Pretty Good House Design Review
I live just West of Salt Lake City in climate zone 5B. 95F and 10F are the design temperatures for my area. Average rainfall is 15”. I am working with a designer and contractor to create my plans and then I will bid them out. Rambler, 2000 ft2 main floor with an additional 2000 ft2 walkout basement. I want to share some details of my home to see if we are missing anything. This is the best time to make changes so any feedback is welcome. I am a bit nervous I am going to create a house I cannot afford….
Roof:
Raised Heel Truss
Blown fiber as insulation R-80 (should I do R60?)
Wall assembly:
Hardie Board siding (no rain screen)
Tyvek (Should I ask for commercial Tyvek?)
Taped 7/16” OSB as an air barrier
12” Double stud wall with blown in fiberglass
Air sealed ½” dry wall
Basement:
R-40 Closed cell in Rim Cavity
Wall: Interior 2” XPS extending below the slab and 2×4 with blown fiberglass
Slab: 4” concrete, 2” XPS, 6 MIL vapor barrier, 4” gravel
Windows:
Anderson 100 Windows
HVAC – Ducted Heat Pump – I am open to ducted mini split but was advised this was the most cost effective solution and installers ability to perform the work. We will be doing a Manual J shortly which will help greatly in making a decision.
ERV/HRV – TBD but the guidance I have received to be economical is to do individual pickups and redistribute in my return duct work
A few specific comments/questions.
- I live in a very dry area of the country (Utah) and my contractor/designer recognizes rain screen is the most robust option but also said he rarely does this do to the low humidity and minimal rainfall in the area. He recommended saving the money.
- OSB vs Plywood, similar to rain screen my contractor/designer said in our dry climate it is just not worth it
- Fiberglass vs cellulose, again I was advised that the local installers are much more competent with fiberglass vs cellulose installs
- My contractor/designer believes the double stud design is preferred due to the simple design. I know there are other designs with exterior insulation but my local contractors I have called have no experience with other designs.
- HVAC system I still have question about the ducted mini split but until I have load calculations it doesn’t seem worth digging any deeper
Thanks again for your thoughts and suggestions.
Matt
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Replies
Good start, obviously it's the details that matter. Two small notes:
Under slab, you want the vapor barrier directly in touch with the concrete, above the rigid insulation. Perhaps that's just how you typed it.
IMO, I'd stick with R60 in the attic and spend any extra money on air-sealing details.
Simple wall outlines and simple rooflines make things easier to seal and cheaper to boot.
Even if you use blown fiberglass packed in the walls, which is the difficult part, I'd consider using cellulose in the attic. It reduces air circulation within the insulation, is very quiet, and I've read that it's more opaque to IR radiation, so even if your attic roof structure is hot, it won't radiate down through the insulation and heat up your ceiling. Perhaps at those thicknesses (R60-80) this is less of an issue.
Thanks for the note on the poly, it was not a typo. I'll discuss with the designer and change it. See attached for how it was called out. I will also look into cellulose as I get closer to that phase of the project. Thanks!
FYI for good reading on this subject https://www.buildingscience.com/documents/insights/bsi-003-concrete-floor-problems
Thanks for the article. I am struggling understanding how to include XPS foam and poly for a capillary break, specifically at the wall transition. Can the XPS foam replace the poly? Also, if I read the article correctly, do I need a capillary break under every pressure treated 2x4 wall that is on my basement slab? Do I need to epoxy the basement floor before I install flooring? We were considering laminate or LVP. Attached is my current drawing.
Hi Matt,
Sounds very similar to our planned build, with main floor (1900 sq) finished and basement (initially) finished as little as code permits and with roughed-in plumbing for eventual ADU-type unit. We're going with 5/8" drywall, which our builder recommended particularly w/ 24" OC studs, as both stiffer, more robust, and quieter. We're also using some interior wall insulation for noise abatement (admittedly a not very efficient approach) in office walls for additional noise reduction, and planning two ERVs (with very large outboard filters; likely Panasoic Intellibalance) for flexibility ventilating main living area vs guest/basement quarters. We're still hoping to break ground this summer. We're _deeply_ into the weeds - if you'd like to exchange plans/ideas and ideas, feel free contact me johngross888 at gmail. I'm working on some aspect of the house literally every day, looking a supplies, home automation, energy modelling, windows, PV, monitoring, etc. We're in CZ 6b Colorado - a bit colder but similarly dry.
A lot of people (well educated and knowledgeable people) will tell you that double wall construction in your area isn't really cost effective - don't listen to them! None of us know when inflation will rise, when the Russians ( or some other evil-doers) will invade, or when some other financial catastrophe will strike - a little insulation overkill works financial wonders when these things happen!
