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Insulate shared wall between basement and crawl space?

Insulate shared wall between basement and crawl space?

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Old 11-11-17, 10:49 AM
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Insulate shared wall between basement and crawl space?

I have an insulation question. First, some background:

I have a heated basement that shares a concrete wall with an unheated crawl space. The crawl space is part of an addition done after the home was built. Over the crawl space is a small master bathroom. The master bathroom gets very cold in the winter because it is not well-insulated and the crawl space is very cold (whatever the outside temperature is). Also, the water pipes run through the crawl space to the bathroom above, and we've had them freeze up in the winter before.

Now, the question: We're finishing our basement and insulating all the walls. We're torn about whether to insulate the particular portion of the concrete basement wall that has the crawl space on the other side. On the one hand, we want the basement to be as insulated as possible, for energy efficiency (it was never insulated before). On the other hand, we don't want to make the crawl space even colder in the winter because it's getting no warmth from the basement through the concrete wall (though we don't know, really, if the warm basement air is helping warm the cold, unconditioned crawl space at all).

So, to insulate the shared concrete wall or not?

Thanks!
Rebecca
 
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Old 11-11-17, 10:56 AM
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It would make more sense to insulate the exterior foundation walls in your crawl space, seal up the crawl space floor, then condition that area.
 
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Old 11-11-17, 11:00 AM
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Thanks for your quick response. I have a bunch of follow-up questions:

1) You mean leave both sides of this shared wall uninsulated, then insulate the rest of the crawl space walls?

2) Seal the floor of the bathroom? How?

3) How condition the crawlspace? I can't imagine a non-MAJOR way to get an hvac duct from our main system into there. Is there another good option?
 
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Old 11-11-17, 11:05 AM
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Yes. If you insulate the shared wall, the crawl space will only get colder. The crawl space foundation walls should have been insulated during construction. If they were not, your options are limited to putting foam on the interior walls... as deep into the soil as possible. The exterior side of those walls can also be excavated and insulated to isolate the frost from the crawl space soil.

Not the floor of the bathroom. The dirt floor in the crawl space. Heavy 20 mil poly sealed to the walls with mastic.

How is the addition heated?
 
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Old 11-11-17, 11:14 AM
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Thanks again! That makes perfect sense. We will be insulating the crawl space from the inside using 2" foam board (CI max, to be specific). To my knowledge, it is not insulated at all.

There is something plastic-like on the dirt floor, but I doubt it's 20 mil. Thanks for that tip.

The bathroom has baseboard heat. It also has one hvac duct/vent in the corner of it it shares with the main house, it but it hardly blows because it's really squished between sill plate and who knows what? It's an old addition that was clearly not the most awesome construction job.
 
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Old 11-11-17, 11:26 AM
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You could see about extending the ductwork into the crawlspace before you completely finish the basement... and add/fix the duct that doesn't work well. It may seem like a waste to add ductwork into the crawlspace but it will keep the floor warm and prevent the pipes from freezing. The poly on the dirt keeps moisture and odor out... but it needs to be sealed.
 
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Old 11-11-17, 12:27 PM
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Thanks again! We actually just had our ductwork replaced and our great HVAC guy did his best, but it's just like a whole separate structure....

One more question. Hubby just discovered that underneath the plastic on the ground in the crawlspace is concrete slab. Is there a need for the plastic on top of that? Any reason we wouldn't add foam board insulation on top of the concrete floor, in addition to putting it on the cinder block walls?
 
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Old 11-11-17, 02:03 PM
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If the floor is concrete that is already covered by plastic, I would recommend you add a minimum of 1 1/2" rigid foam to the floor and 3 outside walls. 2" would be better. Tape all seams.

Regarding the hvac, I would imagine that he would just need to drop the ductwork down and punch some new holes to get it out there. If it is in the scope of work, I'm sure he can do it. He probably was just trying to maintain headroom by keeping it all as high as possible, but sometimes you either have to cut out some structure or drop down and go under it if it can't be cut.
 
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Old 11-11-17, 05:06 PM
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Makes perfect sense about the foam board.

As for the HVAC, because of a lack of space by the sill plate, the only way to get a duct into the crawl space that I can imagine would be to knock a hole in the concrete wall of our basement. I'm not sure my guy does that kind of thing. Is that even possible? Would it affect the structural integrity of the house?

Does all this insulating even make a difference without a heat source in the crawl space?

Thanks!
 
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Old 11-11-17, 05:40 PM
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"Knocking a hole" is a crude way of putting it. But yes, cutting a hole through a wall for a duct is something that is done all the time, with the usual precautions, of course. I can't see your wall or the structure above it, but suffice to say, yes it is likely possible. I assume you have an access door to this crawl space, so if that is possible, then a few holes for ductwork are likely possible too, especially when kept between the joists. Access holes should have been incorporated into the design of the crawl space from the start.

The insulation will help keep the frost outside, heat or no heat, because it should be warmer inside than the ground is outside. Things get cold because of heat loss. This is especially important on your exterior walls and the floor that adjoins them. An uninsulated space can't keep up with the heat loss and eventually all the heat is take from the space, and it becomes cold. The ground is usually a constant 50F or so if you stay away from the frost layer. That's the temperature the crawl space floor "could be" if you could just stop or slow the heat loss. You also have "some" heat loss from the shared basement wall, which is say 65-68F or so... Opening even more ventilation would increase the heat loss from the basement, warming the crawl space. But adding ducted heat to the crawlspace will help your cause tremendously.

If that is not possible, I would suggest you add some form of ventilation from the conditioned space in your basement and draw air out of there by some other means, such as an inline duct fan or louvered crawl space access door. Heat tape on your pipes is also an inexpensive option that would at least prevent the pipes from freezing. You could also ensure the space is air sealed by looking for daylight inside the crawl space under the bottom plates... and caulk those cracks up tight. Also insulate the rim joist (exterior perimeter of the floor framing).
 
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Old 11-11-17, 05:54 PM
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We will follow-up on all this. Thanks!

The space really is quite strange. The bathroom and crawl space underneath it are at the back of an unconditioned garage, which was added at the same time. The only way into the crawl space is through a small window between it and the garage. There is no access from the house into the crawl space.

From the basement, the space in the joist above the sill plate by the crawl space are completely filled with ducting (the duct to the bathroom above) with no room to squeeze more. Any further ducts would need to be run lower then fed through the concrete to the crawl space. I just don't think we have that in us right now.

Hubby insulated the three walls of the crawl space today and will do the floor tomorrow. Is there any reason to have plastic between the concrete floor and the foam board insulation? The 2" foam board will act as a moisture barrier itself, as I understand it....
 
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Old 11-11-17, 06:11 PM
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You will want to leave the existing plastic in place on the floor. it is probably more continuous than the foam will be which has joints... thus not a perfect barrier unless it's been completely taped and sealed.
 
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Old 11-11-17, 08:59 PM
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Thanks! We'll do it! Really appreciate all your help.
 
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