Detailing this Double-Wall Assembly
Hi Folks,
New to this site but seems to have some really good information and a lot of knowledgeable contributors.
We will be building our downsized home beginning next spring in central Maine, Zone 6 (although seems like 5 the last few winters..)
The home will be single story around 1500 sf, probably resting on a frost protected slab on grade foundation depending upon what we find as we choose final site and start ground work.
Primary heating will be radiant in the slab but the house will also have mini-splits primarily for cooling but also convenient for heating during the ‘shoulder’ seasons.
Grid tied PV array will be sized to offset 100%+ of home electrical load.
About 50% of the home will include a hybrid ‘Post and Beam’ interior featuring exposed posts, beams, rafters and T&G wall and ceiling decking.
The balance of the house will be more conventional with sheetrock interior finish.
Exterior of the house will be primarily cedar shingles but may have areas of metal or cedar clapboards.
We will be milling all of the framing, timber and sheathing lumber ourselves and as a result are considering a wall concept that is heavy on these items and uses simple construction techniques and materials that we are familiar with and can do ourselves.
The attached sketch details the wall as it would be built in the area of the house with internal Post and Beam finish.
My specific questions regarding this assembly:
1) Over the ceiling deck is it ok to use 1/2 EXP foam to continue the air barrier to the other side of the house?
2) Is there any thermal benefit to increasing the 2″ air space to a larger value?
3) What should we use as the WRB/Air Barrier on the exterior of the house?
4) Are Rockwool bats an acceptable choice on this design?
5) Most importantly any obvious issues with this concept?
Thanks,
Larry
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Replies
Larry,
1. Forgive me but I don't know from your section what is happening at this "ceiling decking" Is there another floor above, a roof that isn't drawn?
2. An air-space in the middle of a double wall will have convective air movement and needs to be filled with insulation. The wider the gap the more R-value you can add.
3. If you are planning on using the WRB as the primary air-barrier you should use a self-adhered WRB. That's usually the strategy used on retrofits with board sheathing, but for new construction, where you will have a lot of shrinkage, I would include a secondary interior one using a variable perm membrane.
4. Rockwool batts will work fine, although they do not give you the moisture buffering that dense packed cellulose would.
5. The devil is in the details. How these assemblies come together and relate to both the foundation and roof above, as well as what happens in the areas where there is no post and beam are important to clear up.
Hi Malcolm,
Thanks for the feedback....regarding your points:
1) Above the 'ceiling deck' will be an unconditioned attic space..probably trusses and loose fill insulation to get me to ~ R60. My intent was to have a continuous air barrier from the external side walls up and over the 'ceiling deck ' (under the truss lower chords).
In the non post and beam portion of the house the ceiling will either be attached to the truss lower chords or more likely I'll have a ceiling joist system to isolate the sheetrock ceiling from any truss lifting issues..in either case I think I should place the air barrier just below the lower chords.
2) Ok..so is the air space a negative, (R23 +Rair + R15 < R23 + R15) or is it just an opportunity to increase overall R?
3) Where should the secondary Air barrier be located...the inside surface of the external wall seems like to only place it could be close to continuous, at least in the Post and Beam portion of the house.
4) Understood...I'm trying to stay with systems that I can install myself. Do you feel that there is too much risk in going with Rockwool v DPC given the proposed wall construction?
5) Agree...still working through details and looking for feedback from the experts..
Thanks,
Larry
Larry,
1. That makes sense now thanks.
2. The air space is a negative. It should be filled.
3. Yes that's where I'd put it. A fairly tight interior air-barrier stops air cycling in and out of the wall as pressure changes. Usually that can just be the drywall, but as you are using wood a membrane is a good idea.
4. No I think it will be fine, especially as you have included a rain-screen, and your sheathing isn't either plywood or OSB.
5. Good luck with your build!
If you do a good job air sealing and insulating, a heated slab will probably not be a good idea. Water hot enough to give you warm feet will overheat the space. Spending on heat in the floor as well as heat pumps for cooling is pretty extravagant. Our pretty good house in Maine is heated well with minisplits. The slab does have 4" of reclaimed xps foam.
HI Stephen,
Thanks for your input..and I now see that you have built a very similar home concept so I may be leaning on your experience as we progress in ours.
I don't disagree that redundant systems add cost and may be overkill but current thinking is this:
1) We've built our previous 2 homes with in floor radiant (both hydronic based) and absolutely love the 'comfort' this style of heating provides. I'm not sure I'm willing to give that up.
2) Our Maine summers seem to be getting hotter and more humid..I'm not a warm weather guy and decided that this house with have some type of cooling system. For the money it seems to make sense to me to go with mini-splits vs a cooling only solution.
Back to the radiant system..we have not decided exactly how to we will do it in this house.
We are considering typical in floor hydronic with electric boiler (given the PV system ) as one option.
Additionally I'm trying to understand the pros/cons of an electric based system, both resistance wiring within the slab and /or electric mats over sleepers/subfloor and under final floor.
I'm not finding much on the electric options so would appreciate any and all input you or other can give.
Thanks,
Larry
Larry. Putting heat in the floor certainly works. It mostly depends on your budget for construction and whether using additional solar to power an electric heat source is cost-effective.
Our previous house had a propane boiler that heated the floor. It worked ok, but was very expensive to operate. If you were paying for electricity, using resistance heat would cost a lot more that heating with propane or oil.
We don't have a way to know exactly how much electricity we use for heat. My best guess is around 3500 kwh per year. Since our heat pumps have a COP of around 3, using any form of electric resistance heat, we'd need around 11000 kwh for heat. That's around 9-10 kW of solar, as opposed to around 3KW for the heat pumps. At about $ 2.25 per watt installed after federal tax credit, you'd spend about $13500 to buy enough solar just for heat over and above a heat pump.
As you note, air conditioning is nice to have, even in Maine. We use it for 6-8 days every summer. We wouldn't bother installing air conditioning if we didn't have heat pumps that already provide it
We did put resistance heating under the bathroom floor, which my wife is a big fan of.
Where in Maine are you building?
Hi Stephen,
I agree with you, its hard to argue the benefit of a 3 COP Air source heat pump especially in a fully PV home but...
I have not lived in a home heated mini-splits and my assumption is that the experience will be similar to a forced hot air heated home with temperature swings and extremely dry conditions..
Radiant systems in my experience are much more thermally steady state and comfortable than any FHA experience I've ever had.
Seeing as you have lived in both a radiant and heat pump heated home I'd like your opinions on the 'comfort' of each..
P.S. We live in Thorndike and will be building in Montville..
Thanks,
Larry
Larry-My house was finished last year is double stud, zone 7(Colorado Mountains), 1295 sqft. One floor mounted Fujitsu mini split on main level, and another one in the basement.
This is my first winter in the house.
You won't get the benefit of the radiant floor, as it will hardly, if ever, be on enough to feel warm to your feet. Nothing I liked better than my old leaky house with its toasty radiant floors, except my new house that doesn't need them...
The house stays VERY stable interior temp, even when its in the negative double digit temps overnight. The mini split has the ability to put out a very small amount of heat/air, all the way up to blast furnace levels if you'd want. (Just make sure you get a cold climate one that is sized correctly, and has a large "turn down" capability.)
It typically just chugs along with a small amount of heat keeping the house at a very even temp.
I do keep my master bedroom door open, as well as the spare bedroom door.
The rooms tend to say a little cooler than the main living area, which I was initially worried about when designing the house, but its not an issue at all, and nice for sleeping.
I haven't noticed excessively dry humidity- It's basically a high alpine desert where I live (9700'). The house is sealed so tightly that lack of moisture hasn't been an issue with two people living in it.
I'd suggest NOT goin with a whole house radiant floor. Put an electric floor in the bathroom & shower floor, then put all the extra money into very good windows. (I have a mix of Alpen ZR6 for my smaller windows, and ZR9 for my larger ones. If I was going to do it again I'd go with the 9 on all of them. They are so warm when standing directly in front of them in the dead of winter. It's amazing after always living in other houses with average to poor windows.
I also have 7.6 PV on the garage roof. All electric house. My power bill hasn't been any higher than the $5.70 access fee the local power company charges.
mini-split heating in a very well insulated and sealed house has been fantastic so far. :)
Hi Larry. It's hard to compare our houses, since the previous house was not particularly well insulated or air sealed. I think the conventional wisdom is that heat in the floor works well for poorly insulated, drafty houses because it's always on.
Our new house (moved in in summer 2015) is very tight and very well insulated. In early November, we turn on the two minisplits, set the temperature on the remotes to 72° and don't touch them until we turn them off in April. The temperature has never gone below 69 or above 71, except for a few sunny days in early winter when it might get up to 74.
As for humidity, radiant floor heat or minisplits don't really contribute to humidity.
I guess it comes down to budget. It'll cost at least $30 to put heat in the floor, add enough solar to run it and install heat pumps for AC. If cost is a factor, my admittedly non expert opinion is I'd spend the money on a great building envelope and use a minisplit or two.
Stephen/Brain,
Thanks for the insight..
I think you're probably right, my experience is certainly based upon radiant in an average at best home construction. I guess I could verify once I have a better estimate of my heat load but I trust your real world experience.
I'd be interested to hear about your heat pump choices and the reasons behind your decisions.
Thanks again,
Larry
Hoping someone with better knowledge then me can answer op question 3 about which self adhered WRB to use? I assume his sheathing boards will be rough sawn, as well as hign moisture content.
Seven years ago i considered Henry Bluskin for a remodel projecy which had rough sawn boards and gaps as wide as 1/2". At that time the instructions did not allow Bluskin over the rough sawn lumber with large gaps.....however after a quick review of install instructions...i did not see those limitations any longer.