Insulating Basement Walls


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Old 11-12-18, 08:47 PM
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Insulating Basement Walls

I'm starting in on finishing my basement. Right now, it's got foil-faced fiberglass blanket anchored to the walls and unfaced fiberglass stuffed in the rim joist bays. Thinking of replacing all this with 2" rigid foam. Good idea? Unnecessary? I think doing the rim joists definitely would be an improvement, but maybe the walls wouldn't be much of a change, *except* that code requires R-10 for a finished basement, and I don't think the blanket provides that, does it? The 2" rigid would. And it looks a whole lot nicer.

By the way, what kind of caulk should be used around rigid up in the rim joist bays? I've read not to use siliconized latex, because it can dry out, is that correct? Something about the oils in it being absorbed by the raw wood?

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Old 11-12-18, 11:24 PM
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Well the big question is what are you going to do with the basement? If you are going to finish it off with drywall then you need stud walls, if it's just for storage It doesn't have to be changed.

For the rim joist you should use an exterior caulk like Quad!
 
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Old 11-14-18, 07:43 PM
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Yes, I intend to build frame and put up drywall. I'm just wondering if it's fine to frame against the blanket insulation (I know I shouldn't compress it), or whether it's a better idea to remove the blanket, replace with rigid, and then frame against the rigid. My intent right now is to leave the bays between the framing studs empty, so the only insulation will be either the blanket or the rigid against the concrete walls. Although saving money by not buying the rigid insulation would be nice, cost really isn't that big of an issue.
 
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Old 11-15-18, 12:01 AM
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it's got foil-faced fiberglass blanket anchored to the walls
Not really familiar with what you have there, what kind of insulation, thickness/R value. Is it a large blanket?
 
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Old 11-15-18, 04:40 AM
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Here's a link for background guidance. Photo #4 is what I like.

Bud
 
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Old 11-19-18, 09:36 PM
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Thanks, Bud. I think I will go ahead and replace the blanket with rigid. My basement is fully below grade, and my original thinking was to leave the stud bays empty. (Maybe I can repurpose the blanket insulation by filling the bays with it?)

Separate question - say I put rigid foam behind the water piping and shutoffs for the outside spigots. Since the rigid foam is flammable, code requires it to be covered with some fire barrier such as drywall. How would I cover the rigid foam behind the pipes while still leaving the shutoffs accessible? Would I put the shutoffs behind drywall, but put in an access panel so I could get to them when necessary? Seems that's the only way around it.

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Old 11-19-18, 10:02 PM
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If you re-purpose the blanket omit as much of the vapor barrier as possible.

Where drywall will not be covering a small area of rigid I would not be worried.
Two options is covering is necessary. One is an intumescent paint but not sure if available in small quantities. The other would be to use some Dow Thermax that has an approved foil facing that can be left exposed.

I would avoid anything that prevents the basement heat from reaching the pipes. Always insulate between pipes and the cold but not between pipes and the warm.

Bud
 
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Old 11-19-18, 11:32 PM
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Re-purposing the fiberglass from the blanket was just a wild thought...roger on not using the foil facing. Might be really messy/dusty pulling that facing off.

OK, yes - of course I would want to leave the piping exposed to the room air, not put it behind a drywall access panel, I wasn't thinking there.... It looks like between the pipes and the concrete wall there is not enough space to put 2" of rigid. I think only 1" thickness will fit there. I will look into the Thermax and the intumescent paint. Thank you for mentioning these two options!
 
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Old 11-20-18, 04:28 AM
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1" will be fine. With Thermax it is a polyisocyanurate at R=6.5 per inch and the exposed foil adds a bit more. Considering a concrete foundation is close to R-1 the 1" poly is a huge increase, and this is a small area. If you are being inspected the Thermax would be the best choice.

One key step is to foil tape all edges and seams.

If you want to cover the pipes just use something thin and maybe perforated so the heat can get through. I do like access panels.

Note, when I added rigid to my basement Thermax was difficult to get so I used Dow Tuff-R. It doesn't have the Fire rating that Thermax has but I bet it is close, thick foil layer. However I didn't see the same product I used available at orange box. I did see a lot of Thermax so maybe no need to carry the other.

Bud
 
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Old 11-20-18, 10:26 PM
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Bud, you're a great resource. Thanks!
 
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Old 12-04-18, 11:55 AM
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After I've taken down the blanket insulation, I should plug the anchor holes in the concrete walls, right? Is there a recommended material for this?

And...I don't have any water issues now, but should I seal/waterproof the concrete before adhering the rigid foam? If so, is there a recommended product?
 
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Old 12-04-18, 02:10 PM
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Nothing you insulate with will solve a water issue. Rigid foam will not wick moisture from the wall and will not allow air to pass vertically. The foam will create a thermal break and prevent condensation from forming, but it will not address any water infiltration. You would not waterproof the foundation before installing the rigid foam. If you have a water infiltration issue, it would be best solved from the exterior of the foundation.
 
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Old 12-04-18, 06:57 PM
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As stated, I don't have any water issues. I was just wondering if it might be a good idea to proactively coat the walls with something while I have the chance, before they are hidden behind insulation, framing, and drywall, and what that something might be.
 
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Old 12-05-18, 12:47 PM
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I'm sorry, I meant if water finds it's way through the exterior waterproofing in the future.

I personally would not bother with painting on a sealer, as water will just keep moving till it can get past it somehow. One thing you might want to do is a careful inspection for cracks in the foundation walls. Any horizontal ones would be concerning. Larger vertical ones can be ground out and filled, smaller ones are not really an issue if they have not appeared or grown recently. You may just want to map them out anyways before insulating so if you do have a water issue eventually, you know where to start hunting.
 
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Old 12-05-18, 05:59 PM
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I don't think I have any cracks, but mapping anything I find is a really good idea. It looks like any cracks I might find should be filled with an expanding urethane. Is there any such product for a DIYer to use?
 
 

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