Informative New 2024 Eave Baffle Code

[Yankee Chronicler]

The blocking between the trusses.

This?



The blocking occupies 1-1/2 inches of the width of the top plate. Obviously, the insulation cannot possibly extend for the full width of the top plate, so with blocking like this you can't use the reduction in insulation R-value.

 

[Enrgxprt]

My take on this is almost EVERYONE interprets N1102.2.1 wrong.

Where R49 is required and R38 is over 100% of the attic is misunderstood by many.
100% coverage means, no thermal bypass or bridging. AKA Continuous and not in the cavity.
The 100% does not allow for framing.

Then, the insulation must continue over the top plate of exterior walls.

Think about it....
The U for 2x10 @ 16" O/C with R49 = 0.027
The U for R38 Continuous = 0.024
(U values created with Rem Rate 16.3.4 assuming Grade 1 or perfect install)

The exemption for the less "stated" R value provides more of a reduction.
Why would an exemption be given for an area measuring 3.5" - 5.5" x the exterior perimeter ???

Understating basic B.S. allows us to discern the difference from B.S. and the other B.S... ©

 

[bill1952]

My take on this is almost EVERYONE interprets N1102.2.1 wrong.

Where R49 is required and R38 is over 100% of the attic is misunderstood by many.
100% coverage means, no thermal bypass or bridging. AKA Continuous and not in the cavity.
The 100% does not allow for framing.

Then, the insulation must continue over the top plate of exterior walls.

Think about it....
The U for 2x10 @ 16" O/C with R49 = 0.027
The U for R38 Continuous = 0.024
(U values created with Rem Rate 16.3.4 assuming Grade 1 or perfect install)

The exemption for the less "stated" R value provides more of a reduction.
Why would an exemption be given for an area measuring 3.5" - 5.5" x the exterior perimeter ???

Understating basic B.S. allows us to discern the difference from B.S. and the other B.S... ©

Understand your point but sure either doesn't read that way or is just unclear. I do think that in a house with rafters or even trusses without the raised heel that the compressed area is a lot more than just the wall stud depth, especially after vent baffles. A roof at 5/12 or near that and 2x6 or even 2x8 rafters, there's not room for 10 1/2" and 1 1/2" vent very close to wall, let alone exterior face. I finished a garage with trusses and it was only 3 1/2" to deck at exterior, so at least 18 to 19" from exterior surface, if not 2'. (I built a sloped soffit inside so I had 24" parallel to roof and pumped it full of cellulose tight to vent - of course way more than the required R38 or even R49.)

 

[Enrgxprt]

Understand your point but sure either doesn't read that way or is just unclear. I do think that in a house with rafters or even trusses without the raised heel that the compressed area is a lot more than just the wall stud depth, especially after vent baffles. A roof at 5/12 or near that and 2x6 or even 2x8 rafters, there's not room for 10 1/2" and 1 1/2" vent very close to wall, let alone exterior face. I finished a garage with trusses and it was only 3 1/2" to deck at exterior, so at least 18 to 19" from exterior surface, if not 2'. (I built a sloped soffit inside so I had 24" parallel to roof and pumped it full of cellulose tight to vent - of course way more than the required R38 or even R49.)

Your right, and I understand your point. That said, my interpretation is that even having a loss at the top plates (with compression) the area involved is actually not that great, and also against what most believe, the R value in fiberglass per inch actually becomes greater when compressed. Add all of these factors and I intuitively feel like the difference between R38 with no loss at the perimeter is worse than 49 having the same compression (or worse) The Energy Code was actually written by some one who knows there stuff. Id have to do a model and UA calc to determine how much of a difference but again, my knee jerk says the intent is only worthwhile IF its 100% continuous. Again, insulation's resistance to conductive heat loss is like the hull of a ship. A ship with a hull that is 1' thick will sink as fast as the same exact ship with a 1/8" thick hull if we put the same size hole in both. The hole is the framing, and the exception (in my opinion) clearly understands it.

