# Adding insulation to walls in attic



## interplexr (Dec 27, 2008)

I am going to be adding a custom storage space into a knee wall in a 2nd floor bedroom of a cape cod style house. It will be built into the existing small attic space. I figure I will use this opportunity to improve my insulation for all the interior walls accessible from this attic while its open. There is part of my bathroom wall, the rest of that wall is exterior since its a dormer. There is also the unmodified portion of my knee wall to insulate. Framing is 2x4 with R11 insulation. I was going to frame the storage space with 2x6 for more insulation. I was trying to figure on what to do with the existing walls. Would it make sense to put up new 2x4s offset from the existing and fill with an R13 unfaced batt since the existing face is on the interior? I was thinking offsetting the framing would help with heat loss through the existing framing. Is there something better to do? I also planned to lay more batt down on the attic floor where it made sense.

Thanks for the help!


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## Ocelaris (Oct 9, 2012)

I'm not sure your situation is exactly like mine, but I have an uninsulated knee wall that touches the room above the garage. It's standard 2x4 stud bays, so I plan on putting R13 inside, and then using Isocyanite foam sheets (or extruded Polystyrene) over that, and then air seal. In your situation, I would say that fiberglass batts are fine, but probably some physical barrier behind the batts would prove a better insulation than just doing deeper (but unsealed batts). The foam wall will provide some air barrier as well, particularly if you can seal around it.


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## joecaption (Nov 30, 2011)

And add a layer of house wrap on the back side of the wall to reduce wind washing.


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## Ocelaris (Oct 9, 2012)

Is house wrap an air seal? i.e. it would be a lot easier to do a house wrap on the back side of knee walls (with fiberglass batts) than solid foam, but I thought the foam would provide a better air seal? I'm assuming foam is better, but maybe you can get close with a house wrap?


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## Windows on Wash (Aug 30, 2011)

Ocelaris said:


> Is house wrap an air seal? i.e. it would be a lot easier to do a house wrap on the back side of knee walls (with fiberglass batts) than solid foam, but I thought the foam would provide a better air seal? I'm assuming foam is better, but maybe you can get close with a house wrap?


Foam is better but housewrap, detailed and taped properly, can be very effective at preventing the wind washing of the insulation and resultant R-Value downgrading.


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## interplexr (Dec 27, 2008)

If my vapor barrier on the existing batt in on the drywall side facing the interior, don't I have to leave the fiberglass open or would the house wrap still breath?


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## Nailbags (Feb 1, 2012)

Ocelaris said:


> Is house wrap an air seal? i.e. it would be a lot easier to do a house wrap on the back side of knee walls (with fiberglass batts) than solid foam, but I thought the foam would provide a better air seal? I'm assuming foam is better, but maybe you can get close with a house wrap?


Sheetrock is a air seal it seals up nearly 70% of air movement the top and bottom plates and any penetrations need to be caulked. to many times people think that it is the insulation that it so stop the air flow. it is not it is the sheathing the house wrap the siding the caulking of electrical boxes the drywall all those play a larger part the any insulation ever would.


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## Ocelaris (Oct 9, 2012)

interplexr said:


> If my vapor barrier on the existing batt in on the drywall side facing the interior, don't I have to leave the fiberglass open or would the house wrap still breath?


That's a good question, and my wife (an architect/GC) and I disagree vociferously about moisture barriers... My opinion, is that if you stop the air, you stop the moisture from moving into the wall cavity. My wife's opinion is that the surrounding air has moisture, and will get through no matter what, and you should provide a moisture permeable seal... Expanded Poly Styrene is somewhat moisture permeable, and would seem to fit your bill. Extruded polystyrene is moisture impermeable. I would assume any foil faced foam board is also impermeable. 

http://www.jlconline.com/insulation/q-a--insulating-a-knee-wall.aspx

"f you can't get at the underside of the rafters, or if creating a continuous air barrier would be impractical (for example, with truss roof framing), then you'll need to insulate the knee wall itself. In this case, after insulating the stud cavities with fiberglass batts, use 1-inch (minimum) rigid foam insulation to cover the back of the knee wall, which will prevent attic air from circulating around and through the insulation. Be sure to notch the rigid foam over the floor joists and run it all the way down to the drywall or plaster, or tuck separate 2-by or foam-board blocks between the joists to provide air stops in the joist bays, and seal all gaps with canned foam or caulking."

http://blog.energysmartohio.com/blog/bid/211267/How-to-Insulate-and-Ventilate-Knee-Wall-Attics

"Sometimes we will change the Solar Guard out for R-30 fiberglass rolls, which gives a total R-value of R-43. The R-30 will fall over if we don't hold it up, plus it is vulnerable to convection, so we cover it with house wrap, which is an air barrier, but not a vapor barrier."

