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PNWpro

Crawlspace Rim Joist Frosting (cold climates)

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Hi all, new member here. 

I'm in a cold climate (-2 outside as I type this, -30 earlier this month). Over the  years (inspecting for 12 years in AK) I've come across frost behind fiberglass insulation in crawlspace rim joist bays many many times. It's a very common thing. My recommendations typically include better detailing of the ground vapor barrier (GVB) to reduce humidity as well as upgrading rim insulation to spray urethane or well detailed rigid foam. Always checking for low perm covering on exterior to make sure we're not trapping moisture. I'm curious if any other cold climate folks out there can share their experiences with frosty rim joists. 

Looking forward to being a part of this community. Looks like some great discussions going on here. 

Cheers.

IMG_4109.JPG

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Effective wall design for heat and moisture management is on the frontier in an environment as severe as yours, I believe.  Perhaps SIPs (structural insulated panels) would be a better choice.

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I'm in Oregon - no where near as cold a climate as you - and I occasionally see heavy condensation at the rim joists; it's just not cold enough here for it to freeze . Every time I find this, I find a corresponding high moisture condition somewhere in the space.  

Aside from your main question, I'm curious. Around here, foundation footings are supposed to be placed below the frost line. How do you cope with that in your environment? Do you just have super deep crawlspaces? 

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Right, we don't get the frost so much as moisture and white or greenish mold growing on the wood. In worse cases, fungus attacks the wood. Once there, it destroys the wood.

It comes from excess moisture in the crawlspace. I normally would recommend ventilation and improvements to drainage. A crawlspace I remember had been remediated with the sealed liner, sump pumps and dehumidifier. $25G spent but it was all too late. The subfloor was rotten all around the outer walls. The remediation was a ripoff. My clients walked.

I have also seen those OSB I-beam joists they use now with wood-destroying fungi in them. Too late by then to save the joist. Something to watch for.

Your pic shows moisture unable to escape, condensing on the cold surface. One thing we do here is install an electric baseboard heater in the crawlspace with a thermostat to keep the space warm enough to evaporate the moisture. Yes, it costs money to keep rot out of that OSB.

 

 

 

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Fiberglass w/o a vapor barrier in that location is plain useless.  A nicely fitted 1" thick EPS foam board would be a far superior solution and thicker would be better still.

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On 2/1/2020 at 5:28 PM, Jim Katen said:

I'm in Oregon - no where near as cold a climate as you - and I occasionally see heavy condensation at the rim joists; it's just not cold enough here for it to freeze . Every time I find this, I find a corresponding high moisture condition somewhere in the space.  

Aside from your main question, I'm curious. Around here, foundation footings are supposed to be placed below the frost line. How do you cope with that in your environment? Do you just have super deep crawlspaces? 

Jim, we insulate our foundations to compensate for the lack of depth. Basically trying to keep a "bubble" of earth below the home from freezing. We'll install rigid foam insulation at footer depth (typically 4'-8' deep for residential construction). The rigid foam is installed horizontally extending 2'-4' wider than the foundation, and 4" thick or more. ICF's are common. As far as protecting water lines from freezing the rule of thumb in my area is 10' deep for fresh water supply lines. 

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36 minutes ago, Chad Fabry said:

Fiberglass w/o a vapor barrier in that location is plain useless.  A nicely fitted 1" thick EPS foam board would be a far superior solution and thicker would be better still.

Agreed. I typically recommend 4" of EPS/XPS or 3" of spray urethane. I've seen FG with detailed 6mil in each bay, but what a pain in the butt to seal all those joist bays. 

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On 2/1/2020 at 7:49 PM, John Kogel said:

Right, we don't get the frost so much as moisture and white or greenish mold growing on the wood. In worse cases, fungus attacks the wood. Once there, it destroys the wood.

It comes from excess moisture in the crawlspace. I normally would recommend ventilation and improvements to drainage. A crawlspace I remember had been remediated with the sealed liner, sump pumps and dehumidifier. $25G spent but it was all too late. The subfloor was rotten all around the outer walls. The remediation was a ripoff. My clients walked.

I have also seen those OSB I-beam joists they use now with wood-destroying fungi in them. Too late by then to save the joist. Something to watch for.

Your pic shows moisture unable to escape, condensing on the cold surface. One thing we do here is install an electric baseboard heater in the crawlspace with a thermostat to keep the space warm enough to evaporate the moisture. Yes, it costs money to keep rot out of that OSB.

 

 

 

I agree that applying heat to a crawlspace, be it a small boiler zone or an electric baseboard, can improve durability and performance, especially when combined with controlling humidity. There's some lively debate about crawlspace ventilation in this area, as over ventilation can contribute to frozen pipes and high heating bills, and ventilation during the wet/humid months can contribute to mold. I prefer treating the crawlspace like a "mini basement" with a tight ground vapor barrier, good insulation and a small heat source as well as a ventilation fan on a dehumidistat. Really can't go wrong with this method. 

That failed remediation situation sounds like a nightmare!

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7 hours ago, PNWpro said:

Jim, we insulate our foundations to compensate for the lack of depth. Basically trying to keep a "bubble" of earth below the home from freezing. We'll install rigid foam insulation at footer depth (typically 4'-8' deep for residential construction). The rigid foam is installed horizontally extending 2'-4' wider than the foundation, and 4" thick or more. ICF's are common. As far as protecting water lines from freezing the rule of thumb in my area is 10' deep for fresh water supply lines. 

If you're doing a frost-protected footing that's 4'-8' deep, why not just build a basement while you're at it?  It would be the cheapest square footage in the house.  

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