Greetings.
I’m converting my 1920s bungalow attic for additional living space. I may be a little ahead of myself as I try to figure out a way to add more insulation to the ceiling. I have about 9′ from floor to ridge and 5′ knee walls.
The rafters are 2×6. I have baffle vents between all rafters that run from ridge vent to soffit. I have no collar ties. I’ve already installed R-19 faced batts over the baffle vents, but the more I read, I realize I need more. I would like to use rigid foam and realize it can be installed perpendicular to the rafters. My first question is how can I install strapping material to which I can add the insulation and insure strength to secure drywall? I would like to avoid having to scab 2x material as it would require removing insulation I’ve already installed.
My second question has to do with the attic dormer. There is no way to vent the valley, so do I bother installing baffle vents in between the rafters in the dormer, above it and on either side of it?
I look forward to your suggestions.
Replies
Assuming your 2x6 rafters are adequate for the roof span, I would think you could run 2x2s or 2x3s perpendicular to the rafters with 2' x 1-1/2" rigid foam (About R-7, I think) between the 2x's. Use 5/8" drywall to span the 24" strapping.
One way to vent interrupted rafter bays is to drill holes into adjacent vented bays. This might not be a great idea if your rafters are only 2x6, however. Sometimes, you just have to do the best you can.
Al Mollitor, Sharon MA
You don't need strapping. You can screw foam insulation board to the faces of the rafters. Then screw the drywall over it with long enough screws to go through both the drywall and the insulation, and into the rafters about an inch or a bit more. That's just what they make long drywall screws for that you see in the store.
Make sure you push both the foam board and the drywall tight to the rafters before you drive each screw. If you leave it pulled away a bit you are at greater risk for the drywall popping loose around the screws in the future.
If you live in a cold, northern climate you need a vapor barrier on the inside. The best way to do it would have been to install unfaced batts. You should tape the seams of the foam board, and the foam board becomes your vapor barrier. Don't use bead board because it's an inadequate vapor barrier.
Don't forget to mark your rafter locations on the foam boards as you install them so you'll know where they are and you can strike a chalk line between the marks so you know where to drive the drywall screws.
jackson- If you go with the drywall screwed right over and threw the foamboard insulation chances are sooner or later you'll have hairline cracks at your drywall seams if that would be a consideration to you.
A layer of foil backed insulation right under the drywall can be taped with a foil backed tape for the vapor barrier.
If you go this route and have already put up faced paperbacked fiberglass batts in there you can make slightly diagonal slices in the paper backing from one side of each bay to the other which will still keep the fiberglass caged in there and prevent the construction of a double vaporbarrier.
Thanks for your thoughtful suggestions. I know the ideal R value for this area is R-49. Adding the 1.5" foam board would give me R-26. Is it feasible to use material deeper than 2x3 (to beef R value), and if so, what's the best way to attach it on end? Or what about using 2x2s with foam board between and then unstrapped sheets of 1" or so over that. I do want to avoid cracks in the plaster.
Trying to reach near an R-49 in a cathedral ceiling with 2x6s and R-19 6"fiberglass batts already in place is....ah....pushing it a bit.
It might be worth the effort to read up on data involving true R values of insulation and the installation effects of to see how necessary the R-49 attempt might be. Diminishing returns for the effort are waiting out there for you to balance.
If you were to remove the 6in R-19 fiberglass and replace with say 5 inches of foamboard at roughly R-25 or more depending on the board you've gained a solid 7 minimum with a better grade of insulation over the questionableR-19 poorer quality fiberglass batt. Foamboard foamed in place will give you the tightness you are seeking for the more maximum benefit of your insulation. But a lot of work.
Some home stores have dow blueboard readily available in 2 inch thickness or you can order 3 inch. 3inch at R-15 plus your 25 puts you at a optimum real R-40 that is great.
Long screws, special order usually, through 1x strapping and 3inch foamboard into your 2x6s will give you a decent base to work from to apply the drywall and that 3/4 inch air space between the drywall and the first surface of the foamboard will give you a few more R values to throw into the numbers mix, especially if you could manage to put a foilbacked foamboard as the top foam, taped for a vapor barrier and the supposed heat reflective properties of the foil really whip the numbers up. Ka-Ching really sleep well at night then. Roar!