"Bonus Section: If you try this yourself, DO NOT use plastic or you will cause water problems. Foam board, if not sealed very well can also cause problems. A house wrap like Tyvek or a perforated (with pinprick holes) radiant barrier are good covers for fiberglass in knee walls. Don't use the usual radiant barriers sold at Home Depot and Lowe's, they are not perforated. The key is whatever material you use must allow water vapor through it. (I'll deal with double vapor barriers more fully in another post.)"


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## interplexr (Dec 27, 2008)

Those were interesting articles. I'm wondering if it would be best to just put up joist vents to continue down to the soffits so the attic can breath from soffit to roof vent. My closet will take up most of this space so I could just insulate the roof joists also adding more insulation on the attic floor up to the new knee wall. I'll insulate that but there is no access to put up house wrap or anything because of access. I would end up with insulated walls, floor, and ceiling of the attic with a path up the roof joists from the soffit. Does this sound reasonable? The remaining "attic" space would be air sealed from wind but not vapor barrier sealed if I'm looking at this right.


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## Gary in WA (Mar 11, 2009)

If you used XPS for spacers from the sheathing, then added XPS inside cavity for the baffle, you would be good; http://www.buildingscience.com/documents/published-articles/pa-crash-course-in-roof-venting

I would use f.b. behind the built-ins to control the possible condensation there; items blocking warm room air circulating= mold because the air leaks you miss in the attic space will drop the dew-point temps of the drywall there- as f.g is air-permeable. I've been pushing housewrap on attic floor/knee-walls and foamboard for 3 years on this forum now, no call-backs yet, lol.

Gary
PS. where are you located, interplexr?


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## interplexr (Dec 27, 2008)

I'm located in central VA. My existing finished ceiling has faced batt insulation behind the drywall that stopps at the knee wall. The knee wall had R11 batt in it. The floor joists have faced batt as well with the face down on in the floor joists to the ceiling below. The faces of the wall and ceiling batts are facing my room. I'm basically moving this knee wall back and reducing the empty space now. I wasn't originally going to push the entire knee wall back. I'm going to remove the insulation in the existing wall since it'll be an inside wall, cutout and modified for sliding doors. The batt in the floor joists has settled really bad. I was thinking of putting up those vents/spacers for 16" OC joists and put more batt up continuing down the roof like it is done now to the new knee wall. I was going to go to 2x6 knee wall instead of 2x4 that I have to get more insulation in it. Looking at this other document, I seem to closest match the vented roof deck. Would it make the most sense to run the new roof insulation all the way down to the soffit or stop at the new wall? I was going to put new batts in the floor joists to get them filled back up like they used to be. Would you expect issues with this little triangle of attic space? If I stop the roof insulation at the new wall it would vent with the soffits up to the roof vent like it does today but it would seem like I'm loosing a ton of insulation valve with the open batts exposed to this space with lots of air movement. If I run the insulation down to the soffit, fill in the floor and let just the soffit vent up the roof sheathing to the vent, it would seem like that would be better except what will happen in that little triangle. The room isn't bad for temperature in the summer and winter as is. I made an attempt to sketch what I have....

All this has given me a lot to think about when we build a house (hopefully in a year or 2 . I want to do this built in as best as can be done but I feel like the original construction methods have me limited in what I can do.....

I appreciate all the help and information!


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## Gary in WA (Mar 11, 2009)

Your idea to insulate the rafters, yet leave an air space, is fine. That would keep any warm/hot air from settling down against the floor insulation from the top side, giving more warmth to the room below in the summer. It would also increase the "stack effect" made with the sloped ceiling continuous channel; longer/higher/uninterrupted air flow.. http://oikos.com/esb/51/sideattics.html

ADA the drywall to keep the conditioned room air-in and outside air-out.http://www.buildingscience.com/documents/information-sheets/air-barriers-airtight-drywall-approach/

Gary


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## Ocelaris (Oct 9, 2012)

I went ahead and did that today. I basically put baffles to vent between the rafters from the soffit upwards, put insulation batts in, and then put foil backed isocyanurate (sp?) foam, and air sealed the area behind the wall. 

It's hard to see everything in one shot, but bascially there's a door on the left and right, and you can crawl through between the two. The slope goes directly down below those windows. 

I didn't take any final pics after I had foam sealed everything, but you get the idea. Hope that helps.










Before:




















cleaned up, notice the headers for the garage vents right up there:





























Foam almost done:


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## Big N8 (Oct 28, 2009)

Is that space now conditioned?
If not then you have a problem. 
Did you cover the soffit vent? 
if so then you have another problem.


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## Ocelaris (Oct 9, 2012)

Yes, that space is now conditioned. But I put soffit baffles with an air gap in each rafter bay which extends from the soffit up to the attic but I didn't take pictures at that stage. But for reference there aren't soffit vents anywhere in the house, so we will be going back and fixing that when we get to the outside of the house.

It's amazing what a difference that made for that room. Before it was leaking air like crazy, now it feels more or less like the rest of the house. When we first bought the house that radiator had nothing behind it at all, so it was completely open to the attic! Amazing that someone would leave that huge almost 5' x 2.5' gap open... but I kind of understand how nasty that was back there.


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