Crawl spaces in Alaska

[Jamesgang]

My nephew wrote to ask me about a house he bought in Wasilla, Alaska.
I understand the reasoning with the newer building science to seal crawl spaces and not to vent them as I was taught many moons ago.
I can explain that to him, but he asked me if he should insulate his floor joist because the floor is cold on the main floor.
As I write this I realize that I do not yet know whether they have carpet, tile or what ever as a finish floor, not that, that should matter much I suppose.
Would insulating the floor joists lessen the conditioning of the sealed crawl space?
Is pipe insulation enough for water pipes in a cold climate sealed, conditioned crawl?

[Alaskan Son]

There are a lot of variables. How deep is the crawlspace, what type of heat is it, whats in the crawlspace, how well is the crawlspace itself insulated? Is the house on a hill, built on sand, silt, gravel?

The short answer is usually that its not a good idea to insulate the floor around here, but that's not always the case either.

[m beezo]

sounds like the guys in Wasilla need to get together and figure it out. Much better idea than us lower 48 guys trying to figure it out.
I dont know how the houses are built there but for some reason never thought about a crawl space in the frozen north. I figured there was insulation anywhere and everywhere you could put it so what do I know.

[Jamesgang]

There are a lot of variables. How deep is the crawlspace, what type of heat is it, whats in the crawlspace, how well is the crawlspace itself insulated? Is the house on a hill, built on sand, silt, gravel?

The short answer is usually that its not a good idea to insulate the floor around here, but that's not always the case either.

I don't have all the answers, but I'll cover what I can;
Sounds like the crawl is about 3' to 3.5' deep. And the floor of the crawl is about 2' + or - below the outside grade.
I do not know if they are on a hill. He says it's a poly covered dirt floor, which means I couldn't say for sure whether it is dirt, sand, gravel or?? He does say that it does not appear to be wet and the underside of the floor does not appear to be wet or moldy.

Sounds like the exterior block foundation has two horizontal rows of batt insulation which don't reach the ground covered in poly.
Sounds like two non-operable vents in the joist space at either end of the house with a piece of fiberglass batt half assed hung in front of it to block direct wind, I suppose.

Forced air, from his explanation, it sounds like the furnace is in the crawl and the ducts are not insulated (unless it's on the inside and he can't see it. I don't know how common that might be up there)

Without actually seeing it, I could not say how well sealed and insulated it actually is.

[Alaskan Son]

That's pretty typical construction around here with the exception of the lack of insulation down to the poly. That's probably the majority of his problem. The rim and foundation walls should be insulated with at least R-19 from the subfloor all the way down to the poly, and there should be at least a 2ft batt layed on the floor all the way around the perimeter. That transition between the foundation and the floor of the crawlspace is a major heat loss point. It may be okay to insulate the floor, but I wouldn't recommend it. It can lead to moisture issues, and even foundation heaving, especially if the ducts in the crawlspace don't lose enough heat. Its really fairly important to keep the crawl space warm.

If the above recommendations don't help warm the floor a bit, he can either move to a warmer state, install in-floor heating, or if absolutely necessary add a very minimal amount of insulation to the floor, but keep an eye on what it does to the temperature in the crawlspace.

[Jamesgang]

I understand your explanations. Thanks for the advice, I'll pass it along.

If the furnace "is" in the crawl, I wouldn't think insulating the floors would affect the crawl temperature that much? What are your thoughts?

For that area, just how nuts do think he needs to go to make sure all the poly is hermetically sealed?

Should the vents be sealed off?

I would guess that we have a lot more moisture problems in my area than you would up there, but I suppose, like here, it depends on exactly where your at.

[Jamesgang]

Also, if I instruct my nephew to make sure the wall insul is at least r-19, and complete it down to the floor, under the poly as it is above.
The additional 2'wide batt around the perimeter, should it also be covered with poly?
Thanks,
Jim

[Alaskan Son]

Realistically, the poly doesn't need to be sealed off all that well unless he's in an area with high radon levels.

Vents should be left open if there's no other ventilation source to prevent moisture issues, but this isn't a concern with all homes. Ideally a bath fan on a dehumidistat with some air inlets (with P traps to stop warm air from leaking)work the best.

As counter intuitive as it may seem, the insulation on the floor usually goes over the top of the poly. The ground in that area is usually warm enough to prohibit condensation, and you're usually better off trying to keep the vapor barrier sealed to the foundation than to go over the top of everything. Ideally you could run the vapor barrier from the rim all the way down to the floor and tie the 2 together, but its really not necessary in most cases.

[davenorthup]

Here in the land of moldy crawl spaces (but no radon) we still have to seal up the poly in crawls very well.

There are no negatives to sealing them up 'hermetically' so might as well do it.

[Alaskan Son]

Dave's advice is good. Better safe than sorry. I have just personally seen very few crawlspace issues around here, and while I personally try to seal crawlspaces well, I don't like to make people feel like they're houses are going to rot to the ground if they don't. Truth is 95% of crawlspaces around these parts have very poorly detailed vapor barrier and have no real moisture issues to speak of with the occasional exception of the rim joist, but that's kind of another issue.

[Jamesgang]

Thanks so much guys. I think I have the gist of it. Every area is a little different and I appreciate the local advice.
Enjoy the holidays!

[davenorthup]

Truth is 95% of crawlspaces around these parts have very poorly detailed vapor barrier and have no real moisture issues to speak of with the occasional exception of the rim joist, but that's kind of another issue.

We just did a poly job on this one. It was spray foamed to the ground (including the floor joists, no ventilation and there used to be standing water in the crawl. Homeowner had someone regrade it - and within a month the mold had taken over - you can see the stains on the posts.

Homeowner had the mold dealt with, we put in the most expensive vapor barrier I have ever done and the HRV guy is going to mechanically ventilate the space.

We used 10MM poly on the floor - seams sealed with vycor tape. We installed PT 2x4's along the perimeter wall and used Tremco (black death) to attach the poly to the PT. Since the stud bays were filled with foam of varying amounts - I used gun foam between the 2x4 and existing wall foam.

What a PITA. Expensive mistake by whomever put this thing together.

BTW - our home inspectors here are not doing a great job. Nothing mentioned to this owner and heard another story today that the HO crawled in his crawl space and put his hand through a 6x6 structural post. Home inspector made no mention of it. We have a lot of moisture in our ground down here...

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[Alaskan Son]

Very impressive Dave!!

I've noticed most of the rotten and moldy house pictures I've seen are from Homer...

It must be that we have such a high percentage of gravel around here that it keeps the soil fairly well drained. Even on lots with no real gravel it seems to be mixed with gravel and almost always percs out without any problems. I think it may have to do with your precipitation levels as well coupled with the fact that you're closer to the open ocean so the air is usually more humid??

[davenorthup]

That and our expansive clay soil. Not a stitch of local gravel to be found anywhere near Homer. We get all our rock from Anchor Point.

I have learned with the helical pier business that most the soil on the bay side is horrible.... When the glacier formed Kachemak Bay the Homer side has all the mud pushed up (creating the Homer bench). It is now slowly settling back to the bay. No matter where you are in Homer, everything leans towards the bay. The south side (Halibut Cove) is one large rock.... Go figure..

[worthy]

It may be okay to insulate the floor, but I wouldn't recommend it. It can lead to moisture issues, and even foundation heaving, especially if the ducts in the crawlspace don't lose enough heat.


Mighn't it be more energy efficient to retrofit a frost protected shallow foundation?