor transmission rates are difficult at best to really determine. Assuming that the transfer of moisture from in to out is uniform is probably the best and the worst case scenario.

There ain't NO free lunch. Not no how, not no where!

Here's a generally accepted method (post #196031, reply #3 of 15)

That software requires a Ph.D! (post #196031, reply #4 of 15)

Holly cow that software is comprehensive! (although the second demo put me to sleep).

Seems like using finite element analysis to size a window header or "30# sledge to crack a pistachio" though.

 

Alternatively is there anywhere I can read about wall assemblies/insulation/dew point etc specifically for Seattle's climate (figuring its nearly identical to Vancouver's (Canada)). From what I can gather that would be "Zone 4 - Marine".

 

By way of background I am designing a smallish tight envelop house with air tight drywall (called ADA here), wet blown cellulose, and either "Moony type wall" or more likely continous exterior XPS insulation. Its got to breath inwards (at least) hence the ADA.

 

thanks

 

John

You here in WA? Whereabouts? (post #196031, reply #7 of 15)

I've been getting to know the good folks at the WSU Extension Energy program down in Olympia. There are several people there who could field questions for you. You want I get a phone number or two?

You are unlikely to have any problem at all if you install 2" of exterior foam. What's the reason for the wet cells? You can net the walls and blown in dry material, and avoid having to try to dry the stuff before you rock.

There is a lot of good wall detail discussion over at GBA. Here are a couple of pieces on exterior foam and condensation:

http://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing

http://www.greenbuildingadvisor.com/blogs/dept/musings/using-rigid-foam-water-resistive-barrier

http://www.greenbuildingadvisor.com/blogs/dept/musings/how-avoid-condensation-your-walls

You Should Need to Only Contact Your Code Office (post #196031, reply #5 of 15)

You really should need to do nothing more than contact your muncipal building code office to determine under what energy code your state or province operates, and then insulate accordingly.

 

All the differing scenarios will be already worked out for you in your local code.

Manhattan42 I have the (post #196031, reply #6 of 15)

Manhattan42

I have the local (Provincial code) - all it says is framed walls are to be R20. I talked to an official and they just look at the number on the bag of insulation. If I frame 12 o/c and stuff it casually they rate it as R20 when in reality its probably R15. So having said that I am looking for ways to get the most bang for my buck without compromising the durability of the wall with condensation or lack of drying ability.

My current thinking is 2x4 @ 16 o/c with 3.5" wet blown cellulose, 1/2" plywood sheathing, 1.5 to 2" XPS on the exterior, rain screen and Hardi Plank siding.

Thanks

John

My current thinking is 2x4 @ (post #196031, reply #8 of 15)

My current thinking is 2x4 @ 16 o/c with 3.5" wet blown cellulose, 1/2" plywood sheathing, 1.5 to 2" XPS on the exterior, rain screen and Hardi Plank siding.

That'll do.

Don't forget vapor barrier on the inside, however. In your climate, a Class III VB will work, which can be 2 coats of latex paint over drywall, which is probably part of your plan anyway.

It might be less work and $ to go with 2 x 6 framing, 24" o/c along with 5.5" of blown cellulose. You could skip the XPS, which would make for easier siding attachment, and one less time of going all around the house.

How about (post #196031, reply #9 of 15)

2x4 or 2x6 @ 24" O.C. with 2x2 horizontal strapping on the interior @ 24" O.C.? Or did he already talk about that above?

2x2 interior cross strapping (post #196031, reply #11 of 15)

I looked into that because its "one pass" around the house rather than something inside AND something outside. Problem is 2x4 with 2x2 cross strapping (5" thick cavity) won't yield R20.

2x6 with 2x2 cross strapping - starting to consume too much floor space. This is an urban lot and I can't have 12" thick walls (drywall, sheathing, rainscreen, Hardi Plank clapboard etc).

Good idea though. Thanks

John

2x2 on 2x6 doesn't yield 1 (post #196031, reply #12 of 15)

2x2 on 2x6 doesn't yield 1 foot thick walls. Besides, the 2x2 could likely be an extension of the base plan, no a subtraction unless you are splitting hairs w/ e.g. a setback like and maximum plan width. Your foundation/slab dimensions are the same and the 2x2 adds to the exterior of that. The 2x6 @24oc is superior in terms of framing, cost, energy, etc.

There ain't NO free lunch. Not no how, not no where!

Any thermal FEA program will (post #196031, reply #10 of 15)

Any thermal FEA program will spit that out for you in seconds.

ok, 11-1/8" (post #196031, reply #13 of 15)

RE is so expensive here that some people energy retrofit with poly iso just to save a few inches.

 

2x2 over 2x6 ~might~ add up as follows:

1/2" drywall

1-1/2" 2x2

5-1/2" 2x6

1/2" sheathing

2" XPS

1/2" rainscreen

5/8" siding

= 11-1/8"

 

That could represent a large cost impact to the available FSR.

 

That said I would love to be able to purchase a half acre lot and orient/build whatever I wanted :-)

I used to calculate this (post #196031, reply #14 of 15)

I used to calculate this stuff in college but that was 20 years ago and I can't remember how now!..

This handy little calculator tells you whether there's a condensation risk but that's all:

http://www.vesma.com/tutorial/uvalue01/u...

Nice site, if it's halfway (post #196031, reply #15 of 15)

Nice site, if it's halfway accurate.  Too bad it's metric (or, perhaps, too bad we aren't).


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville