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Old 09-22-2016, 01:48 PM   #1
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Caulk... so many types... which to choose?


Hi all,

I'm reinsulating one of the upstairs bedrooms in my Cape. So far I've installed 2" thick XPS foam baffles and spray foamed the sides using GreatStuff Windows and Doors. I trimmed off the excess spray foam and now I'd like to also caulk the edges to ensure everything is completely air tight. I know the spray foam should have taken care of that but I don't want to take any chances.

This leads me to my big question... what caulk should I use? I didn't realize there are so many kinds out there! I used Big Stretch (acrylic elastomeric) to caulk where the roof joists and decking meet but now I'm not so sure that was the best choice. It seems others may be more flexible and less prone to cracking. Either way I can't go back since that part is now covered by the XPS panels but I want to make sure I use the right caulk for this part of the job. Would a silicone like GE Silicone II Window & Door be a better choice? It says silicone is permanently flexible and doesn't shrink/crack. I thought the downfall was it wouldn't adhere to wood but apparently this does.

All this caulk needs to do is air seal potential gaps between the foam insulation and the wooden roof joists. It won't see any moisture. Does anyone have recommendations?

Here's a pic of the trimmed spray foam.



Here is one a little further back so you can see what the project looks like. The spray foam wasn't trimmed in this pic.



Thanks,
Mark
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Old 09-22-2016, 03:46 PM   #2
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Re: Caulk... so many types... which to choose?


Since it's something no one will see and it doesn't have to be paintable, this is one time where I would use pure silicone, white or clear.

Also consider OSI sealants. They make some really good stuff.
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Old 09-22-2016, 03:46 PM   #3
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Re: Caulk... so many types... which to choose?


I applaud your effort but I think caulk is going to fail before the spray foam.
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Old 09-24-2016, 09:21 AM   #4
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Re: Caulk... so many types... which to choose?


I am not an insulation expert, but I have a question/concern about what you are doing. If those collar ties (hotizontal framing members) are going to hold the drywall for the ceiling, then I believe that the rafters and roof deck above that ceiling should not be insulated. That never really made sense to me, as long as you left an air space above the rafter insulation to connect the soffit vents to the ridge vent, but i believe i read that many times when i was insulating my cape cod house. Hopefully one of the experts here can shed some light on this.

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Old 09-24-2016, 09:48 AM   #5
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Re: Caulk... so many types... which to choose?


You've done as much as anybody can, diy or pro. Don't know how you did it, but I'd have cut the panels 1/4 short on all sides so that the spray foam fills the thickness of the panels. If the panels were tight and the foam is surface only, I guess there is more chance of leaking through the foam esp as cutting off the access means damaging the bead. Little voids in the foam is surface only. Smoothing out the air bubble voids is absolutely not necessary unless you want the look of air tightness, which is illusion.
My choice is osi quad as well, but it stinks for 1-2 days. I wouldn't recommend using that much indoors. Cheap dap caulk remain flexible. I don't think any caulk these days hardens like a rock.
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Old 09-26-2016, 01:45 PM   #6
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Re: Caulk... so many types... which to choose?


Quote:
Originally Posted by jeffmattero76 View Post
I am not an insulation expert, but I have a question/concern about what you are doing. If those collar ties (horizontal framing members) are going to hold the drywall for the ceiling, then I believe that the rafters and roof deck above that ceiling should not be insulated. That never really made sense to me, as long as you left an air space above the rafter insulation to connect the soffit vents to the ridge vent, but i believe i read that many times when i was insulating my cape cod house. Hopefully one of the experts here can shed some light on this.
I don't have soffit vents yet, only a ridge and gable vents, but I plan on installing them in the future (too much work right now). The baffles go a little higher than the collar ties so the insulation above the ceiling won't block them.

Quote:
Originally Posted by carpdad View Post
You've done as much as anybody can, diy or pro. Don't know how you did it, but I'd have cut the panels 1/4 short on all sides so that the spray foam fills the thickness of the panels. If the panels were tight and the foam is surface only, I guess there is more chance of leaking through the foam esp as cutting off the access means damaging the bead. Little voids in the foam is surface only. Smoothing out the air bubble voids is absolutely not necessary unless you want the look of air tightness, which is illusion.
My choice is osi quad as well, but it stinks for 1-2 days. I wouldn't recommend using that much indoors. Cheap dap caulk remain flexible. I don't think any caulk these days hardens like a rock.
Thanks carpdad! I did cut the panels 1/4 short and spray the edges with Great Stuff Window and Door foam. Then I trimmed the excess foam and lightly sanded so it's flush with the XPS. I'm installing fiberglass batts next and I want them to have a perfect surface so they don't get bunched and lose r-value. It's also why I'm really focusing on making everything airtight... fiberglass is worthless if air gets to it.

I couldn't find any OSI Quad that was compatible with XPS at my local home improvement stores so I'm using some 100% silicone instead. I know this step is probably unnecessary but I've done so much work trying to make everything as airtight as possible, if it takes care of even 1 small leak, it will be worth it.

Last edited by Marky82; 09-26-2016 at 01:50 PM.
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