Insulating a house (remodeling project)
Some background information:
Zone – 5A – Shoreline Connecticut
Construction Year – 1958
House is framed with 2×4″ studs, sheathing is 3/4″ tongue-and-groove.
I’ve begun remodeling and I’m currently at the point where I’m ready to re-insulate my master bedroom. My plan is to attack one room at a time and the first is always the most difficult, so anything I do in this room I plan to transition into the other rooms (more-or-less).
My question is, what is the best approach to tackling this project?
I would like to air seal the best I can before I insulate, is canned spray foam an acceptable way to seal the top and bottom plates as well as where the studs meet the sheathing? Is there a better way?
As far as R value goes, I know fiberglass can only get me to R-13 in a 2×4″ cavity, is that enough along with air sealing? Can I do better without breaking the bank?
A lot of the information out there can get a bit confusing for me, I’m an avid DIYer but never knew the science behind insulating a home before.
If anyone can offer some advice or point me in the right direction I would appreciate it.
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Replies
OBG 8,
First of all, can you tell us your name?
Q. "I would like to air seal the best I can before I insulate. Is canned spray foam an acceptable way to seal the top and bottom plates as well as where the studs meet the sheathing?"
A. No, because your sheathing consists of individual boards, and every crack between the boards amounts to an air leak. The two best approaches to air sealing at the sheathing level in your case are either (a) the flash-and-batt approach, or (b) the cut-and-cobble approach.
For more on the flash-and-batt approach, see Flash-and-Batt Insulation.
For more on cut-and-cobble, see Cut-and-Cobble Insulation.
Q. "As far as R-value goes, I know fiberglass can only get me to R-13 in a 2x4 cavity. Is that enough along with air sealing?"
A. Whether it is enough depends on your goals and your budget. An R-13 wall is certainly a poorly insulated wall from the perspective of the building code -- it's less than minimum code requirements in your climate zone. One approach you might consider is to install a continuous layer of interior rigid foam. (If you take this approach, the interior rigid foam could be the air barrier.) Here is a link to an article with more information: Walls With Interior Rigid Foam.
Thank you for the links and reply Martin, my name is Chris.
The flash-and-batt method is definitely appealing, but from my understanding there is a chance of condensation forming.
"The flash-and-batt approach can be risky in very cold climates, unless the thickness of the foam layer is carefully monitored. If the foam layer is too thin, condensation can form on the interior surface of the cured foam, especially in late winter."
According to that article 1" of closed cell foam would be adequate for 2x4" walls to prevent condensation?
I am planning on re-doing the siding on the house, (not before winter but probably next spring) would it be worth while to air seal with rigid foam board on the outside instead of trying to do it on the inside? I know there are a few different types of foam board products, which is best? If I housewrap and rigid board seal the outside, would R-13 batts do a better job than flash-and-fill or cut-and-cobble?
Thank you again.
Chris,
As my article of the flash-and-batt approach noted, the minimum thickness of the closed-cell spray foam layer for a 2x4 wall in Climate Zone 5 is 1 inch. As long as you follow that recommendation, the flash-and-batt approach is not risky.
Q. "Would it be worthwhile to air seal with rigid foam board on the outside instead of trying to do it on the inside?"
A. Yes. For a variety of reasons, a continuous layer of exterior rigid foam is preferable to the flash-and-batt approach. Here is a link to an article with more information: How to Install Rigid Foam Sheathing.
With high density batts in 2x4 framing you can get up to R15 in either fiberglass or rock wool. The R-value is one thing, but the air retardency is another. R15s are far more air retardent than R13s, and with a nearly-impossible-to-perfectly-air-seal plank sheathed house it's well worth the upcharge to go for R15s (rock wool preferred.)
It's tough to fully air seal a plank sheathed wall with rigid foam from the exterior, but it's still a huge improvement. The seams of the planks can still leak air, so take pains to seal the edges of the foam well.
In your area there are multiple vendors of used roofing foam, at a significant cost savings to virgin stock goods. But an exterior foam-over is a much bigger job than a flash'n'batt- be sure you're up for it! (Martin can point you to the relevant pages on this site covering the topic.) Sheathing a 2x4/R15 wall with 2" of reclaimed roofing polyiso DOUBLES the performance of the wall, and beats the IRC 2015 code minimum for zone 5 (R13+ R5 sheathing) with a bit of margin.
Thank you Martin and Dana,
I appreciate the information and resources provided.
One last question, what is the best product to seal a mudsill/rim joist from the outside as well as the inside? Spray foam? Caulking?
I know using spray foam on the outside is not a good idea.
Thank you again!
edit: One last question, I think I plan on going with 1" XPS rigid foam on the outside sheathing with high density R-15 on the inside, do I need to consider having a vapor barrier on the inside? Do paper faced batts provide a sufficient barrier? Or should I go with unfaced batts and a type of plastic barrier. I'm not really sure if it matters at all but I figured I would ask! Thank you again!
Chris,
With a mudsill, all you are doing is reducing air leakage through a crack. With a rim joist, you have two tasks: air sealing and insulating. So you need different approaches.
The best way to seal air leakage at the mudsill is with a European tape like Siga Wigluv, although most builders use a high-quality caulk like silicone or polyurethane caulk. Using caulk is usually fine, unless you are aiming to comply with the Passivhaus standard.
To insulate a rim joist, you can either use a layer of rigid foam on the exterior side of the sheathing -- always to best approach -- or you can insulate on the interior with (a) cut-and-cobble rigid foam, or (b) closed-cell spray foam. For more information, see Insulating rim joists.
Q. "I think I plan on going with 1-inch XPS rigid foam on the outside sheathing with high density R-15 on the inside. Do I need to consider having a vapor barrier on the inside?"
A. The type of wall you describe should be designed to dry to the interior, so you don't want to install any interior polyethylene. All you need is an interior air barrier (usually, the taped drywall is sufficient). If a building inspector insists on a vapor retarder, either vapor retarder paint or a variable-permeance barrier like MemBrain will work fine.
Q. "Do paper-faced batts provide a sufficient barrier?"
A. The kraft facing is unnecessary but harmless. Kraft facing is not an air barrier.
Q. "Or should I go with unfaced batts and a type of plastic barrier?"
A. Unfaced batts will work fine, but you definitely don't want a plastic vapor barrier on the interior side of your wall if you have exterior rigid foam.
Despite the labeled R5, as it's climate damaging HFCs leak the 1" XPS will drift down to ~R4.2, which is lower than the IRC chapter 7 prescriptive for dew point control on a 2x4 wall (with a presumptive R13, not R15 cavity fill). It might be enough given that you're on the warm edge of zone 5, but it's not a slam-dunk.
If there is only an inch to work with, use foil-faced polyisocycanurate, not XPS. It's labeled-R is typically R6+, but on the cold side of the assembly it can only be counted on for R5- R5.5 in your climate, but even the derated value offers some margin.
As an additional vapor-retardent interior side air barrier something like 2-mil nylon (Certainteed MemBrain) or Intello Plus can work, but it's not absolutely necessary for moisture diffusion control as long as there is 1" polyiso on the exterior. Detailing the wallboard as an air barrier and painting it with any latex/acrylic paint would be sufficient.
Dana, according to one of the articles Martin linked it stated that XPS outperforms polyiso is colder climates?
https://www.greenbuildingadvisor.com/blogs/dept/musings/cold-climates-r-5-foam-beats-r-6
Also I think I read that XPS holds up better against moisture than polyiso? I could be wrong, it is still early in the morning here.
So you think that 1" XPS and R-15 dense fill will not be enough to prevent condensation on some portion of the wall?
Chris,
There are two issues of concern when evaluating XPS: (1) Environmental damage caused by the blowing agents used in its manufacture, and (b) The gradual reduction in the R-value of the insulation due to the escape of the blowing agents.
For more information on these issues, see:
Choosing Rigid Foam
Thermal Drift of Polyiso and XPS
New XPS will outperform polyiso when it's cold enough, but after 20 years of outgassing that is far from guaranteed. On the fairly temperate coastal CT warm edge of zone 5A polyiso may even outperform new XPS, inch for inch, even if it doesn't in northern New England.
Looking at Karagiozis' test samples the performance crossover point between fresh XPS & polyiso occurs when the mean temp through the foam is about +10C/50F,, but the crossover between high density EPS (which represents where XPS will eventually be) occurs at a mean temp through the foam layer of +7C/45F:
https://www.greenbuildingadvisor.com/sites/default/files/Karagiozis%20-%20thermal%20conductivity%20of%20a%20variety%20of%20insulations%20as%20a%20function%20of%20mean%20temperature.jpg
The binned hourly mean January outdoor temp in coastal CT is about 30F, and over the whole winter season about 40F. eg:
https://weatherspark.com/y/24702/Average-Weather-in-Bridgeport-Connecticut-United-States-Year-Round
With R5 on the exterior and R15 in the cavities the temperature on the warmside of the foam is about 1/3 the total inside to outside difference above the outdoor temperature, so when it's 40F outside, 70F inside, the warm side of the foam is 50F and the average temp through the foam is 45F. So when it's warmer than 40F outside Karagiozis' polyiso test sample beats EPS (and aged XPS), and when it's warmer than 45F outside it beats fresh XPS, That covers MOST of the heating season in your location.
[edited to add]
Dumb math error (not enough coffee?) The temp on the warm side is about 1/4 the difference not 1/3, but hut the gist is the same.
Hello again, hopefully this will be my final question regarding this subject! (Until it's time to tackle the attic)
Is there anything I should do to my tongue-and-groove sheathing before installing my rigid foam and WRB? Caulking, sealing, taping, etc?