Or special order a 3 inch foilbacked foamboard for ease of installing. Or the readily available 2 inch board with a 1inch foilbacked, not as easy to place your strapping over.
Or if you really wanted to go for it you could frame another light ceiling and wall inside what you have and insulate between. Money and effort and continually shrinking your available walking floor space by lowering the head room of the ceiling slopes.
Or with what you got: R19 + 3inch@15 +air space and drywall@?5 =39 and good enough.
good posts by several here, what i have done on cathedrals with success in the past is use the foam board and strapping. i like to rip sheets of 5-ply 1/2" ply to 3" wide strips for the strapping, and use the long screws through the strapping and regular to secure sheetrock to strapping. what i like best about this method is using long sheets of drywall so you have no butt seams in the field, only the beveled edges, allowing for an easier mud job. also you are able to check with straight edge and adjust strapping in and out (foamboard gives a little) to get it to plane in very nicely, often helpfull on old rafters.
since it is now conditioned space i would eliminate venting. you may still need it behind knee wall? but i think venting should stop where the cathedral ceiling is a conditioned space.
skid- did you really say venting. He's got fiberglass up there.
yes i did say venting. original post says he has baffle vents from ridge to soffit, not sure if thats a baffle at ridge and soffit or if it runs continious but i am an advocate of no venting a cathedral ceiling that is also an exterior wall. simply closing the openings on the outside of the structure would accomplish this. if the knee wall itself is uninsulated then no venting at all, if knee wall is insulated then treat area behind knee wall like any attic.
would you vent an exterior wall? if you wouldn't vent an exterior wall why would you vent an exterior wall just because it happens to be overhead and at a pitch? depending on climate you may need vapor barrier, on either hot or cool side, but no vent. there are those that disagree with this, but i have done it on my house in california with no problems.
Hi skids, thanks for your post. When you use the 1/2" ply, I take it meaning it's across the top of the rigid foam every 24" . What I would end up with then is the faced fiberglass between 2x6 vented rafters, 2" rigid foam running perpendicular, then 1/2" x 3" ply strapping, and drywall. I plan to run the drywall from ridge to knee wall - 12' cut to length eliminating any butt joints.
Do I have it straight?
yes that is it. check with straight edge to make sure everything planes in, adjust if not. the 5 ply has less of a tendency to split than say a 1x4 would, and provides a better hold for the drywall IMHO. you could also do it without the strapping as another poster suggested, its a personal preference thing. the advantage to straping is a huge area to hit wood with a screw, and a rigid surface that planes in nicely. with the longer screws and no strapping you have to make sure you are going straight and the you get a good bite into the rafter, so a little more attention has to be given to marking where to put the screws on the face of the drywall. as long as you are using a drywall lift you could do it either way. the strapping is time consuming. the carefull attaching of drywall with long screws also time consuming.
jackson-
so now that a few days have passed and time to mull over a decision, what'd you finally come up with?
Hey Rez!
After lots of mulling, I decided to go with the 2" rigid foam over the existing fiberglass (I'll slit the kraft-face) and 1/2 plywood strapping. I'll use polyethylene vapor barrier before installing 5/8" drywall.
Sound kosher?
Well, I ain't a pro but think the poly over the foam is overkill since taped or foamed seams tween the foam boards should act as your vaporbarrier especially since your vented way up on the top.
I like the idea of adding another 1/2 inch foilback foamboard to the face of your 2inch foam beneath the strapping and taping the seams as in the earlier posts. More insulation value since you've cut back on your original aim.
But that sounds good enough without getting into extremes and not overly complicated to get an ok result.
So how'd you treat the sidewalls and end walls since the whole thing sits like a cap on the house?
In these parts, Wisconsin, exterior walls are spec'd for R-11. I will use the same for the knee walls, and leave the area behind uninsulated (much like the attic was before I started the project).
Does the poly vapor barrier along with foam lead to trouble, or just overkill?
