Adding additional insulation to kneewalls of attic wall bonus room
I have learned a lot on this forum that has helped as we been working to improve a room addition that is terribly hot in summer and cold in winter.
Northwest Arkansas Zone 3-4
800 sq foot addition in vented unconditioned attic with dormers and knee walls.
Had R-13 in walls with R38 batts on ceiling of addition except for some areas of R-19 in slopes.
Attic floor (over 1st story) is blown cellulose R38-49.
We have caulked/sealed wall penetrations, top& bottom plate seams in drywall, outlets, can lights, worked to add blocking under knee-wall floor joists.where accessible. Added 3/4″sealed foil faced foam board over back of batts on knee wall, bringing those up to about R17ish?
While there seems to be some improvement (especially the difference of temp at baseboard from bottom plate air leaks sealed) we had our 1st “warm” day around 70 degrees and it was still very hot upstairs. Can’t imagine when summer temps get here.
Insulation person has recommended stacking R19 unfaced batts to the back of the foam board we already added and covering batts with housewrap and blowing an extra 4-6 inches of cellulose over the flat portion of the ceiling over the 2nd floor addition (unfortunately no room to add anything to the slopes.
Does this sound like a good idea or is there any other preferable way to add to the work already done? Another 1″ of foam board is almost as expensive as the R19 but would only be about an R6 addition.
Any suggestions would be welcomed.
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Replies
Mark,
It's hard to diagnose the defects in your thermal envelope over the internet.
Here are the principles:
1. Your thermal envelope needs to be as airtight as you can make it. There are a couple of red flags in your description. One red flag is that you have chosen to insulate the kneewalls instead of insulating the sloped roof assembly down to the soffits. (The method you have chosen complicates air sealing immensely.) Another red flag is that you "added blocking under the kneewall floor joists where accessible" -- implying that you weren't able to install blocking everywhere. That's not good.
2. You need to aim to install code minimum levels of insulation or more. Your sloped roof assembly is insulated to R-19, but the 2012 IRC calls for R-38 in Climate Zone 3 or R-49 in Climate Zone 4.
You haven't yet told us whether you have a ventilation channel between the top of your R-19 insulation and the underside of your roof sheathing in the sloped roof assembly. Such a vent channel is a code requirement. One typical problem occurs when builders use ventilation chutes (baffles) that aren't airtight. This complicates efforts to make your envelope airtight.
Here are links to a couple of articles that you may find helpful:
“Two Ways to Insulate Attic Kneewalls.”
Insulating a Cape Cod House
Thanks Martin for your response. I have read through both of those articles and every post on GBA about knee walls and bonus room insulating & air-sealing, which have been very helpful.
We bought the house last summer with the room addition that had already been done with the poor insulation and air sealed problems. Since it already had existing insulation on kneewalls and ceiling, we are doing our best to work with whats already there. With the inaccessible & hard access areas, it would be a huge challenge (and expense) to go back and insulate the roofline at this point.
Had we built it ourselves and known what I have read on here, we would have certainly tried to insulate the roof line instead.
We can access 3/4 or more of the attic but the dormers & framing limit access in a few areas, so we have been trying to improve all the accessible areas the best we can. we may need to to cut through drywall from the interior to access 1 section of the room and seal up those joists to address what you have pointed out.
没有发泄的渠道上的几个小斜坡sections with R19 over the dormer windows which is something we are also working on - there are only about 4 small areas with the R-19 with - maybe only 30-50 square feet. The majority of the top slope sections are open to the attic with R38 batts and 3-6 feet of clearance between the insulation and the roof deck, so mostly not a problem.
So with all of that in mind if there is anything else you can recommend in addition to getting access to and sealing the rest of the joists, which we will plan to do.
Specifically, whether you think that the additional layer of insulation over the back of knee walls that already have R13 batts + R4 foam board, or capping the top of bonus room ceiling with 4-6 inches of additional cellulose might produce additional benefit from reducing attic heat transfer. We are right on the border of region 3-4, so I am thinking the R38 ceiling batts improved to R49 in the hot attic (and be the correct code under 2012.)