You are proposing a fairly large house. Houses this size (2 floors, 2000 sq. ft. on each floor) commonly have total heating/air-conditioning costs above $3000 per year - (it depends on the type of fuel (elec. or gas) and your life style of course but double wall construction, good glass and intelligent design will reduce this by a really large amount (perhaps in excess of 90%).
Let me provide an example - I designed , built and lived in three double wall houses between 1973 and 2003 - these were in located in average every day subdivisions in the Rocky Mtn. west and averaged around 2300 sq. ft. Total time living in them came to a little over 26 years - most of these years were high interest rate years - as I recall, nearly all of them were well over 10% one of them climbed up over 20% - When I Started No One Saw This Coming! Other houses (same size in these subdivisions) averaged $1600 to $2000 per year in heating costs - Mine averaged $100 to $300! I invite you to find an interest rate calculator on the web and run the numbers for a house such as yours and input say $2600/year for 25 years or so and your guess as to what interest rates might be for that period - you won't believe how much money you will have at the end of that time if you are able to put it all in the bank for the entire period. If we have another high inflation rate period like we had in the 1970s to the 1990s you will generate an amount of money just from interest on your yearly heat savings to nearly equal the total construction cost of the house. The extra costs to over insulate now won't amount to chump change then! Will we have another high interest rate period like that? Nobody knows, not presidential advisors, not world class economists and not me.
Now lets discuss your proposed wall - a 12" wall with fiberglass blow-in will provide an R rating of about 44. There is a better way - Build a 10" double wall with 2x4 walls filled with mineral wool bats and fasten 3 inches of foil faced polyisocyanurate insulation to the inside of the exterior studs then build the interior 2x4 stud wall right up against it (a polyiso sandwich).
The R rating will come to around 42 and the wall thickness (stud face to stud face) will be 10 inches. There are several advantages to this - the polyiso becomes a perfect air/moisture barrier, it's protected from damage by the 2x4 stud walls on both sides (nobody will poke a hole in it hanging pictures or fastening something on the outside of the house) and you will never have condensation problems inside the walls - last you gain an extra 2 inches in each room with an outside wall. The only downsides are it may be slightly more expensive than blown in fiberglass and the contractor has to be willing to construct the interior walls after the roof trusses are in place (in fact it's actually easier and better to have the roof complete before installing the polyiso and constructing interior walls). Last I agree with Andrew C in post #1 above.
Have you considered Alpen windows, out of Colorado? I don't know how they compare in cost to Anderson, but you might want to take a look at them. Also, are you planning on PV on your roof? Something to do, if at all possible, even if it means making some concessions in other areas.
Even though the climate is dry, combining a double-stud wall with osb and no rain screen still seems risky, based on the numerous articles and discussions here over the years. Others with practical experience might want to chime in. A fairly easy alternative to double-stud walls would be 2 x 8 walls, with Zip r-sheathing.
1. I agree. Rainscreen is probably unnecessary. But make sure to do excellent water management (flashing) details. With walls that thick, there's not much energy available to promote drying.
2. OSb is fine.
3. Cellulose is probably a better solution for environmental and other reasons, but if there's no local talent pool, FG will work too. Cellulose is probably a little less risky with the wall system you've specced. That said, Utah is so dry the wall system is probably fine.
4. Some analysis shows that double stud walls can be somewhat at risk for internal condensation against the sheathing. In practice, this rarely happens when the inner/outer walls are well air-sealed. An interior smart membrane as air/vapor retarder would help, but pricey. Even VR paint would reduce risk a bit.
5. +1 on load calculations. You can't even start thinking about equipment without them. Make sur they're done by someone competent, preferably independent of the HVAC contractor.
Just a comment about the smart membrane - I don't think 20 cents a square foot is expensive. Let's say 40c installed. 60c installed if you want a more expensive brand. Definitely a good idea on a double stud.
+1000. Manual S comes after manual J. If you switch them around, you might as well skip both and spend the money in Vegas.
Just a little more, you don't need to fool around with smart membranes if you put a layer of polyiso in between the double walls. Apparently quite a few double wall houses (with a polyiso layer in the middle) have been designed and built in Canada - by an architectural firm called Passive Design Solutions in Nova Scotia (Natalie Leonard, P.E. is the founding partner in the firm). You can see examples of her work on YouTube and on the web if you're interested.
Why don't you want to buy a ready-made house? After all, the projects of most houses are typical, and you can always find a house in your area that suits you. It is possible to change it somewhat, but not build a house starting from the foundation and ending with interior design. I was also thinking about building my house. But then I realized that it would be easier and cheaper for me to buy it. I chose a great house with a great layout. Moreover, I liked the https://onstage-online.com design solution in the interior design of the house, and I already knew at the time of purchase what kind of furniture I wanted to see in my house. It's much easier that way.