SIP vs 2x6 with spray foam (energy efficient home building)

A common issue among homes that were built too-tight, is the need for make-up air, and the need to reduce moisture inside.

Heat-Recovery-Ventilators are designed to fill that need.

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We built a new house using 6" SIPs about 6 yrs ago. Some walls are 2x6 frame but most of the house is SIPs. I was on the construction crew and got to be part of all of it. The advantage of the SIPs is that they go up really fast, are very strong, and are super insulated. Ours are rated at R40, which may be overkill, since at a certain point all your heat losses are going to be thru the windows, doors, and floors. But they look nice, with the thick walls around the windows and doors. It's very important that your building, electrical, and plumbing contractors are familiar with SIPs because things run thru the walls differently than in a stick-framed house. Another real advantage with SIPs is that doing stucco on the outside, hanging sheetrock on the inside, and then putting up pictures and things later are much easier. You don't have to find studs. The advice we got when we designed the house was that the SIPs, tho more expensive material-wise than 2x6 framing, went up so fast that the cost was about the same. I think that was true. And the walls are stronger and better insulated. So far we have no complaints about the SIPs. This house has some passive solar concepts built into it. Heavy insulation, south facing windows to gain heat in the winter, and thermal mass in the form of ceramic tile floors and some adobe walls inside. This house gets about 1/2 its heat from the sun, and 1/3 from the wood stove. Radiant electric heaters come on occasionally if we don't make a fire at night. But we can turn all that off and leave for vacations in the winter and the house won't freeze up, just because of the passive solar heating that's built into it. For ventilation in the winter we just open a couple of windows for an hour or so during the sunny part of the day. You may not be able to do that in MN. For summer cooling we have a "swamp cooler" or evaporative cooler that we almost never use. The R-40 SIPs and the thermal mass inside the house keep it from getting very warm, even when the outside temps are 90-100. Just opening the windows at night lets the house cool down and when all that thermal mass gets cool it tends to stay that way. So the thermal mass works in the winter in the summer. The nice thing about passive solar design is that there are no moving parts, electricity, or fuel required to make it work. Just the sun, insulation, and thermal mass.

We had another house with radiant in-floor heating. I would never do that again. The heating system is complicated and a lot of energy went down into the ground rather than up into the house. That was because there was no insulation below the concrete. Insulation below the concrete would have helped a lot, but your concrete guys need to know what they are doing or you may have a lot more cracks in the slab than if you hadn't put insulation under it. Also, we've always gone for as much passive solar gain as possible (south facing windows) and that made the in-floor heating system shut off during the day when the sun was shining, and the slab got pretty cold before night when the heating system kicked on again.

This house has a crawl space with engineered floor joists, plywood, hardi-backer, and ceramic tile for a floor. 8" of fiberglass insulation is stuffed between the joists from below. This floor isn't nearly as cold and hard as a slab.

What to do to save energy costs? Here in NM anybody who's designing a new house and doesn't seriously consider the abundant sun we have, and getting a lot of passive solar gain, is dumb. Up in MN maybe it's not worth it because you have so little sun. I don't know. Some things I would consider up there, anyway, are:

- Minimum windows and doors on the north. You only lose heat, never gain it in the winter, with north facing windows and doors.
- Shade trees on the west to block summer afternoon sun from the house. Minimal, or shaded west windows for the same reason. If you put the garage on the west side that will keep the sun off the living area of the house in the summer afternoons, too.
- Entrance to house and garage not on the north. It will stay icy and muddy a lot longer on the north side of the house.
- It still may be worthwhile to do some passive solar. All that amounts to is south facing windows to gain heat in the winter, and thermal mass inside the house to store that heat. That can be concrete floors well insulated from below and around the perimeter, or like in our case tile on wood floors, and rock, adobe, or brick walls inside (like around a woodstove area). Just don't cover the floors with carpet or they won't work as thermal mass. The south facing windows need a 12-16" overhanging roof to shade them in the summer.

Same with every system that has thick walls. Our walls are a foot thick.



Woodstick is a fairly expensive method of home construction. About 3X more expensive than steel.



You walls are R-40. And your walls are better insulated than any other R-40 walls? How does that work?

I thought R-40 would equal R-40. But if R-40 with SIP is better than all other forms of wall with R-40. Could you please explain.



I have never heard of a radiant floor where the floor did not have insulation under it. I did not know that anyone does that.

You say it is complicated, there is generally one circulator pump. The pump is 'on', or it is 'off'.

My radiant heat system has 2 pumps, I guess it is double complicated. One pump circs water from the wood stove to the thermal-bank. The second pump circs water from the thermal-bank through the floor. 2 pumps, so 2 switches.



Your 'passive-solar' heats the floor, but the floor cools. What is your thermal-bank made of?



It would be warm if it were heated.



MN is different, over here in ME there are passive-solar homes that use no other heat source.

In my home the passive-solar keeps us above 50F, but that is about all it does.

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In a 4x8 section of wall, the SIP has insulation across the whole 4x8 section except the edges where it's joined to the adjacent SIPs. The same area of a stud wall has about 2 square feet of wood surface area instead of insulation. Wood is an insulator, but not a good one.

R-40 is R-40. But a 2x6 wood stud wall is only about R-19. That's the comparison I was trying to make.

The reason I think a radiant heat floor system is complicated is because it has some kind of boiler, with all the associated plumbing, valves, burners, etc. plus a pump and a controller unit and bunch of thermostats and solenoid valves to control the water to the different zones, plus tubing all over the place in the concrete. It's complicated, expensive to install, and expensive to repair. You are probably right that nowadays all slabs with radiant heat are poured over insulation. The house we had was 20 yrs old. And with a smarter (and more complicated) control system, a radiant floor heat system would keep the heat on the floors during the day (even tho the thermostats don't call for it) even when the house is warming from the sun. That way the slab wouldn't cool down so much during the day.

