New Build Insulation Question
Hello, my wife and I are building a new home in Missouri, right on the northern edge of zone 4. I have spent the last few weeks sifting through forums about how to best insulate a new house. I am slightly overwhelmed with the back and forth I find in the forums and want to post my questions here. Our house is a hybrid steel wood building. Picture a red iron building, except wood 2×6 girt and 2×8 perlins on 2 ft centers. I want a tight envelope and plan to sheet with Zip panels and would like to put foam on top of that to increase efficiency. It makes me anxious when I read about how foam can cause moisture and rotting issues when not done properly. Here are my questions:
1. Should there be an air gap between the sheathing and foam? Or if I use EPS is that permeable enough to let any moisture out that could make it between it and the sheathing?
2. Since the foam acts as a vapor barrier, should the insulation bats be unfaced to allow to dry to the inside. We will have a combo of painted drywall and painted or sealed carsiding on walls and ceilings.
3. The ceiling is vaulted and an unvented roof. Do I follow the came principles as my walls(foam min 27% R value, faced or unfaced bats) or is it different?
4. I plan on using a house wrap on top of the foam per a GBA article I found today unless anyone advises against it?
5. The whole reason I went down this road is to provide a thermal break over the steel. Can I simply achieve that by putting 1″ of foam under the sheathing and be done? These building have steel cross braces and do not rely on sheathing for strength as metal panels can be attached directly to the perlins and girts.
Thank you in advance!
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Replies
Energy efficient steel building is almost an oxymoron.
The fact is steel conducts heat from the inside to the outside so well that insulation between the steel supports is almost a total waste of time and money. The only affective place to insulate a steel frame is 100% on the exterior of the steel.
You may find these articles interesting.
https://www.greenbuildingadvisor.com/article/insulating-a-metal-building
https://www.greenbuildingadvisor.com/article/insulating-pole-barn
https://www.greenbuildingadvisor.com/article/building-with-steel-framing
Walta
Walta, thanks for your input. This is a hybrid wood/steel building. All of the clips that hold the wood perlins and girts are on the exterior of metal frame, so all of insulation in the cavity is outside the metal.
I wouldn't be so harsh. Steel buildings can pose challenges but they can be made to work with a bit of upfront design effort.
I'm not following exactly where the steel is in your assembly, can you do a quick sketch of what you are proposing.
Thank you everyone for your replies. I dont want to open up a can of worms on steel buildings, so let me simplify this a little! Lets forget about the steel frame for a second, as the wood wall sits outside of the frame. What would recommendations be for the following questions for a house with 2x6 walls and 2x8 vaulted ceiling with unvented roof in zone 4.
1. Should there be an air gap between the sheathing and foam? Or if I use EPS is that permeable enough to let any moisture out that could make it between it and the sheathing?
2. Since the foam acts as a vapor barrier, should the insulation bats be unfaced to allow to dry to the inside. We will have a combo of painted drywall and painted or sealed carsiding on walls and ceilings.
3. The ceiling is vaulted and an unvented roof. Do I follow the came principles as my walls(foam min 27% R value, faced or unfaced bats) or is it different?
4. I plan on using a house wrap on top of the foam per a GBA article I found today unless anyone advises against it?
5. The whole reason I went down this road is to provide a thermal break over the steel. Can I simply achieve that by putting 1″ of foam under the sheathing and be done? These building have steel cross braces and do not rely on sheathing for strength as metal panels can be attached directly to the perlins and girts.
No need for an air gap. EPS is not a vapor retarder. You don't say what thickness rigid foam, but you won't need much in zone 4. I'd use XPS, as the new formulations are more green and XPS is a vapor retarder. You can put housewrap where you want. I used Zip sheathing with no housewrap. You do want to plan on a continuous air barrier that merges with window and door flashings, so it's easiest to make face of sheathing the location for air barrier. Somewhere on this site is a chart showing how much insulation on exterior of roof for each zone. You can use Kraft face batts if you want. They're not much of a vapor retarder so your wall will still dry to inside. If you want, you can slice up the paper face.
BYutz,
1. No air gap is necessary as long as you follow the the recommended foam ratios. the walls will dry to the inside.
2. Yes unfaced batts are preferable.
3. The ratios for roofs can be found in assembly #3 in this link
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
4. The benefit to having the WRB over the foam is that you can locate your flashing there. If it is behind the foam the flashing needs to go back to there to be lapped.
5. You could. That will leave you with an around R-4, which may not be enough to prevent condensation on the steel, or the whole wall if you are close to climate zone #5. If you prefer to use less than the recommended ratio of foam you should follow the advice in this link: https://www.greenbuildingadvisor.com/article/rethinking-the-rules-on-minimum-foam-thickness