I plan to install the windows using the "innie" approach so I will place my WRB and then foam over that. Should I use some type of caulking around the seams of the rigid foam when attaching it to the WRB? If so, what product is recommended? I've done a few searches online and I do plan to call DOW chemical company but if anyone has any suggestions that would be great.
As far as house wrapping, it's conventional to just staple the house wrap to the sheathing and tape all the seams correct? I'm not sure if there is a better or tighter method to installing it.
Thank you all again!
Chris,
Q. "Is there anything I should do to my tongue-and-groove sheathing before installing my rigid foam and WRB? Caulking, sealing, taping, etc.?"
A. Board sheathing is so leaky that it's not worth caulking. Your air barrier needs to be created at a different layer -- at the WRB layer or the rigid foam layer.
Q. "Should I use some type of caulking around the seams of the rigid foam when attaching it to the WRB? If so, what product is recommended?"
A. To limit air leakage, most installers tape the seams of the rigid foam. Foil-faced polyiso is the easiest type of foam to tape, but there is a tape for every purpose (and every type of rigid foam). For more information, see Return to the Backyard Tape Test.
Q. "As far as house wrapping, it's conventional to just staple the house wrap to the sheathing and tape all the seams, correct?"
A. No. Housewrap is usually installed with cap nails, not staples. Once you have chosen a brand of housewrap, you can go online to read the manufacturer's installation instructions if all of this is new to you.
So I have most my supplies ready, unfortunately I already framed out all my windows before I even learned about making little boxes to meet the end of the polyiso (only 1" though) so hopefully I will be okay :)
My question is, I got double foil faced polyiso sheets, now on the sheets it was mentioning about leaving a 3/4" air gap to perhaps boost the R value. Now this gap will be created by the furring strips of 1x4" boards I put down that I will eventually nail the siding to, correct? So my air gap will be between the siding and the outside of the polyiso.. Is this where I want it? I assume so but I really don't know.
Layering will be
Siding | Furring Strips | 1" Polyiso | Tyvek Housewrap | 3/4" Sheathing | R-15 fiberglass | Drywall
My other question was, can I leave the polyiso exposed during the course of winter or should I just leave the WRB up and forgo installing the polyiso until spring? I don't plan on putting siding up until I finish installing windows in all rooms and that will definitely keep me busy until next summer I'm sure. My house does have a 18" overhang so it would only be susceptible to the wind blowing up stuff. Would it be beneficial to install the polyiso and then install the house wrap over that? I would love to benefit from the added R-value over the course of winter but not if it will damage the foam, that stuff wasn't cheap!
Thank you again.
Chris,
Q. "On the [instruction] sheets it was mentioning about leaving a 3/4 inch air gap to perhaps boost the R-value. Now this gap will be created by the furring strips of 1x4 boards I put down that I will eventually nail the siding to, correct?"
A. This is correct.
Q. "So my air gap will be between the siding and the outside of the polyiso. Is this where I want it?"
A. Yes.
Q. "Can I leave the polyiso exposed during the course of winter or should I just leave the WRB up and forgo installing the polyiso until spring?"
A. The answer to that question can be found by consulting the installation instructions provided by the manufacturer of the polyiso. For example, I just consulted the installation instructions for a brand of foil-faced polyiso called Johns Manville AP Foil-Faced Sheathing, and this is what I learned: "Once installed, AP Foil-Faced Foam Sheathing may be left exposed for up to 60 days."
So if that's the brand of insulation you are installing, you wouldn't be able to leave it exposed to the weather all winter long. Other brands of polyiso may have different requirements.
Hello I've made some great progress with my installation so far but I fear I may have made a few mistakes and I'm wondering if I should re-do some of the work before continuing on... So I realized the windows I purchased have a pre-attached window frame about 1/2"-5/8" thick (for vinyl siding), that wasn't going to work with the 1" of polyiso and furring strips... so we went with the dudley box approach and installed a box around the rough opening that is an 1.75" thick to meet the 1" foam and 1x4" furring strips. Now I saw a Tyvek video where they also followed this same approach and I thought I was in the clear, turns out they were using Tyvek Drainwrap instead of Housewrap... I've already wrapped a good portion of my house in Housewrap and installed 3 windows using this approach (no foam installed as of yet)... Will this installation be okay?
Chris,
It sounds as if you have installed housewrap on the exterior side of your board sheathing, and that you plan to install rigid foam on the exterior side of the housewrap. Your plan will work. The housewrap doesn't need to include wrinkles, and you aren't restricted to a single brand of housewrap like Tyvek Drainwrap. Other brands of housewrap will work.
Here are the key points:
1. Your window rough openings need to have sill-pan flashing.
2. The sill pan flashing that you installed in your window rough openings needs to drain to the exterior side of your WRB. (For more information on this issue, see All About Water-Resistive Barriers.)
3. The exterior rigid foam layer has to be thick enough to keep the sheathing above the dew point during the winter. For 2x4 walls in Climate Zone 5, your rigid foam needs to have a minimum R-value of R-5.