Attaching Foam board to masonry

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Old 01-10-15, 03:53 PM
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Attaching Foam board to masonry

I know this information has been discussed before, but I'm trying to get a more specific answer than I've heard previously. I am not sold on using adhesive to attach foam board to my crawlspace/basement walls because there's no guarantee that adhesive will be a permanent attachment and I don't want to risk problems. So I was pointed to this company called Rodenhouse (Insulation Fastening Systems - Rodenhouse Inc.) that makes easy fasteners for 2" foam board. You just predrill 5/16 holes and tap in the fastener. So I emailed them for a quote on their product and their reply was below.

Hi Dale, thanks for reaching out to us!
For 2" rigid board I would suggest our 3.5" PMF anchor. On a 2x8 sheet, I would probably use at least 4 anchors in order to attach ends and center to keep it from bowing in the middle.

Pricing is $115.00 / 250 pcs.
Shipping from Grand Rapids, MI - charges would be about $17.00


My question for those who have experience with foam board is if you think it could warp with less then four fasteners per 2x8? That seems like a lot to me to have a fastener at each end a foot apart. What if I used three fasteners spread right down the middle from end to end? Or just two? I don't think it would be cost effective to use both adhesive AND fasteners. This is important to know ahead of time so I can calculate how many to order, although it appears this company does not sell specific numbers, only by the 250. Are there any cheaper methods that are just as effective? I have about 800 sq. ft to deal with between basement and crawl.
Another question is do I have to frame out in front of the foam board? I understand that code says the board has to be covered but do I have any other options besides framing and drywall?
 
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Old 01-10-15, 04:01 PM
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If you use an adhesive made for foamboard, it will not come off the wall, unless the wall is so convoluted that the board can't get a flat space to grab. Using those fasteners is fine, and will give you a warm and fuzzy, but the adhesive will do all the work.

You can't leave foam board exposed. It must be covered for fire purposes. Not that it burns so readily, but that it gives off toxic fumes when exposed to flame, and you don't want that in an open area. Building a stud wall in front of the foam will allow you to run any plumbing and electrical with ease, as well as providing a surface to attach your sheetrock/plywood/T1-11, or other wall covering to. Any other method will negate your efforts at having proper space to put the needed options in.
 
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Old 01-10-15, 04:32 PM
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Thermax (foil face) does not have to be covered. Seams should be taped with foil tape. 2" foam isn't going to warp provided the wall is flat and you've pinned the corners.

You can fasten foam without using glue. Predrill the right sized hole with a masonry bit and use a cap nail, or use a fluted masonry nail with a disk washer. Or use Nydrive (nylon drive rivets) and disk washers, also in a predrilled hole. If you plan to frame over it, use your framing to hold it back to the wall.
 
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Old 01-10-15, 04:38 PM
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Thanks Chandler, so do you recommend cleaning the walls prior to the adhesive? I know it will likely say that on the tube. I know specifically there are a few smaller areas where the cinderblock is crumbling/bubbling out and maybe a few areas where old paint is coming off. This is what I'm afraid of and don't want a lot of extra scrubbing work if it can be avoided. Also, what if in the future, additional paint starts to come loose where I glued the foam board? The walls are in reasonably good condition for a 75 yr old house. I just want to cover all the bases.
 
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Old 01-10-15, 04:42 PM
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Thanks Xsleeper, as I mentioned, I thought four fasteners was a lot for a 2x8 piece of foam but if I have to hit all corners, I guess its the best.
 
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Old 01-10-15, 04:56 PM
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Heres another question, besides having a fire resistance, is themax foamboard a better insulation than the Owens corning Pink or Blue Foam board? I noticed at Home depot, you can buy a 4x8 sheet of 2" Rmax Thermasheath for 30.00 where the owens corning is 20.00 for a 2x8 sheet. So the thermax is cheaper. If its better quality or just as good, Ill use it. It appears that the Thermasheath is R14 also where the Owen pink is R10.
 
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Old 01-10-15, 05:14 PM
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Oops, you didn't say anything about painted walls, so yeah, adhesives are probably not going to be your best bet. Any of the fasteners Brant mentioned would work as well as the ones you described. Brant's are probably a lot cheaper, and just as functional. The R value is the difference in the cost factor, IMO.
 
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