# Attic insulation - Replacing fiberglass batts with ROXUL?



## Windows on Wash (Aug 30, 2011)

No need to put rigid foam over the top plates unless you really want to or if the gap between the roofline and outer top plate is really tight (i.e. you won't be able to get good depth there). 

No to the last question. 

Just loose fill blow cellulose up to R-50 (14" total) and be done with it once you have completed air sealed the attic.


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## VitoB (Nov 30, 2012)

Windows on Wash said:


> No need to put rigid foam over the top plates unless you really want to or if the gap between the roofline and outer top plate is really tight (i.e. you won't be able to get good depth there).
> 
> No to the last question.
> 
> Just loose fill blow cellulose up to R-50 (14" total) and be done with it once you have completed air sealed the attic.


It's a mansard roof so it's very tight on the edge, there is not much height clearance, I can barely each the edge even lying down.


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## Windows on Wash (Aug 30, 2011)

Rigid or CC SPF is the right application there then.


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## VitoB (Nov 30, 2012)

Windows on Wash said:


> Rigid or CC SPF is the right application there then.


OK so I will apply the rigid foam on the edges, complete the air sealing and add my cellulose on top of my existing fiberglass batts.

Your advice is greatly appreciated, I would have wasted a lot of time and money removing the fiberglass batts for nothing.

Thanks for your help and Happy Holidays to you.


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## Windows on Wash (Aug 30, 2011)

Glad I could be of service. Merry Christmas and happy holidays to you as well.


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## Gary in WA (Mar 11, 2009)

Keep in mind the FG will compress about 40% (depends on amount), lowering the total R-value figures. http://www2.owenscorning.com/litera...ul Compressed R-Value Chart Tech Bulletin.pdf IMO,keep it for the vapor barrier, be careful; the drywall will only support (recommended) 1.6# IF 1/2" thick and cj spaced 24"oc. my 2 cents, take it or leave it for those that want it.... Enjoy the work break, maybe not from the attic, lol. Gary


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## VitoB (Nov 30, 2012)

Gary in WA said:


> Keep in mind the FG will compress about 40% (depends on amount), lowering the total R-value figures. http://www2.owenscorning.com/litera...ul Compressed R-Value Chart Tech Bulletin.pdf IMO,keep it for the vapor barrier, be careful; the drywall will only support (recommended) 1.6# IF 1/2" thick and cj spaced 24"oc. my 2 cents, take it or leave it for those that want it.... Enjoy the work break, maybe not from the attic, lol. Gary


Do you mean 16 inches of cellulose provided the drywall is 1/2" thick and the ceiling joists are spaced 24" on center?


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## Gary in WA (Mar 11, 2009)

No, 1.6 pounds max. for 1/2" drywall ceiling, 24" on center (only) fastened. http://www.greenfiber.com/step_one_-_calculate_your_need_how_to_install.html

Gary


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## VitoB (Nov 30, 2012)

Gary in WA said:


> No, 1.6 pounds max. for 1/2" drywall ceiling, 24" on center (only) fastened. http://www.greenfiber.com/step_one_-_calculate_your_need_how_to_install.html
> 
> Gary


OK I understand now, forgive my ignorance. :laughing:

Some of my ceiling joists span even less than 24" OC, I see some that are 16" OC depending on which room they are over, so I should be OK to add R38 which is what I would need, I am in Montreal, Canada which is close to the coldest zone, they recommend an R50.

My problem now is getting to the top plates of the exterior walls to insulate them properly due to the low pitch of my mansard roof.

The only option that is remotely possible is using spray foam which is fine with me. The issue I have is I can't even reach the end with my arm to pull back the insulation where it stops at the exterior top plate.

It has the paper backing which is stapled to the ceiling joists so I can't just slide it out without cutting it.

Would anyone have suggestions as to what I should do?