Hi jackson,
let me make a disclaimer since some guys here frown on amateur posters giving advice. I am a amateur homeowner and all advice is just in my humble opinion. Although one does learn a few things standing around while shuffling his feet and asking questions.
Oft times folks might post an ideal answer to a question that can have a dozen answers in a real life environment that can give success. Sometimes their time involved and money have no limit to achieve the perfect ideal.
Sometimes a homeowner just wants an average success for his endeavors without reaching for the nirvana building technique.
Me, I've got a cold, a toothache, and it's Saturday night on my third beer so I'll tell ya what I know in my limited experience. Ok, enough of the disclaimer.
<and leave the area behind uninsulated... I'd want to know more of what you're saying here since I'm not clear on it. I hope no area of your walls are being left uninsulated as that could undermine the heat containment of the room.
Not just from heat convection thru the walls but chances are you'll have air currents making their way in from somewhere outside in the building envelope. And that area where the top plate of the wall meets the roof can be a nasty area to insulate tight. Foamboard over studs and plate are ideal there.
The walls in the upper room here should be treated with as much respect as your attention to the roof.
I can't wager an opinion on trouble from the poly over foam except that if a double vapor barrier is enclosed in a wall there is the possibility of moisture leaking thru the first barrier and being caught between the two barriers, not being able to migrate to either the inner or outer surfaces and cause mold uglies and the like.
However, it has been said that that isn't always the case. Dealing with the location of the barrier inside the wall and the percentage of the total insulation sum found on the inner and outer sides of the inside vapor barrier. Those numbers slip by me now but aren't important in your scenario.
Then again in would seem that multiple layers of foamboards would perform the same thing as a double vaporbarrier and there are those who say to apply some sort of solid adhesive or foam spray between the faces of the foamboards to prevent any moisture migration.
Ideal or practical function? The poly on the foam might be a risk but probably not. I wouldn't do it and would just tape the seams of the foamboard. I think either way would be alright.
I recently came to the conclusion that fiberglass installation should never be used in a cathedral roof situation. But that's just my opinion.
With easy access to foamboard products and the whole venting controversy ie: venting lets in more moisture/ you must let moisture escape etc. and the nature of fiberglass to carry moisture I'd opt for foamboard. But with yours already done Id just let it fly as is.
Rez,
Sorry about the cold and toothache. I've stayed healthy since Thanksgiving. You might want to switch to Jim Beam. That's my preventative of choice.
I see your point about the uninsulated portion of the attic and easy to remedy. I definitely got ahead of myself with the insulation. I had a different, albeit incorrect, idea in my head about how I was going to use the space a year or so ago. Since initial post, I've learned a lot about fiberglass and its appropriate/inappropriate applications.
Skål,
jackson
jackson-
Just another question out of idle curiosity.
I'm supposing you have been reading some of the other insulation threads running here and those in the archives. Some regarding the cons of fiberglass insulation.(oh gawd, here we go he says)
My question is in regards to your thinking about your present 6inch fiberglass insulation between your 2x6 roof joists. And the fact that even tho' it is already up there it is a small area to contend with.
I only say this because it seems as a homeowner doing this himself you present a detailed oriented approach to wanting to do your project right.
And seeing the great grey areas involving pros and cons of fiberglass batt insulation and what can occur if a failure is involved meaning wet insulation, worthless r-values and mold. Not to say this will occur but just the idea of limiting possible scenarios in the event.
Just wondering where your thinking has evolved to now that you have saturated yourself with pertinent data.
inquiring minds want to know
Rez,
I have thought on it, not so sure of much evolving happening--grunt, grunt. What experience I'm working from has been a few varying applications using the vent baffles, unfaced fiberglass (deeper than 6" as I remember) and the poly vapor barrier covered by drywall. Only recently (postings and others) have I learned of the moisture trapping qualities of fiberglass. I have thought about replacing it with the rigid foam and the like. I understand, too, the greater R value benefit of doing so. Two things make me stop and reconsider this -- $$ spent and getting a good fit with the foam board between the rafters.
