Insulating basement with existing VB, Worthy...

kparenteauJanuary 23, 2010


I have followed you postings and you have some great professional solutions.

For more reasons that I can shake a stick at, I needed to install a partial only interior drain system. The system is only on the front part of the basement and 6' on each side. All this feeds into a sump pump. When they built my house, they poured the slab at the same height as the footings because the floor height would be too high. Yup, there is only like 1" of slab concrete sitting over the footings at the walls. So the only solution was to put down a 20Mil. vapor barrier on the wall and drape it into the trough system they put in place. The also drilled (2) 1/2" holes into each first course of block. So any water that rises below the floor and and water coming into the block weeps behind the VB and goes into the gutter trough, which then flows to a 3 pump sump pump system.I had the system installed in late 08 and has been working great for its purpose. I can access the gutter system via 2 access ports and the gutters have been dry as the sump pit takes care of the ground water.

I now want to finish my basement. I already started a small section of the basement by putting down platon (1/4" dimpled membrane), taped the seams and tapcon'd 3/4" TG OSB. This has been installed for 6 months and doing good.

My question to you is what is the best approach to insulating the walls that have the VB on the wall. I planned on 1-1/2" XPS on the walls, followed by FB in the stud cavities. I live in a area of New England / North Mass. that only sees 20f-40f degrees for about 3-4 months of the year. I have asked the guys on the Holmes on Homes forums and all they do is bash me for my interior weeping tile system. They were usually helpful. My option that I needed to take was an interior system and I need and want to move forward.

I also plan on tiger foaming the joist cavities and foaming down the sill plate till the foam meets to top of the ridged XPS. I need advice on the insulation portion so I can move forward before I start studding the walls.

Here are some pics of what I have installed.

In this pic I replaced the 1" foam floor with 1/4" Platon membrane, taped, with 3/4" TG on top. The rest is that I think I should do??

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If you have so much water running down the walls that you need that sheeting to keep it under control, I couldn't in good conscience recommend covering it permanently with anything.

On the other hand, maybe the only water you're getting is from water vapour trapped by the vapour barrier turning to liquid water as it enters the basement in the summer. (The vapour drive is from warm to cold.)

If you only have a small amount of water on the wall, you could adapt the Building Science suggestion on page 5 ofRR-0509c designed for rubble or cobblestone walls. They use a proper waterproofing membrane attached to the interior basement wall and coming under the framing;

In that case, you could insulate with XPS and forget the fg altogether.

My own preference would be to excavate the exterior and install waterproofing. From what I can see on your Photobucket album, you seem to have easy access and the basement is not high.

    Bookmark   January 23, 2010 at 7:13PM
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Thanks for the reply Worthy.
There is actually no water running down the walls. They put the VB up only because they could not get there gutter system up against the floor / wall joint. The only way for the water to get from the holes drilled in the first course of block was to put in the VB to channel any water that "would" come in goes to the gutter system that is under the concrete at the floor / footer / wall joint.

I have checked the access ports over the past year and they are always dry. The sump pump is not busy winter and late spring through fall. So the only action is sees is the wet spring and a good T storm in summer. After that it is quiet.

If I have minimal water coming from the wall and all the action is from a high water table, how to proceed with insulating the basement..

I just want to insulate the best I can given my circumstances and move forward the best I can.

An additional exterior system is water well under the bridge.

    Bookmark   January 23, 2010 at 7:38PM
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That vapour barrier makes little sense, especially above grade.

If you still insist on walling it in--and I'd certainly hate to explain that vapour barrier if I were selling the home--I'd use only XPS friction fitted between the studs. I presume you're using a dehumidifier to keep the relative humidity below 45%.

    Bookmark   January 23, 2010 at 8:24PM
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Thanks Worthy..

So your saying to put the studs against the VB on the walls that have the VB. Then cut strips of XPS and friction fit between the studs against the VB? Should I put FG infront of the XPS so I have a "decent" seal before drywall?

At that point would it be easier to stub against the VB, then 2" spray foam to ensure that I get a good seal?


    Bookmark   January 23, 2010 at 8:41PM
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Hi Worthy,

I think I can adapt the page 5 of RR-0509c. This will keep the walls semi-warm in the winter. And also the walls will be less likely to get condensation on the VB as the foam will keep that part of the walls normal temp in the summer.
The rest of the basement that does not have the white VB on the walls will get the normal 2" XPS PL300'd to the wall. This will be connectd to rim joist foam sprayed down to meet the top of the wall that has the XPS. THis will create a no thermal break.

Your thoughts are greatly appreciated.

Again, only the front of my house has this white VB, the rest is dry, non leaking block which will get the XPS treatment.

    Bookmark   February 4, 2010 at 7:11PM
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To be on the safe side, I wouldn't put fg in the cavity with the 20mil poly. Better, use some more XPS or even EPS.

When you're finished, keep the relative humidity below 45% with a dehumidifier.

    Bookmark   February 4, 2010 at 8:18PM
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Thanks so much for your insight...

Yes, Any wall that has this VB against the block, I only plan on +/- 2" spray foam against the VB. And nothing else on those walls and no additional FG.

ON the non-VB walls I plan on 2" XPS + FG to fill the cavities. Not sure if I would need normal 4 mil poly after the additional FG before the drywall layer.

Thanks thus far Worthy. I'm sure you know your the person people look to on this forum for your expertise.

    Bookmark   February 4, 2010 at 8:27PM
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Also Worthy..

In the photo on page 5, what thickness in inches of sprayfoam would be a good thickness. I guess I would want a minimum in order to keep the wall from freezing , but also keep some of the coldness from leaching into the room once the wall is sprayfoamed from rim joist to the bottom of the wall..

    Bookmark   February 6, 2010 at 9:34AM
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I only plan on +/- 2" spray foam against the VB.

Spray foam willnot stick to the polyethylene. Use the foam boards there.

Not sure if I would need normal 4 mil poly after the additional FG before the drywall layer.

It's not needed, according to Building Science Corp. But inspectors will usually insist on it. The good thing is that, because you have the XPS on the wall, the added vb won't cause damage either.

what thickness in inches of sprayfoam would be a good thickness.

Enough to yield R-18-R-20, usually 3 inches.

    Bookmark   February 6, 2010 at 4:03PM
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