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## Gary in WA (Mar 11, 2009)

Utility knife taped to a stick, use XPS over top plates in 1 or 2' wide strips, stack them at the very perimeter to use as insulation stop so nothing touches roof sheathing;use foil faced for the ignition barrier required-- http://bct.eco.umass.edu/publications/by-title/reroofing-and-residing-to-save-energy/

Gary


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## VitoB (Nov 30, 2012)

Gary in WA said:


> Utility knife taped to a stick, use XPS over top plates in 1 or 2' wide strips, stack them at the very perimeter to use as insulation stop so nothing touches roof sheathing;use foil faced for the ignition barrier required-- http://bct.eco.umass.edu/publications/by-title/reroofing-and-residing-to-save-energy/
> 
> Gary


Sorry for all the questions.

How far do I make the rigid foam pieces extend over the top of the drywall?


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## Gary in WA (Mar 11, 2009)

Questions are good.... we like questions. I would add enough FB to stop your other insulation from getting past, yet leaving the airspace intact, Fig. 13 OR 14 (memory fail).... here; http://www.buildingscience.com/documents/bareports/ba-1006-ba-high-r-roofs-case-study-analysis

The attic portion over exterior wall should *at least* equal the wall R-value, add the foam board (or SPF- though not in your app.) R-value when calculating; second picture; http://www.buildingscience.com/documents/published-articles/pa-crash-course-in-roof-venting

Gary


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## VitoB (Nov 30, 2012)

Gary in WA said:


> Questions are good.... we like questions. I would add enough FB to stop your other insulation from getting past, yet leaving the airspace intact, Fig. 13 OR 14 (memory fail).... here; http://www.buildingscience.com/documents/bareports/ba-1006-ba-high-r-roofs-case-study-analysis
> 
> The attic portion over exterior wall should *at least* equal the wall R-value, add the foam board (or SPF- though not in your app.) R-value when calculating; second picture; http://www.buildingscience.com/documents/published-articles/pa-crash-course-in-roof-venting
> 
> Gary


You are a wealth of information Gary. LOL.

OK so I guess it doesn't matter if my rigid foam boards overlap my existing vapor barrier a little in the ceiling right?


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## VitoB (Nov 30, 2012)

I was looking at this rigid insulation, it has foil on one side, it's 3 inches thick and rated R15.5 which is already higher than my walls which are R12:

http://www.isolofoam.com/english/isofoil


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## Gary in WA (Mar 11, 2009)

And probably pricey, as well. If a vapor barrier is existing, no need for another one there.... thank you for the compliment, much appreciated! The fruits of my labor... retired now and time to spend on 2hrs a day in research... plus my kit trailer is waiting for parts from China so I can continue to lake fish, lol. Gary


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## VitoB (Nov 30, 2012)

Gary in WA said:


> And probably pricey, as well. If a vapor barrier is existing, no need for another one there.... thank you for the compliment, much appreciated! The fruits of my labor... retired now and time to spend on 2hrs a day in research... plus my kit trailer is waiting for parts from China so I can continue to lake fish, lol. Gary


Fishing sounds good!

Thanks for your help, much appreciated. 

Happy New Year!


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## AlphaPilot (Aug 6, 2012)

As far as venting the attic goes, is the idea to have baffles just above the insulation in the attic, so air freely moves from soffit to *above* the attic insulation, or are you supposed to have it sealed off so air is flowing ONLY through the roof trusses, and blocked off from the attic air?

In the PDF's Gary posted it showed air movement with arrows, and it appeared there was rigid foam nailed to the trusses along the kneewall, so air was uninterrupted by insulation, but then the air mixed with the attic atmosphere right after the rigid foam ended (what appeared to be 16-24" of insulation). Won't that cause frigid air in the winter that passes through the soffit along and along the baffles to chill the top-plate area of the exterior walls all around the house, since they could not be insulated by any more than say 2" of rigid foam before the contact of the frigid air due to conventional framing methods of how trusses rest on the top-plate?


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## Gary in WA (Mar 11, 2009)

The second diagram is the same as the first, but with added FG insulation. With a passive system air will move from soffit (low vent) along the the bottom of sheathing (roof deck) to exhaust high due to stack effect. The baffle is* needed* only at the FG layer (top) to prevent air flow through it as it's air permeable- it also prevents conduction from FG touching sheathing. Soffit air replacing ridge vent air exhausted out follows the roof plane; with or without a baffle, just not needed- because the FG is on ceiling level. Because of the warm/cold temp differences in an attic, they are inherent with convective loops robbing your R-value 24/7. A 3" layer of cellulose will stop them, as will house wrap over the top of the FG. Did this explain it.... or which link? 

