I have been reading with high interest, the discussions related to applying rigid foam insulation to the exterior of the sheathing. I also read the recent article in FHB about applying foam insulation on the exterior.
My particular case is retrofitting an existing structure (location: New York State, 1988 construction, 2×6 construction, kraft-faced fiberglass batts) to be more energy efficient. My 2×6 walls with FG batts is nominally R19, but efffectively it is likely equivalent to r13 considering thermal bridging and other effects.
I read the FHB article showing 2″ and 4″ exterior foam applied as a retrofit, but do not see how this is practical in most aplicatons. I am considering 1″ foil-faced poly iso foam board at R6.5, which effecively will increase my total R value by about 50%. I plan on a Tyvek barrier detailed carefully. My theroy is that sealing the sheathing properly will minimize air movement and vapor intrusion into the wall cavity. Some would think the Tyvek would be overkill, but I have read a study which details shrinkage of rigid foam panels over time, so I do not want to rely on the foam as a water and air barrier.
Will the 1″ foam be enough? Is my theory about air sealing on the right track? I can see retrofits of this type becoming more popular as opposed to the 2″ or 4″ foam method, which although effective, may be daunting to a homeowner.
As for siding, the other issue is nailing. I am considering fiber cement and am wondering what are the opinions about using a 1″ longer nail to put up the siding (without furring strips)?
Thanks for your help…
Replies
When you apply less than (thumb suck here) about an R15 to the exterior you need to worry about condensation in the existing insulation due to the new insulation trapping moisture. How serious a danger this is depends on the relative R values of the foam and existing FG, the type of foam you use, and what sort of vapor barrier (if any) is on the inside of the house.
I would guess that your house has a poly vapor barrier on the inside, so probably less of a condensation worry than a home a few years older.
Dan,
So you are saying that unless there is a poly vapor barrier on the inside, 2" minimum exterior is recomemnded? I only have the kraft faced batts.
If you go with a poly vapor barrier on the inside, you need a breathable foam like EPS or XPS on the outside. If you do the foil-faced polyiso on the outside, you can't do a poly vapor barrier on the inside or you'll trap moisture in the wall, but like Dan said you'll have condensation issues if there is not sufficient R-value in your exterior foam.
If you want to leave your fiberglass in place and don't want to deal with a thick buildup on the outside, a thin (1") layer of either EPS (white beadboard) or XPS (blue or pink board) on the outside would be your best bet.
Running a housewrap on the outside of that may make flashing easier to keep moisture from getting in from the outside, but it's not stricktly required on way or the other.
I'm saying that it gets iffy and complicated. Draw a cross-section of your wall with the inside surface on the left and the outside surface on the right. Under that draw a plot of the temperature inside the wall on the coldest day. It will obviously be warmest on the inside surface and coldest on the outside.Next draw a plot of the dew point inside the wall (easy to say, hard to do). The dew point will be highest on the inside surface, lowest on the outside surface, like the temperature, but the shape of the curve will be vastly different. (Dew point will be essentially constant across standard fiberglass insulation, eg.) If the dew point at any point in the wall is higher than the temperature at that same point in the wall, then condensation will occur.A vapor barrier on the inside of the wall will keep moisture from entering the wall in the first place and hence reduce dewpoint across the entire wall. Foam on the outside will trap moisture in the wall and raise the dewpoint, but it also raises the temperature inside the wall. How much effect the foam has on both dewpoint and temperature depends on the type of foam, and it's thickness.
What is wanted is not the will to believe, but the will to find out, which is the exact opposite. --Bertrand Russell
Dan,
What's the difference on the wall I got? At some point somewhere in the wall, the dew point will be reached, even with no fom on the outside. Is it that the exterior is leaky enough so that the inner cavity will dry up? Or is it that the moisture will remain as a vapor as it passes thru the sheathing?
It's not a given that at some point "the dew point will be reached". This is because both temperature and dew point vary across the cross-section of the wall. If I had a decent draw tool I'd draw some graphs for you to show you what I mean.OK, here's a very crude drawing:The temperature plot is fairly constant across the inside drywall, drops gradually through the fiberglass, is constant across the outside sheathing, and then drops more rapidly through the outside foam. The dew point plot (same crude scale) drops rapidly across the inside drywall, barely at all across the fiberglass, then rapidly through the outside sheathing and foam.In this case the two come closest at the outer edge of the fiberglass, but they don't cross, so no condensation will occur. Though it's hard to show here, with such a crude diagram, were the foam a little thinner then the two lines would cross and condensation would occur.The temperature chart is easy to produce for a given wall configuration, at least in theory. The problem is predicting the dew point chart with any accuracy.
