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Basement finishing Insulation question.

2873 Views 12 Replies 6 Participants Last post by  Gary in WA
Hi all at DIY. I've been trying to find a forum for home renos for a while and not many exist.

I'm redoing my basement. I live in a back split in Ontario Canada and I'm redoing the lowest level basement. The fiancées father did it before and he did a horrible job... (running wires outside walls, unfinished drop ceilings, poor framing and drywall work etc.)

So needless to say I'm ripping everything down and am starting from scratch...

I've redone 2 full bath rooms before, a powder room, and most of a kitchen before so I feel I got a decent understanding of how things are done though I've never taken on a project as big as a whole basement before.

I've been doing some research and I'm thinking of doing 2 in rigid foam on the concrete walls and filling the vats on the wall with your standard fiberglass insulation then 1 in on the floor..

I read and they seem to suggest the same from what I can tell after doing my best to decipher all the technical stuff.

My question is with the rigid foam is it simply glued to the wall and floors with a construction adhesive approved for foam?

I saw this video and was wondering if its as simple as it looks?

To me it looks like its done in the following steps.

1) The foam on the floor is laid without any fastening or adhesive. The joints are sealed with im assuming is either silicone or construction adhesive

2) Cut the 1/4 in expansion slot

3) fill the expansion slot with foam (wouldn't that take away from its ability to expand though?) and then glue the wall boards to the wall with just a few dabs of glue and seal the joints same as before.

4) Tape all joints.

5) The plywood subfloor is screwed to the foam and then you're off to the races framing, insulating vats, drywall etc.

Is that pretty much it?
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"My question is with the rigid foam is it simply glued to the wall and floors with a construction adhesive approved for foam?"-------- Yes. The adhesive pattern is important against air movement;

Hence the caulk/canned foam and tape, I recommend. F.b. on the rim joists, also;

The foamboard "expansion" gap is not required with f.b. as it shrinks over time, not expands. Your local AHJ may require fire-stopping every 10' horizontally and at top of wall per minimum code;

ADA the drywall as a first defense against air to the concrete;

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Basement Insulation Studies

The two places I have seen studies of insulation in below grade interior basement insulation for perimeter walls are Building Sciences and a lesser known one, "The Challenges of Basement Insulation," by Aldrich and Zuluaga from 2006.

As discussed many times on this forum, circulating air is the biggest culprit, i.e. air getting behind the insulation systems, and where FG fails. Both camps agree that rigid foam insulation is superior, however, Building Sciences advocates rigid foam to the floor, while the "A-Z" study advocates stopping short and leaving a 6 inch space before the floor. Their reasoning is a small leak will run down and have excess water quickly dry.

Has anyone had experience with the rigid foam to the floor, sealed, and then a leak develops? Does the water just continue to collect behind the sealed foam until the water finds a way out, maybe through a taped joint?

Then A-Z study mentioned how trapped moisture (not vapor) was a possible issue with rigid foam to the floor and they advocate leaving a 6 inch gap.

I am trying to reconcile these two studies, had anyone else examined both?

I know this has been covered ad nauseum on here but I haven't seen mention of this other study.

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this is good topic

for new construction.. since about 2005 code requires that all basement slabs have " foam and sealed plastic below the slab, this creates a thermal break and stops moisture from coming up through.. for older homes having a 6" air space just isnt possible most of the time for head room reasons.. so using a floating interlock subfloor system works better

regarding a leak the key is to stop the leak at the source if it ever occurs, othewise your always going to have issues
"My question is with the rigid foam is it simply glued to the wall and floors with a construction adhesive approved for foam?"-------- Yes. The adhesive pattern is important against air movement;
That link doesn't describe adhesive patterns.....

Hence the caulk/canned foam and tape, I recommend. F.b. on the rim joists, also;
Do I need to just glue it at the rafters in the same manner as the walls and then seal with spray foam?

GBR in WA;1082212The foamboard "expansion" gap is not required with f.b. as it shrinks over time said:[/URL]
Is that all foam board? Should I just get it as close to the wall as possible and then fill it with spray foam then?

GBR in WA;1082212ADA the drywall as a first defense against air to the concrete; [url said:[/url]
I was planning on a drop ceiling.... That would make it difficult to seal at the top... Any other suggestion?