Gary


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## AlphaPilot (Aug 6, 2012)

In the document you posted : crash course in-roof venting you will see on page two a diagram of what I am questioning. 

It appears the air moves from the farthest outer edge of soffit (as documented, you want the soffit vent opening to be as far away from the warm wall) and air travels along what looks like the roof's truss spacing that is sheeted with rigid foam or something that serves as an air-barrier. This is where I question 1.) is using a standard 'attic baffle' between every truss along the attic knee wall the same thing as shown in the diagram? 2.)it appears the air channel created by that blue sheeting stops after the top of the pink insulation, is that sufficient or was the idea to carry the vented channel all the way up to the ridge? 3.)It appears the roof trusses are 2x10 or 2x12, and have a shelf notch in them so that they can rest on the top of the exterior wall top-plate, while still having additional length to provide a decent eave which soffit then covers. To me, that is not normal, the truss to be that size. There will be this issue where there is no room for rigid foam and cellulose or FG insulation to be decent R value above the wall top plate. The photo shows an imaginary 3" of pink FG and then 1 or 2" of seamless blue rigid foam that creates the barrier between the chilly venting air and the conditioned space below.


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## Gary in WA (Mar 11, 2009)

1. Yes, an air (insulation) baffle will separate the air flow from the insulation at the exterior wall. As will XPS (unfaced) foam board (the blue stuff pictured). 

2. No the baffle ONLY covers just past the insulation to prevent wind washing= R-value severely handicapped.

Rafters are usually much smaller, or trusses and a raised heel truss costs more for a production builder. Just as low density FG costs less to DIY'ers and give attic convective loops, buy the medium/high density or cover with a house wrap or cellulose. 
Gary


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## AlphaPilot (Aug 6, 2012)

Gary in WA said:


> 1. Yes, an air (insulation) baffle will separate the air flow from the insulation at the exterior wall. As will XPS (unfaced) foam board (the blue stuff pictured).
> 
> 2. No the baffle ONLY covers just past the insulation to prevent wind washing= R-value severely handicapped.
> 
> ...


Interesting.

So, with your knowledge, what do you find to be the most effective way to address this venting situation, while maintaining excellent R-Value, even though the vent process allows cold air to be running just inches away from the wall's top plate?

You noted rafters are smaller like my assumption. I wasn't aware of the term, but raised heel truss would not seem economical. I would like to shy away from using poly spray foam along the attic knee wall, but I don't see a better alternative since dense cellulose (which is what I think I will opt for in the attic) would only have about 3 inches between the top plate and attic vent baffle. :huh:

edit:I can draw a quick photo if my question doesn't make sense.


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## Windows on Wash (Aug 30, 2011)

Fabrication of the baffles out of rigid foam is the best but is more time consuming by comparison. 

The styrene chutes are crap and don't do a very good job of protecting the insulation or insulating.


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## VitoB (Nov 30, 2012)

Windows on Wash said:


> Fabrication of the baffles out of rigid foam is the best but is more time consuming by comparison.
> 
> The styrene chutes are crap and don't do a very good job of protecting the insulation or insulating.


Very interesting. 

So the baffles made from rigid foam can actually serve a dual purpose, maintaining the air flow and also insulating to prevent some cold air from moving down to the top plate?

I always figured it makes no difference since cold air will always drop to the bottom of the attic.


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## Gary in WA (Mar 11, 2009)

Just checking if everyone was awake.... I was not stating it complete--- actually; the baffles are required by industry to direct air upwards (and protect insl.) and it will then continue rising and not come down. Not true, except for air coming in the soffits which is warm on the sunny side of the house and rises to wash the roof plane. But- when cooler air is coming in (night or non-sunny sides or ambient low temps), it separates; warmer air up the roof, colder air sinks to the attic floor (above AND IN the insulation when low density fiberglass) and gets warmed by the room heat rising from below-- and forms convective loops to exit out the ridge without washing the roof plane. So, some of this cold supply air being pushed in attic through soffits due to warm exhaust air out through ridge (pressure diff.) is constantly feeding the loops. Hence, cover the attic insulation (when LD fiber). 