All this being said, and also whats is lost in the code comes down to does it really make enough of a difference to get stupid over it? I think not. Sure, if we are sending the house to the moon in a space ship we might need to worry. May be if its going to be -98 Deg F it would matter a whole bunch more. My knee jerk reaction to this is that it might account for a few hundred btu's per 1,000 sq ft of attic. 0.027 and .024.. so the difference is .003. For 1,000 sq ft that's a UA of 3.
We have a 10 deg winter design, so at an indoor design of 70 is a difference of 60 degrees. BTU loss per hour is U x A x Temp Diff, so 60x3 = 180 Btus of loss between the two (R38 continuous and R49 Cavity) Kinda stupid to be even thinking about this, and even more stupid to worry about it.
Code should focus on the important stuff. Build it tight. Ventilate it right. Make sure mechanical equipment is commissioned and not confined. Make sure Manual J/D/S is accurate (Addendum A is followed) Make sure equipment is installed properly. Make sure walls and ceilings have a vapor profile and can dry in or out. done.

 

[bill1952]

the R value in fiberglass per inch actually becomes greater when compressed

Yes but that 8" batt compressed to 6" has less overall insulation than that same batt uncompressed. A 14" R49 compressed to fit in a 2x10 is R37. So yes R3.5/in versus R4 compressed, but a 25% reduction.

I think ice dams plays into the incentive for insulation to the edge, and the insulation weakness of the typical double top plate. Add the convective loss within a typical exterior stud wall and the eave is weak point.

 

[TheCommish]

The top plate is part of the wall assembly and is accounted for in the wall assemble U value calculations.

The reason for the wind wash baffle and insulation thickness for R-38 outside of plate to outside of plate is to first prevent air circulation in to the end of the insulation decreasing its value, second prevent the wind from displacing insulation at the eves, third to proved full thickness insulation to the outside of the plates. The code tries to make up for reduced insulation thickness (and R-value) at the eves below R-38 by increasing the R-48 for the balance of the ceiling- roof.

Ventilation chutes installed in cold roof assemblies combined with attic ventilation is maintain the attic at close to exterior temperature in the winter to prevent ice damming, to provide a path for moisture escape.

Note, hot roof assemble are another seperate discussion.

 

[Enrgxprt]

Yes but that 8" batt compressed to 6" has less overall insulation than that same batt uncompressed. A 14" R49 compressed to fit in a 2x10 is R37. So yes R3.5/in versus R4 compressed, but a 25% reduction.

I think ice dams plays into the incentive for insulation to the edge, and the insulation weakness of the typical double top plate. Add the convective loss within a typical exterior stud wall and the eave is weak point.

100% correct with compressed values at and above the plates. I see what your saying with the overall reduction, and do not disagree.
What Im trying to point out is the delta in btu's.
Sure its a 25% reduction, but here is an honest question, A reduction of what ??

Because we use R value's, a convenient metric, a 25% reduction in R49 sounds really bad.

So lets see what a 25% reduction of U 0.020 means and what the result is....

At R37 the BTU loss per degree of temperature difference, per square foot is already a super tiny number.
U x A x dT = btu loss..
.027 x .61 (2x6 top plate area) x 1 = 0.016 BTU's lost per degree of temperature difference.

My area has a 60 Deg temp diff, so
R37 looses less than 1 btu per HOUR...

Not accounting for framing, since we are talking about fiberglass wind wash baffles and framing is air impermeable anyway..
0.99 BTU per hour is not enough heat to melt anything,
Maybe a blanket of snow's own R value would be something to think about, since it would prevent that single btu of heat
from moving into the air or space, but I think that's an exercise for someone who is Reealllllly into it lol

What causes Ice damming as stated by others (and myself) is outdoor air mixing into the fiberglass and severely reducing its r value.
R37 with a wind wash baffle is so much above what would be required to create an ice damn..

In my area, ice damning can occur when that R value drops to about R1.75
( not exact, and without other factors, but just a quick and dirty to get the idea).

That's wind washing, causing a layer of snow to melt, only to freeze up later at night.
A double top plate alone has about this R value, so the wind washing appears to drop the R value to 0...
We don't often get ice damns in our area. That's not saying it doesn't ever happen.

It pays to know which battles are worth a fight, which ones are silly to engage in.
Again, if your design temperature is -4F my opinion may be (only slightly) off but that's something the CEO and AHJ have to decide.