What is wanted is not the will to believe, but the will to find out, which is the exact opposite. --Bertrand Russell
Also, upstate NY has a climate much luck here in Maine, where most vapor transmission problems come from inside the house--warm, moist air trying to find its way to the exterior. Any housewrap needs to keep rain out, but vapor coming in from the outside is less of an issue.
Twang ... Mike is right and wrong. On one hand, he implies moisture coming from/migrating into the wall from the outside, then he says it right ...
With few exceptions (the SE being one), moisture always is higher inside than out ... even when it is cool and raining.
A craft faced batt has an acceptable perm rating ... trouble is, it tends to not be continuous ... broken twice at each stud. Yes, the standard wall breathes so much that moisture moves through it w/out being trapped. As you tighten the envelope, you risk the condensation ... Under your worse case scenario of weather ... you could determine what the dew point temp is w/ an assumption of interior RH (e.g. 50%).
Personally, I disagree how 'easy' it is ... easy if you do it (calc) daily. On one hand, you have to determine the temp profile and then drag out your psych chart and map temp/RH changes at the key locations in the wall section to determine how close the dew point is reached at any particular surface.
Tighten the house up ... don't forget that you may need to now alter your mechanical ventilation (either the schedule of use and/or the volume of air). This could be a classic ... tighten the old house up and then watch it rot due to the condensation you create. Whether you put 1" or 4" won't really alter that need, really. You may find your house zooming from 30% RH in winter to 70-80% depending on lifestyle, etc. This is due to reduced natural ventilation (air leakage).
It is cheaper to mech ventilate than to leave it up to nature (energy wise).
Don't want to discourage you, but remind you that what you do may have other ramifications that you haven't considered.
Clewless,
Moisture in the form of rain is trying to get in from the outside.
Moisture in the form of water vapor is trying to get out from the inside.
Except in humid summers with A/C running, when inside R/H might be lowered to 30% while outdoor R/H is much higher. In that case moisture in the form of water vapor will be trying to get into the house. It's not a significant portion of time in twang's case but it does happen.
Right ... but assuming you have a way to shed basic rain from getting into the construction (e.g. lap siding), then rain on the outside is not much of an issue ... unless a hurricane drives it horizontally ...
thanks for clarifying ...
I've only worked where horizontal rain is frequent, so I consider lap siding only an initial rain deterrent. It's the next layer down that's really important for keeping water out.
Moisture isn't driven by relative humidity, it's driven by absolute humidity. In the winter when it's 30% inside and 60% outside the drive is still from inside to outside.
It is an ironic habit of human beings to run faster when we have lost our way. --Rollo May
Right on, Dan ... thanks for the clarification/addition. A psych chart will show you that many times, even if it is raining ... the 'force' is from the inside, not the exterior.
What is it in summer when it's 65° and 30%RH inside and 90° and 90%RH outside?
IOW, what about Florida? Yes, in such climates the drive is the other direction and a vapor barrier (if any) should go on the outside.We do see brief periods of such inverted temp patterns even this far north, but not often enough or for long enough to be of concern.
It is an ironic habit of human beings to run faster when we have lost our way. --Rollo May
You're referring to the SE summers (but as Dan says, it can happen elsewhere, too albeit for short periods of time).
I think much of the discussion was with respect to heating climate ... cool or winter conditions. In the NW, we have a lot of rainy days in the 40 degF range, so outside air is 90%+ RH and cool while inside might be 70 degF and say 50% RH almost twice the absolute humidity.
If you allow the inside space to get to 70% RH, you begin to ask for trouble. If outside air temp drops, you might reach dew point on a wall surface ... I've seen this happen w/ brand new houses w/ poor bath fans. The vapor pressure inside wants to get outside ... the potential for condensation rises. Got an air leak in your ceiling? Then it goes up and condenses on the nearest cold surface ... the underside of the roof sheathing and exposed roofing nail points ... I've seen baseball size ice on nail heads ... then when things warm up you end up w/ major problems!
I'm digressing, though ... sorry
I'm not a pro, just a FHB fan- I built a 1000 ft addition to my house using an article in FHB as a guide to insulating the exterior. I can't reference it at the moment. The Author uses (from outside in) foil face foam- pink foam staggard over joints- tyvek-1/2 in sheathing cdx- blown cellose- NO VAPOR BARRIER- drywall.
The article explains that condisation has to have a way of evaporating either inside or out.
I went a step father and left the holes to blow the cellose in open and used a drop ceiling so the cellose (3" hole) is exposed to the inside air,
Its my guess if you have fiberglass insulation, then the tar paper on the FG is a vapor barrier trapping the moisture inside the wall.