Over the knee wall top plates? With the chutes installed - starting a foot before the wall in side attic and low clearance above the plate? Yes, foam board is perfect for max R-value and minimal thickness. Cover (house wrap) the attic side of knee wall FG to prevent wind washing and air degrading (air barrier as FG is air permeable). Air seal/insulate the attic access door, air seal under the knee wall to stop side attic air from reaching uninsulated joist cavities. Caulk under the wall bottom plate or at least in plate/flooring joint after removing wall trim baseboard. http://oikos.com/esb/51/sideattics.html

Gary


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## Gary in WA (Mar 11, 2009)

Good catch VitoB, you were awake and sharp!

Gary


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## Gary in WA (Mar 11, 2009)

The cheap foam baffles do the same as rigid board unless you rip them to pieces when installing on the protruding roofing fasteners. You aren't after the R-value, they are not going to give conduction from the roof framing- not much more than rigid, IMHO. Some suggest installing them continuously - overlapping shingle fashion to drain the condensate, others say leave a 1" gap to let warm convective loops/hear/moisture rising to the roof plane airflow-- more reasonable-IMHO.

Gary


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## Gary in WA (Mar 11, 2009)

WoW, perhaps you were thinking of the 24" baffles that break in two for the 16" oc. (multipurpose ones)? Those wouldn't cover the whole vented area, rafter-to-rafter. And how many install them with caulk/canned foam,; not many. In that regard, I agree.

Gary
VitoB; http://digitalcommons.unl.edu/cgi/v...y rising` attic air from passive ventilation"

http://web.byv.kth.se/bphys/reykjavik/pdf/art_085.pdf


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## AlphaPilot (Aug 6, 2012)

That oikos site was helpful but also lead to more theories. I will plan on blowing in cellulose in my attic when the time comes for insulating the attic. So I am not sure the same rules apply as you said with FG (i.e. using house wrap to create air barrier)

In this truss example, it seems a bit more realistic. The oikos site gave insight, but there is not that much room for insulation along the very start of the eave, where the truss rests on the top plate with a standard 5/12 truss system. In the photo I use as an example, you will see my confusion. :huh:

The idea is to use rigid foam. It comes in 4x8 sheets as standard. I would rip the sheets so they form 2x8 sheets. From discussion, it sounds like they could simply be fastened to the trusses, running horizontal. So technically a 2x8 sheet would span 6 to 8 trusses. This would create a cavity the width of each truss for air to move freely, between the rigid foam and the roof decking.

An issue arises when it comes down to the top plate. If the bottom chord of the truss rests on the top wall plate, then that leaves a triangle area that must be insulated. Assume the bottom chord is 5 1/2" thick, with a 5/12 pitch, that would leave very little room for 1.) rigid foam to serve as the baffle in that tight of an area and 2.)little room for insulation directly above the top wall plate. :huh:

Soooo....What am I getting wrong here, that continues to seem like an issue? I have been up in a few attics but they are not the new vented design. Insulation is simply blown in to the tight knee walls. With an infrared camera you can see the cold air in the winter all along the outer walls up against the ceiling because the insulation is obviously lacking r-value at the knee wall.

Sample truss: Note how it is a tight fit for say rigid foam to serve as the baffle, and also have decent insulation in that triangle area.


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## Gary in WA (Mar 11, 2009)

At the exterior wall or rafter bearing, use foam board laying flat with notches for the wood rafter on the plate. You may only get 2" or so of height above plate before you start to lose your 1" (min. code) or 1-1/2" air space. You could go to Polyiso board for max. R-value there, but as soon as able, use XPS (or EPS) for the rest. Way more work to fit in FB between rafters, but it is what it is on existing houses. IF pieces of FB are let in to the rafter bay, you get more insulation but less airflow.... oxymoron. Your choice... not much else unless you go with a conditioned attic; http://www.buildingscience.com/docu.../irc-faq-conditioned-attics?searchterm=attic+

It will remain under-insulated at exterior plate line, air seal first and foremost, Fig.14; http://web.ornl.gov/sci/buildings/2012/2013 B12 papers/007-Parsons.pdf

Gary
PS. Glad I don't have much snow here.... http://www.foam-tech.com/case_studies/part2_ice_dam.htm


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## AlphaPilot (Aug 6, 2012)

Interesting article. That finally shows that I am not driving myself crazy for no reason. It seems raised heel trusses or extended bottom chord trusses are the smart option. Each one will be more costly. I believe extended bottom chord would be cheaper.

