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Old 01-07-2019, 03:28 PM   #1
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R38 Spill Over? or R30


DIY Pros,

I'm redoing our family room and took down the old, nasty insulation that was above the ceiling. Above the ceiling is a triangular dead space. The room is old and is framed with TRUE 2x joists by 9 in tall. So the span is more like 14" rather than the modern 14.5".

My QUESTION - I'm planning on putting in fiberglass batt insulation, but should I stick to R30 which will fit the depth of the joist more traditionally, or should I use R38 which will spill-over, or "muffin top" over the tops of the joists.

Since it's an unused space, I figured spill over isn't an issue, and thought it might even be better to insulate over the joist a bit too..

Any input?

Thanks!

Jay
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Last edited by javelin; 01-07-2019 at 03:32 PM.
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Old 01-07-2019, 08:23 PM   #2
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Re: R38 Spill Over? or R30


More insulation is better as long as it stays in and doesn't get compressed and you can vapour barrier over it. (if you're in a cold climate)

Insulating a roof deck incorrectly can cause the deck to rot out, be very careful.

A complete foam thermal break is far superior to batts when putting insulation against a roof deck.

Fiberglass works fine in exterior walls because there's drying potential.

Normally, there's a vapour barrier but some moisture can still get in.

The problem with roofs is, the roofing material doesn't allow moisture out. Unless you have some type of ventilation with baffles creating channels for air movement.
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Old 01-07-2019, 08:36 PM   #3
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Re: R38 Spill Over? or R30


Missing some information.
The picture looks like a shallow attic space, you describe it as "a triangular dead space". The insulation you want to add is in the ceiling joists and not the rafter, correct?

What is missing is any mention of ventilation and Canada still requires a vapor barrier in some cases and that would need to go at the bottom of those cavities, against the drywall ceiling.

I'll stop here in case I'm lost and wait to here what you have for ventilation.

Bud
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Old 01-07-2019, 09:11 PM   #4
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Re: R38 Spill Over? or R30


i just assumed it was the roof deck.
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Old 01-07-2019, 09:24 PM   #5
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Re: R38 Spill Over? or R30


That was my first thought from the picture, but aftet a couple more readings I think he is just replacing the ceiling insulation.

But, the reference to closed space and no mention of ventilation needs to be determined. Some of those rafters have a much darker color than the others.

Bud
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Old 01-08-2019, 09:05 AM   #6
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Re: R38 Spill Over? or R30


Thank you for your responses!

Yes, I'm just replacing the insulation at the ceiling level (not the diagonal rafters). My question was really weather it was worth it to spend the extra money on R-38 and let it compress a bit (at the low side of the roofline - realizing it will really only be R-32ish there since it's compressed) and let it "spill over" or "muffin-top" in areas where there is a bigger uninsulated void above.. Maybe the attached drawing I made will shed some light?

Thanks again!

Jay
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Old 01-08-2019, 09:23 AM   #7
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Re: R38 Spill Over? or R30


Nothing wrong with the r-38 where it fits without being in contact with the bottom of the roof. As to whether it is worth the added cost I don't see your climate zone or nearby big city. Maybe I missed it.

As for ventilation you don't seem to have any on the lower end of that slope and the single roof vent will do little by itself. Getting a vapor barrier in there might help as it would slow the moisture coming through. But ventilation is the higher priority.

Bud
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Old 01-08-2019, 09:29 AM   #8
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Re: R38 Spill Over? or R30


Thanks Bud - I'm in the Philadelphia area.. I'll plan to put some of those plastic baffles against the roof deck where the R38 might hit it (or else use lesser R there). As for the vents, you're correct, there are no soffit vents - only these static ones (x5) in the roof.. Given the construction of this OLD house with stone exterior/etc. I think it would be hard to add any real soffit ventilation aside from those little hockey-puck round cutout ones, and I've heard they don't do much and are very ugly.. Still, maybe a few is better than nothing?
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Old 01-08-2019, 09:31 AM   #9
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Re: R38 Spill Over? or R30


https://www.homedepot.com/p/Master-F...LSC2/100090036

Do you think these would be worth adding? at least to some of the bays in the soffit being that they would share the intake air?
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Old 01-08-2019, 09:36 AM   #10
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Re: R38 Spill Over? or R30


Sorry.. one more question.. I thought the vapor barrier went TOWARD the living area (the craft faced batt) - but it sounds like you think I should put it on TOP of the insulation (toward the unconditioned space)?
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Old 01-08-2019, 09:55 AM   #11
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Re: R38 Spill Over? or R30


No VB goes to the warm side of the cavity.

Your high vent will have about 50 inē of NFA (net free area) so your lower vent should have similar or more. We can calculate the target amount if needed. I missed the first post so now see you have 5 of the roof vents.

But the pop-in vent you linked has 1.5 inē of NFA so you would need about 32 of them to get 48 inē of NFA. They make long continuous vents where you drill similar holes and cover them with the long vent.

The real problem is the lack of height difference between low and high. 4 or 5 feet would be good, obviously not there. In some cases a raised stack can be built, like a covered chimney to act as the high vent.

The good news is, apparently this has been in place for some time with just the upper roof vents. Given that, add as many of the pop-in as you can and the baffles you mentioned in every rafter bay. Results should be better than what you have and that doesn't seem to have been a problem.

Another approach would be the r-30 fill to the top and then 6" unfaced diagonally across the top of everything. Trimmed as necessary at the bottom. I prefer Roxul insulation but FG works and in Philly you will want the extra, I think they recommend R-38 to R-60.


PS in Philly a vapor barrier is only needed if antiquated code enforcement requires it. Citation available.

Bud
Dug out the link I mentioned
https://www.energyvanguard.com/blog/...rrier-Probably
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Old 01-08-2019, 10:05 AM   #12
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Re: R38 Spill Over? or R30


Thanks Bud - yes, as you said, it's been this way since the 1940's and apparently has worked pretty well... The old insulation I took out was pretty nasty and compacted (maybe rockwool batts? or maybe old fiberglass.. I couldn't tell, but itched a bit, so assuming the latter), but I chalk that up to just being very old, and unknown (but now fixed) small roof leak and critters of the years maybe...

I've done exclusion to hopefully prevent any squirrels or anything from entering and will add as many of the small "hockey puck" soffit vents as I can easily.. I'll probably go the route of R-30 as you said, then with a supplemental unfaced roll criss-crossing the opposite direction where space permits.. that seems like it would work best.

Thanks for all of your help!

Jay
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Old 01-08-2019, 10:08 AM   #13
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Re: R38 Spill Over? or R30


Note the link I added about not needing a vapor barrier. Paint is actually a vapor retarder anyway. That will make the installation much easier.

Bud
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Old 01-08-2019, 10:11 AM   #14
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Re: R38 Spill Over? or R30


thanks - I wasn't going to add an additional vapor barrier, but rather just rely on the craft-faced of the batts overlapping at the joist seams/staples, then getting further sealed by the drywall boards pressing against it. I think that seems like it will be more than ample, especially considering probably not needed at all per your link... Thanks again.
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Old 01-08-2019, 10:26 AM   #15
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Re: R38 Spill Over? or R30


Didn't realize you had the drywall down. From below using the Kraft facing to install the insulation is probably the best reason for having it.

Note, you could probably install the upper layer diagonally across the joists before you install the r-30. Might fight you a little but a lot easier than crawling into that thin triangle.

Bud
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