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I just wanted to point out my other thread which has this link

which disagrees with ceiling to floor continuous rigid insulation...unless I'm reading it wrong.

If a leak develops....and we all know homes never spring a leak :whistling2: a sealed continuous rigid foam insulation method will damn up the excess water against the concrete wall with no escape other than vapor.

I was all set to do the Building Sciences method when I discovered this article done around Chicago....which is my zone.


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Don't caulk horizontally. It will drip down and show if caulk vertically.
Ok so verticle strips for the glue..

I read that article and they seem to think leaving 6 in from floor is best... But how are you going to finish the floor then? I was under the impression that with the f.b. On the walls and floor its one continuous system.. So any moisture if any will condense on the concreat and run down the walls being evaported or for major issues down the drain. Under the floating floor since its all sealed...
It appears you are skipping some important steps in your reading. Foamboard installed air-tight to concrete will insulate and vapor retard any cold moisture coming through the concrete. If you air-seal the f.b. and glue as mentioned; with the "closed grid pattern" (horizontally and vertically) which limits the surface area of the air movement and wetness to diffuse through-- rather than drain-down to collect/pool at the bottom- possibly wetting wood framing (to mold, being organic) or cavity insulation from wicking- degrading it's R-value- if fiberglass;

If you have a perimeter interior drainage system, the application would be different. Because it is an insulating sheathing board- it raises the dew-point of the cavity materials- wood framing/fiberglass or Roxul insulation- so you don't get condensation there. With plain f.g. batts, the dew-point would be the temperature of the concrete wall. For those locations with a deep frost-line and above-grade wall areas, foamboard is essential to control condensation because of the cold temperatures at those areas. Leaving a 6" gap at the bottom will create convective loops behind the drywall, warming the top of the wall/ceiling joists rim area 24/7 when the heat is on. The gap promotes air flow, much better to have dead-air (whole purpose/design of fiberglass insulation); The foamboard is the vapor retarder mentioned on the first page of that link.

Compare the different basement insulation strategies listed on pp.7, and the moisture potentials of that wall throughout the article;

Using your 3 yearly average low temperatures= 14* at 70* interior basement temperature, using R-5 f.b. and R-15 (3-1/2") cavity insulation, you would be safe at 23%RH and below. With R-10 XPS and R-15 cavity, above grade and to frost-line = safe at 48%RH, much better.

PS. I merged your other similar thread with this one.
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I applied foamboard adhesive (ML-300?) to the cleaned (make sure no loose debris) walls in a grid pattern (say 4 strips top to bottom and 6 or so horizontally). I braced each sheet overnight. I framed 2x4 tight against the XPS. For the rim joists, I put in 3 1" sheets snug against the rim joists (no gaps if possible) and foamed, and/or caulked. It has been said that it is better to caulk in case of expansion/contraction.

Ok sorry for being difficult but I'm getting confused now because of the two threads merging :huh:

So my basement is mostly below ground. I got maybe 1.5-2 feet or so above ground. the news/HomeEnergy_The Challenges of Basement Insulation.pdf

This link mentions stoping the insulation about 6 inches from the floor to let the moisture dry out.

this one mentions that leaving any air space would lead to more moisture.

I was also talking to another friend and he was saying that in his reno he was just going to put dimple mat or another subfloor smiler down because if in the event you do get water their its not air tight and can dry out...

Let me know if this plan is good and if I'm on the right track

Would a better option be to use a Dri-Core subfloor? My lowest level of the basement which im working on is only 340 square feet +- so its not a huge space...

1)I'd lay the dri-core sub floor or some other .

2)then glue the R.F.B to the wall using a grid pattern (in order to not allow airflow on the walls and moisture to develop)

3) go from subfloor to the rafters and then glue a piece of a foam in between joist sealing it with caulk or spray foam.

4) Seal all the gaps with spray foam and tape all joints between boards and the floor

5) frame walls with 2x4's now and then fill bats with another form of insulation. Probably fiberglass at this point.

6) I don't put plastic sheeting on at this point I just drywall right over right?
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Do I leave a gap between the wall and the foam board? Im assuming there's a .5 to 1 in gap to allow you to make the walls square and what not.
The wood frame wall has a gap (to plumb the wall) the cavity insulation fills when touching the foamboard (glued tight to the concrete wall) and the drywall. No plastic sheeting anywhere;

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