You're suggesting to use at least 2" of poly iso rigid foam along the top plate, notched for each bottom chord that rests on the top plate. I understand that, since there will be very little room after accommodating for a 2" air channel for the venting process. Curious as to why you suggest XPS or EPS next after using the poly iso for the top plate? :whistling2:


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## Gary in WA (Mar 11, 2009)

Mainly because of price. Be certain to air seal first, before any rigid insulation.

VitoB, any other questions after your thread was hi-jacked....?

Gary


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## VitoB (Nov 30, 2012)

Gary in WA said:


> Mainly because of price. Be certain to air seal first, before any rigid insulation.
> 
> VitoB, any other questions after your thread was hi-jacked....?
> 
> Gary


Actually maybe just one last question if that's OK with you. I wasn't able to post them previously due to some issues posting pics. 

I was just wondering what you think of the attic chutes I currently have:

http://www.diychatroom.com/albums/t...3801decf22d277ccb9ec3f_7240.jpg?dl=1417031484

I still can't post the full size pics so I'm sorry if it's hard to see. 

The OSB panel is kind of sitting in there loosely between those 1x3s, it is only held in place by the fiberglass batts over the top plate. 

There is also a large gap between the 1x3s and the rafters. 

When I cut my rigid foam blocks they need to fit nice and snug from rafter to rafter correct?


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## Gary in WA (Mar 11, 2009)

The OSB is rather poor as it won't direct all the cold air, some will degrade your FG by up to 40% by wind, pp.5; https://www.google.com/url?sa=t&rct...pCFvoJlgzLf5dFA&bvm=bv.83134100,d.cGU&cad=rja

Yes, cut the FB between the wood rafter/ceiling joists, some notching is involved, lol. Been there-done that. The OSB is also compressing the insulation lower total R-value. Cellulose will form a crust on top that air seals fairly well (if not disturbed later), but you will air seal before insulation as it is more important than insulation; http://www.google.com/url?sa=t&rct=...w4HQDw&usg=AFQjCNHw8aGbLEJ7HIbHQNHKwJ2BS-kZEw

Easy to find the air leaks from the dirty FG, great filter as in furnace filter.

Gary


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## VitoB (Nov 30, 2012)

Gary in WA said:


> The OSB is rather poor as it won't direct all the cold air, some will degrade your FG by up to 40% by wind, pp.5; https://www.google.com/url?sa=t&rct...pCFvoJlgzLf5dFA&bvm=bv.83134100,d.cGU&cad=rja
> 
> Yes, cut the FB between the wood rafter/ceiling joists, some notching is involved, lol. Been there-done that. The OSB is also compressing the insulation lower total R-value. Cellulose will form a crust on top that air seals fairly well (if not disturbed later), but you will air seal before insulation as it is more important than insulation; http://www.google.com/url?sa=t&rct=...w4HQDw&usg=AFQjCNHw8aGbLEJ7HIbHQNHKwJ2BS-kZEw
> 
> ...


Please don't tell me I need to replace my existing chutes. 

Very interesting articles, thanks again for sharing your knowledge and expertise.


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## Gary in WA (Mar 11, 2009)

Alright, I won't tell you to *replace your baffles*...... any other thoughts?

Glad we could help. 

Gary


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## VitoB (Nov 30, 2012)

Gary in WA said:


> Alright, I won't tell you to *replace your baffles*...... any other thoughts?
> 
> Glad we could help.
> 
> Gary


I should be good to go as soon as the weather warms up a bit up here in Montreal, it's been really cold lately.

Thanks again Gary.


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