I have not worked with the foam before, so I don't know how easily it is to rip to fit. I also know that the rafters aren't necessarily evenly spaced, and there is fluctuation in single cavaties. Fiberglass, on the other hand, is more easy to conform or fill voids if necessary.
Man, you're right. Foamboard between the rafters is a job and can't say it is a labor of love. Roar!
I think maybe the baffles and unfaced fiberglass batts were made solely for the ease of installation. I did a cathedral ceiling very much similar to yours in configuration several years back now. My whole thought at the time was insulation value with 2x6 rafters and man the torpedos!
I looked at those baffles as they were the general consensus for cathedral venting at the time and thought why spring the bucks on those when I can cut foamboard to fit and gain some R.
I nailed 2x2s up the sides of each 2x6 against the roof sheathing and found the 1/2" celotex foilback board to slice easy with a utilityknife and a straightedge. Laid a good bead of caulk where the 2x2 met the joists and pressure fitted the half inch foamboards on there. Reminded me of the old schoolboy analogy about tighter than a nun's.
Then I got lost in tinytown with a handsaw and dow bluefoamboard of varying thicknesses to get the same tight pressure fit hammered in. If I were to do it again cannedfoam would ease the time and labor fitting tremendously.
Applying foamboard over your rafters under the sheetrock will make a big difference in the tight heating envelope you're trying to achieve in the old house.Good luck Jackson, it can be done.
Keep in mind that the difference between R-26 and R-49 is not all that it would appear to be. 'R' represents radiant losses. The bulk of heatloss in most structures is convective, so a continuous and effective air barrier is far more important than the incremental (diminishing) returns going from R-26 vs R-49.
The real world thermal performance of foam board is significantly better than fiberglass because there is no air movement within the material.
I designed an enormous home (in New England) with a 2-inch layer of foam beneath the drywall on every exterior wall. No strapping, drywall screwed through foam. Continous thermal break with the framing. The thermal performance is amazing, and there is no drywall cracking after 6 years. The principal downside to this approach is the extra effort in trimming out the windows and doors to account for the extra thickness.
csnow... ah... doesn't R-value refer to RESISTANCE to heat movement.. not radiant..?
and.. though you may like screwing thru foam.. i certainly don't.. i would strap my foam anywhere i use it..
get better plaster jobs.. at a lower cost.. and plenty of blocking for trim... base, windows .. chairrail.. doors.. you name it.. the 3/4 strapping can fix it
your point about a diminishing return between R29 & r45 is confusing.. what are you measuring... money ?... btus ...
what about the difference between R-2 & R-3.. is that a diminishing return.?. all of these numbers are relative..
one true way is to use different scenarios and evaluate them for overall heatloss of the envelope.. or evaluate in terms of occupant comfort
but hey, whadda i no ?
Mike Smith Rhode Island : Design / Build / Repair / Restore
"doesn't R-value refer to RESISTANCE to heat movement.. not radiant..?"
In theory, but it is essentially a Radiant test. It does not translate well to the real world conditions where convective and conductive factors kick in. It does not translate well across insulation materials.
"...though you may like screwing thru foam..."
Works for me. 1 less step. More than one way to skin a cat.
your point about a diminishing return between R29 & r45 is confusing.. what are you measuring... money ?... btus ...
Cost/Benefit ratio.
one true way is to use different scenarios and evaluate them for overall heatloss of the envelope.. or evaluate in terms of occupant comfort
Yep. Agreed.
csnow.... if you do a whole house heat loss to project your annual heating bills.. the R-values of teh various sections will be the only element to calculate btu/h... not radiant values ...or convective values... unless you are rewriting some of the ASHRAE calculation methods
if you want to do a blower door test so you can calculate air-exchanges.. that would be valid...
this is starting to sound like the old envelope house theories
Mike Smith Rhode Island : Design / Build / Repair / Restore
"if you do a whole house heat loss to project your annual heating bills.. the R-values of teh various sections will be the only element to calculate btu/h... not radiant values ...or convective values... unless you are rewriting some of the ASHRAE calculation methods"
I agree with that statement. The thing is that R values are used here as a matter of convenience, not optimal accuracy. You can determine R values based upon tables and such without any special equipment, and you may even hit a reasonable average heatloss value.