 

[Enrgxprt]

Forgot to mention, Joe Lstiburek has a great video out where he talks about even with well insulated homes, the warm air that builds up against the outside wall rises up, heating the soffits and causing ice damns! So, wind wash baffles, full height insulation etc isn't always a guarantee for solving ice damns. The sunny side of a wall is another issue.. Anyway, this B.S. stuff is never ending. Regardless of what power magnifying glass you use, there's always microscopes that reveal more, and then even more powerful electron microscopes, and then there is ..

Another question to ask yourself, What is the purpose of building codes, and how far into the weeds should "code" go ?

Happy labor day to all!

 

[Enrgxprt]

The top plate is part of the wall assembly and is accounted for in the wall assemble U value calculations.

Its called the framing factor and is an estimate based on averages. (Why exterior continuous insulation makes such a huge difference)
Im unsure if heat loss up through the top plates is part of the percentage.
My hunch is no, because when entering wall areas into software most only want to know overall square footage's.
None to my knowledge care if the 100 Sq ft wall is 10x10 or 1x100 which would be a 10x difference for say top plates..
Rem-Rate says 16" oc framing is 23% framing and 24" is close to 20%
Good stuff, and glad this forum is here.... Not many folks speak this language ;-)

 

[TheCommish]

I built my house with 1/2 continuous insulation long ago, whish I had done a better job air sealing.

I talk to builders all the time about change how they build instead of grousing about the energy code requirements, not they would rather keep building 2x6 walls 16inch on center, sheath them and spray foam everything, rather than use continuous insulation and 2x4 studs.

 

[jar546]

I am thrilled with this thread, which is why I marked it 'Educational' and hope it continues to grow. With that being said, I want to bring up a variable.

My bubble was always retaining heat in the home for energy code compliance, as I grew up in Pennsylvania. Now that I have been in South Florida for the past 13 years, and my AC never shuts off year-round, what is the relevance to a climate zone 1 such as mine, which does not have issues with ice dams? Our concern is keeping heat out and cold air in. This is why we don't have a requirement for a window U-Factor, but we do have a maximum Solar Heat Gain Coefficient (SHGC). We are seeing more unvented attics in new construction. Can someone elaborate on the relevance in this climate?

 

[Enrgxprt]

Now that I have been in South Florida for the past 13 years, and my AC never shuts off year-round, what is the relevance to a climate zone 1

Hmmm...
I almost always have thought about B.S. from the climate zone 4 and up...
But, here on Long Island our CDD is almost equal to the HDD...
So, in your zone, heat almost always moves from outside to inside.
Water vapor same direction.. so outside is wet, in is dry.
That said you want your vapor barrier on the outside so condensation wouldn't occur in the cavity.
Cold inside surfaces could be well below the dew point of outside RH.
Moisture moves the same direction as the heat (Hot to cold)
Yea, Im getting it now lol...

Your attics are still vented ? Soffit vents and ridge/gable vents ?
The convection of hot air into the soffit, getting heated by the attic even more and leaving the upper portion...
Keeping the hot outside air from mixing into the edge of the insulation allows the insulation to keep the indoor surface cooler.
The thing of it is, which is bugging me, is that the condensation point is now moved further towards the outside with a wind wash baffle.
It bugs me because Im wondering if the baffles in your area should be part of the outside vapor barrier/drainage plane.

If warm humid air moves through a vapor barrier, the air is now less humid once its on the other side. Now its dryer and its dew point lowered.
But the collected water has to go some where...
on a humid and hot day, in a well cooled house, do you ever see (from the outside) evidence of moisture on the top plates, outer edges of insulation ?
Is there ever a problem where the wall drywall meets the ceiling drywall has signs of moisture or damp corners ?
Id be curious to see if thermal imaging or even a moisture meter measurement shows the corners more wet than 3-4' in..

If anyone has any one in warm and wet climate zones has any "issues" as described above, please pipe in as everything above at least is conjecture if evidence never exists ;-)

 

[jar546]

Your attics are still vented ? Soffit vents and ridge/gable vents ?

My home was built in 1969 with a combo of soffit and gable vents. Since gable vents are rare in new construction because of the wind zone which limits the percent of gable ends (most are all hip) we see mostly unvented attics but there are millions of homes with vented attics.

I just want to make sure the discussion is well-rounded and takes into consideration different climates within Canada & the US.