The floor is part of the thermal mass. The rest of it is some interior adobe walls. You warm it up in the winter, it tries to stay warm. You cool it in the summer, it tries to stay cool. It's called thermal inertia. What I'm saying is that what makes passive solar work in winter also makes it work in the summer.

Heating a crawl space to keep the floor warm doesn't make much sense. The floors (and all the other thermal mass inside the house) will stay about the temperature of the air in the house if they are insulated well.

Keeping your house at 50 degrees just using the sun, instead of letting it get a lot colder if you don't, is a huge benefit. Now all your heating system, whatever it is, only has to raise the house temperature 20 degrees instead of 30, 40 or more.

Yes, I understand. That is what we have been calling 'thermal-bridging'.

When you calculate R-value you subtract the thermal-bridging, before you get the final value.


Say you install insulation to R-50, then when you subtract the thermal-bridging, you might only get R-45. [or whatever]

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A woodstick house has a R-value of whatever you make it to be. There is no set value, because you are free to design it, and to install as much insulation as you want.

You can have a woodstick house with R-10, or with R-20, or R-100.

Say you use 2X6s, you put fiberglass between them, then you hang 2" foam sheets on the outside, before the tyvek, tar-paper, fur-strips and siding, and you hang 2" foam sheets on the interior, before you do paneling. The thermal-briding is still there. But no sane person uses 2X6s and has only 6" thick walls. Your going to have 8 to 10 inch walls. 5" for the 2X6s, plus 2" for the foam on the exterior, 2" for the foam on the interior, and the exterior and interior wall surfaces.

I did not use wood, but my walls are a foot thick. I paid way less then what wood costs.



Our woodstove heats water, we have a thermal-bank, two pumps and two switches. No burners, no controllers, no thermostats, no solenoid values, no zones.



Kind of. With other heating systems you heat the air. But with radiant flooring you heat the floor, so you use much less heat.

The air temp in our home can be 5 to 10 degrees cooler, but we feel warm when our feet are warm.

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I've never heard of subtracting the thermal bridging...I always hear people refer to the R value of a wall as the R value between the studs without subtracting anything. I wouldn't even know how to calculate the thermal bridging amount to subtract. Anyway, thanks for the explanation.

I have seen 2X6 walls where they do two layers of studs framing. The second set of framing, the studs are set between the first set, so they do not line-up. It cuts down on the thermal-bridging, and only really gives you a 10inch thick wall [before you add the usual foam sheeting on exterior and interior sides].

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Really, something like SIPs should virtually eliminate thermal bridging, though.
For all intents and purposes, it creates a solid envelope, right?

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Thermal-bridging is a greater issue with woodstick construction.



A 'solid envelope' minus doors, windows, floor and ceiling.

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If your going to build a stick build home with 2x6's only put 1 to 2 inches of foam in and then use fiberglass batting. My BIL is an engineer and did a lot of research before he build his home and this is the most energy efficient way. His house was too air tight so he had to put in venting for fresh air. His house is in MT and it can get cold where he is at.

Bob

Is he saying to put the foam board on the exterior? Or on the interior?

A total of 2 inches of foam is not very much.

How many inches of fiberglass is he recommending? I put 9 inches of fiberglass in our walls.

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Sorry about not being specific.

He sprayed 2 inches of foam inside then used 4 inches of fiberglass. From what he found out and the insulation guy confirmed is the foam seals all the drafts. The fiberglass has more air pockets in it and thus insulates better.

Bob

It greatly reduces thermal bridging. The SIPs I've seen use basically a stud along the vertical edges of each panel to connect them together, so there would still be bridging at each of those connections. But there may also be different ways of connecting them that I'm not familiar with.

Spray-on foam does seal 100% against all drafts.

I sprayed 2 inches of foam on the interior of our home's outer skin. Before I added 9 inches of fiberglass batting.

Your talking about 2X6 framing. I think your saying that he sprayed foam in the spaces between the studs. Which makes to source of the thermal-bridging.

2inches of foam, 4 inches of fiberglass, then subtract 5% [guess] for the thermal-bridging; may be plenty in his region.

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I usually spline my sips with 3" osb or plywood strips and foam between the panels so there isn't any thermal bridging. They make a very strong and tight house.

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Once you've achieved R-19 or so in your walls, I can't see how spending a lot of money and effort to increase the R value of the walls is going to pay off. You're going to lose so much heat thru your < R4 windows and doors that losing a tiny amount more thru your walls isn't going to make much difference in your utility bills. And the bigger the windows, the more heat you're going to lose in the winter, and gain in the summer, thru them. On the other hand, any windows that you can gain solar heat thru in the winter will count in your favor. And not gaining heat thru the windows in the summer will also work in your favor. So I think it's more cost effective to design a house so that you gain heat from the sun in the winter, and block it in the summer, than it is to make super insulated walls. Unless you can do it for very little additional cost. Or just don't have many windows and keep the ones you have really small.

I like that the look of that building...do you have any pictures of the finished project?

There's probably a lot of truth to what you say. The only caveat is that lots of places have very different climate than yours so it may not be as true some places. I think R-40 is probably overkill in most places though. I would add that sealing air infiltration is more important than super-insulating. I would rather have 1" of insulation in a very tight building (with controlled ventilation) than 12" in a very leaky drafty one.