As you mention, the blower door test is more meaningful for most structures, because convective losses are the most significant heatloss factor. A heatloss calc based upon R values is going to come up short on a leaky structure. Standards are fine and good, but any given laboratory condition is not going to exist in the real world.
"In theory, but it is essentially a Radiant test. It does not translate well to the real world conditions where convective and conductive factors kick in. It does not translate well across insulation materials."
What "test" is that.
From what I quickly gathered R values are found by two different ASTM testing methodes.
In both test the SURFACE of the material is matained as controlled tempaturers. And then the heat transfer through the material is measured. That is purely a conductive test.
http://www.fullback.com/pt_rvalue.php
"R"-value is based on a mathematical term known as "R"-factor. The term "R"-value was developed to represent the ability of an insulation material to restrict heat flow. It is determined by placing test specimens between two plates in a laboratory apparatus and measuring heat-flow through the insulation. The test specimen usually consists of a square foot of material exactly one inch thick whose surfaces have a temperature differential of 1 degree Fahrenheit. The thermal conductivity (k) of a material is expressed as the rate of heat flow in BTUs per hour.
Thermal resistance (R) of a material is its resistance to heat flow, and "R"-value is expressed as the reciprocal of the material’s thermal conductivity. Simply put, the greater the "R"-value the better the insulation.
Fullback contoured EPS foam products cannot be tested under ASTM C177 or 518, which are prescribed for testing only flat (consistent thickness) sheets. Fullback must be tested using the "Guarded Hot Box Test" outlined in ASTM C236. This same test method is used to test windows, etc., where various thicknesses and components are involved.
http://www.buildinggreen.com/features/tm/thermal.cfm
R-values are measured by testing laboratories, usually in something called a guarded hot box. Heat flow through the layer of material can be calculated by keeping one side of the material at a constant temperature, say 90°F (32°C), and measuring how much supplemental energy is required to keep the other side of the material at a different constant temperature, say 50°F (10°.C)--all this is defined in great detail in ASTM (American Society of Testing and Materials) procedures. The result is a steady-state R-value ("steady-state" because the difference in temperature across the material is kept steady). R-value and U-factor are the inverse of one another: U = 1/R. Materials that are very good at resisting the flow of heat (high R-value, low U-factor) can serve as insulation materials. So far, so good.
That is an interesting article on how mass affect dynamic performance.
Also see http://www.buildinggreen.com/features/tm/sidebar.cfm
Building systems with high mass in or on the build (vs interior high mass) do benifit from the high mass where you have large daily temp swings such as in the dry SW. The far north and SE where you are cold or hot most of the day the mass adds little.
And I came across this site which will calculate whole whole effectice R-values based on several different common types of contruction.
http://www.ornl.gov/sci/roofs+walls/calculators/wholewall/
I have not dug into it, but it looks like the info at this site on how whole walls and whole building perform and not just on the characteris of FG, foam, and wood.
Hiya Jackson- Just thought I'd step in here again to fill you in on the venting comments seeing you are fairly new to the forum.
I forgot that there were certain topics better not mentioned in January and venting is one of them. :)
A few years back here on Breaktime I understand there was quite a war involving 'to or not to' vent and these winter months generally have posts that can get rather exciting seeing those normally outside find themselves in and also find Breaktime a viable outlet for energies.
So, it's a good time to just kickback and gather data. A fella can learn a lot around here.
Howdy rez,
Yeah, I'm gettin' a real education here. I'm fascinated by the exchange of ideas and have a lot to chew on before I get down to business.
I failed to mention I'm in southeastern Wisconsin, and have always been under the impression that it's necessary to vent roof sheathing. I'm going to stick with the vents, but I like what I'm hearing about a variety of foam products and their performance. I kind of knew that I'd never likely reach R49, but these products may offer good possibilities for making it comfortable in cold winters and humid summers.
Thanks for the input and seasonal guidance.