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Planning a new double-stud house — what should I change?

My wife and I are planning a 2,100-square-foot house in Massachusetts. Here's the plan: a south-facing wall with glazing equal to 13% of the 1st floor area. A 5" radiant slab on grade with 4' frost wall on footings insulated with 4" of xps underneath with a stepped foundation top to minimize thermal bridging.

We were planning a 10 1/2" cavity double-stud wall with dense-pack damp-spray cellulose and lots of air sealing. Huber Zip System exterior sheathing with strapping and vertical pine siding sealed on all sides for a rainscreen wall.

The ceiling on the second floor would be mostly vaulted with 2x12 rafters with dense-packed cellulose. Our plan was to put 2" of isocyanurate on the outside of the sheathing and do a cold roof of exposed fastener metal roofing.

For systems we would have all radiant upstairs and down, heated by a Polaris propane water heater. Hot water would be tankless on-demand propane. We would install an HRV system with intakes in the upstairs and downstairs baths as well as the kitchen and supply lines in the bedrooms and living room.

We are also toying with doing photovoltaics due to the availability of Solar Renewable Energy Credits here, and also a solar hot water pre-heat system, in which case we would eliminate the tankless and use a heat exchanger with the Polaris.

I don't really know what I'm doing so any and all advice would be appreciated before we start and I royally screw this up!

Thanks in advance. - Noah Kaput

Asked by noah kaput
Posted Thu, 09/15/2011 - 11:30
Edited Thu, 09/15/2011 - 12:09

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10 Answers

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I should also mention that we have appropriately angled awnings on the south face and plan to do cellular blinds on the interior as added insulation. The windows will be Pella architect series unless Marvin beats them,

Answered by noah kaput
Posted Thu, 09/15/2011 - 12:00

2.
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Noah,
Some feedback:

1. I'm not in favor of in-floor hydronic heat. It's a very expensive way to heat a house. If you improve your thermal envelope, you can install a much cheaper heating system.

2. Propane is a very expensive fuel -- much more expensive than fuel oil.

3. If you buy a Polaris water heater for space heat, you don't need to buy an on-demand water heater for domestic hot water. The Polaris can do both. (It better -- it's really expensive.)

4. You don't mention what type of windows you are installing. Think about air sealing; think about thick insulation; think about really good windows; and then you can have a much cheaper mechanical system.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 09/15/2011 - 12:13

3.
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Thanks for the response! Everything ties together so that if we don't do radiant heat then we'll have to eliminate the slab so we're not walking around on a cold stone all winter. If we eliminate the slab then we need a crawlspace and 1st floor deck with it's associated costs and we need to raise the house higher due to a high water table, more fill, foundation, etc.
Propane was chosen in the hopes that not much would be needed! I was under the assumption that it burns cleaner but also, due to space concerns, I liked that the fuel is stored outdoors.
My thoughts on the on demand was that it would be more efficient than heating a tank full of water all year round for us to use a pretty small amount of hot water. Here in Mass., we would need a heat exchanger to use the same appliance for heat and hot water and the costs for the heat exchanger is similar to that of the on demand unit.
As far as the windows go, I would love to have triple pane, krypton filled windows but I have always been scared off by the price, the style/size/color selection, and the time-testedness of the products as well as the longevity of the manufacturers.
Thanks again, I've already gotten so much useful info from this site! What a resource!

Answered by noah kaput
Posted Thu, 09/15/2011 - 12:31

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Noah,
If you install continuous horizontal foam insulation under your slab, as well as vertical insulation at the perimeter of the slab, you won't need in-floor heat.

If your house is well designed, your in-floor heat won't come on for very many hours each day anyway, so don't expect the "warm-toes" phenomenon. The slab will just be at room temperature for most of the time anyway -- unless you've made some serious errors with your envelope.

With radiant heat, here's the rule: The warmer the floor, the worse the house.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 09/15/2011 - 12:37
Edited Thu, 09/15/2011 - 12:38.

5.
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That is great to hear! What type of heat would you recommend then? Regular old baseboard? or Runtal style radiators? We are not into mini-splits, and need something thermostatically controlled for the bank. If not for the bank, I'd have a sealed combustion wood stove and call it a day!
Also, do you think my use of Pella or Marvin windows is going to be a weak enough link to really hurt my envelope?

Answered by noah kaput
Posted Thu, 09/15/2011 - 12:52

6.
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Noah,
A furnace is cheaper than a boiler, so there is no reason to install hydronic heat.

I wouldn't build a house in Massachusetts without triple-glazed windows. I'd make the house smaller if I had to, and I'd order the windows I wanted.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 09/15/2011 - 13:05

7.
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I would definitely not do all radiant floor up and down but rather limit the radiant slab to 30-50% of the floor area, smaller as the envelope improves. I like doing radiant in the entry, kitchen, bathrooms and in front of the fireplace in the living area. It creates a "virtual wood stove" where you have warm areas that send heat out to cooler areas through natural air circulation. If you want more slab area for thermal mass consider having warmer areas and cooler areas where you have some radiant manifolds circulating warm water and some circulating the return water on the way back to the heat exchanger. Too much radiant floor area means that if the floor feels warm your house is over heating.

I agree that propane is expensive fuel. Ted Clifton has been using split heat pump water heaters with a COP in the range of 4 to heat a tank of water to heat the floor. This combines well with grid-tie solar PV but I've found it challenging to implement. If sticking with propane take a look at A O Smith's Vertex condensing heater. If you separate the floor from the water in the tank with a flat plate heat exchanger you can use a single water heater to do the floor as well as the domestic hot water.

I'm guessing that the 10 1/2" wall thickness is from two 2x4 walls with a 2x4 purlin at the mid-level. I prefer to use a split 2x6 purlin so I can use 2x10 lumber for the upper top plate which gives me a 9 1/2" wall 9 1/2" also rips conveniently out of 4x8 plywood but I think 2x10 top plates are easier to work with in the field (and I prefer JM Spider but I'm in the south) I very much agree with Martin about the importance of the windows.

Just a few immediate thoughts, I hope they are helpful.

Answered by Michael Chandler, GBA Advisor
Posted Fri, 09/16/2011 - 20:41

8.
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We've built several double-wall homes. I would make the walls thicker - we build them completely separately, no common plates. And I would size the rafters for structure and then gusset down for insulation - you can eliminate both the bridging & the foam that way. And I'd use dense-pack instead of damp-spray.

I wrote a piece in Journal of Light Construction a while ago on one of our houses - e-mail me at dan at kolbertbuilding dotcom and I'll send you a pdf. Or you can find it here - http://www.jlconline.com/cgi-bin/jlconline.storefront/4e751b980022e98e27...

Answered by Dan Kolbert
Posted Sat, 09/17/2011 - 13:38

9.
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what are people's thought on cellulose vs blown fiberglass? I know the obvious 'greener' aspect, as well as densities, however fiberglass carries a slightly higher r per inch, and I know every insulator I have talked to will blow fiberglass any day over cellulose, so around here they tend to price it almost the same.
will an r-40+ double stud cellulose perform better then similar thickness fiberglass however?

Answered by Jesse Lizer
Posted Mon, 09/19/2011 - 09:18

10.
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Jesse,
Choosing a contractor with experience installing blown-in insulation in a double-stud wall is more important than whether the insulation is cellulose or blown-in fiberglass. There are several ways that such installations can go wrong -- so get a good contractor.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 09/20/2011 - 04:19

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