Piffin, who is the site expert, will tell you foam on the outside depends on the climate you're in
I've been through one season with this new addiition and all I can say is "WOW" its really working well. My only regret is I wish I went with a "Mooney wall"on the inside
Last note- I am using a rain shield for my siding. Furring strips vertically attached to the house and siding nailed to the furring. Air then can traverse up the wall behind the siding and dry out the wood siding
GO Cubs----------
"The Author uses (from outside in) foil face foam- pink foam staggard over joints- tyvek-1/2 in sheathing cdx- blown cellose- NO VAPOR BARRIER- drywall."The foil facing on the foam has a very low perm rating, that's where your vapor barrier is. Your drying will be to the inside now, or not at all.
Jon Blakemore RappahannockINC.com Fredericksburg, VA
Yes, the way the article was describing, the house dries to the inside.
"Dries to the inside" is a misstatement when the moisture drive is toward the outside. What's really being said is that at equilibrium (with essentially no moisture movement either direction) there's no condensation in the wall.
It is an ironic habit of human beings to run faster when we have lost our way. --Rollo May
Hey, I didn't write the article. I'm now trying to fix my piece of #### lawn tractor. If it get some time I'll get the FHB issue and page for you. I know your line of thought is the dominate one here at BT. I thought the article made sense to me. Either the condensation dries in or dries out.
It's a convenient fiction -- a mental model -- but just not how it actually works. The model unfortunately doesn't tell you whether a particular configuration will have condensation problems or not.
It is an ironic habit of human beings to run faster when we have lost our way. --Rollo May
Like I said to the orginal post. I never claim to be a pro like the others at this site. I am a big fan of Fine Home Building magazine. I just told the gentleman I used an article from FHB and applied it in my situation with good results.
One thing not mentioned before, if there is R-15 on the sheathing with foam and you dense pack the wall with cellouse, how can any possible airflow get to the sheathing which wouldn't get that cold anyway (with R-15) to cause any condinsation???
Go Cubs
Generally, with R15 in the sheathing there'll be little chance of condensation from outside temps, assuming the sheathing is a vapor barrier and the inside insulation is under R30 or so. But the outside had better be sealed tight, because the tinyest air leak could cause condensation. Also, if the sheathing is only an R8 or so, there's a good chance of condensation.
It is an ironic habit of human beings to run faster when we have lost our way. --Rollo May
Your terminology is off is all. It's vapor pressure, but your idea is generally right ... i.e. you understand sort of. Someone needs to step in here and set us all straight ... where is Listbruek when you need him ... doesn't he read this stuff?
Do you know where Curley lives?As I understand it, there can be a moisture drive to the inside. The conditions just have to be right. Maybe it doesn't happen in Minnesota, but it does in Mobile.BTW- I agree with Curley. Go Cubs!
Jon Blakemore RappahannockINC.com Fredericksburg, VA
How would I know if he doesn't update his profile?
It is an ironic habit of human beings to run faster when we have lost our way. --Rollo May
That's my point. I thought that you were saying that "'Dries to the inside' is a misstatement" applied to Curley.Isn't that kind of assessment climate-specific?
Jon Blakemore RappahannockINC.com Fredericksburg, VA
I said dries to the inside is a misstatement when the moisture drive is towards the outside.
It is an ironic habit of human beings to run faster when we have lost our way. --Rollo May
I see.
Jon Blakemore RappahannockINC.com Fredericksburg, VA
I've followed this discussion and now I would like to throw another curve. I'm in
Minnesota in a 45 year old house with R11 FB walls kraft face then sheathing then about
20 year old tyvek then cedar.
The cedar is good and I do not want to remove it. If I try to improve R value in the
wall by simply applying rigid on the inside directly to the sheetrock and then sheetrock
again over the rigid, what's the trap?
I've got a very inefficient vapor barrier now and I'm drying to the outside every
spring, wouldn't the rigid on the interior help reduce my moisture transfer?
I've got the moisture from the sill below stopped with spray urethane even into
the tip openings of the block wall.Your thoughts please.Navion
Should be no problem with foam on the inside.
It is an ironic habit of human beings to run faster when we have lost our way. --Rollo May
Thanks Dan,seems like a reasonable way with 1.5 inches of wall space to pick
up 5 or 6 R's.Navion Ed
Aside from having to pack out all your trim and adding rings to your electrical boxs, and a bunch of other minor details, foam on the inside is theoretically fine. Although it's not necessary, you might consider stripping off the existing sheetrock and putting the foam directly on the studs.
yeh Red you have a point. The wall in question has no
windows etc.,.. Just a few outlets. The other thing I like about
this idea is that it produces a thermal break from the studs.
I'm big on spray poly when I can afford it, but I have a couple
of walls in the new additions where I will use rigid foam board
on the interior for the thermal break. Even with spray poly between
the studs, if you SR directly to the studs you get cold conduction
through the studs and across the SR.Ed