Requesting comments and suggestions for a “pretty good” house balancing performance, cost, and windows with views
We are working on house plans with our designer in a rural area of Northern Virginia (zone 4 mixed humid) for a 2800 two story rectangle. It is a south facing stick framed home with a poured foundation basement and no roof over-hangs. We will have sliding shutters on the exterior of large south facing windows. This is an electric/solar home using the south facing rooftop with 5 kilowatt array.
• Sheathing will be huber zip–sealed with their liquid flash.
• Roof also huber zip and liquid flash. Vaulted ceiling with rockwool bats between trusses and polyiso on the exterior of the sheathing for 38 R. Metal roof.
• 2×6 framed walls with six inches of rockwool between the studs for R24 and an additional inch and half on the exterior to reduce thermal bridging for a total of R30.
• Concrete foundation walls with outsulation of three inches of recycled foam XPS for R15 and a dimpleboard for drainage.
• Anderson fibrex 100 double pane low e windows.
• Open jointed rainscreen vertical battens over the rockwool and a UV rainscreen fassade fabric.
• Ducted mini split with ERV and a dehumidifier. Wood stove for back up heat.
• Will get a blower door test.
Thank you for sharing your wisdom with us!
Lisa and Paul
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Lisa and Paul,
Sounds like a great house to me! Two very minor things that may or may not be problems:
- I don't know if Huber insists you use their tape on the Zip, or if you can substitute a liquid flashing (or whether you care what Huber wants).
- You might want to run the roof assembly past your solar installer to make sure there is sufficient anchorage for the panels on a roof that is fastened though rigid insulation. They might want blocking, or...?
Since you noted you have views from your large south-facing windows, its likely that you have mostly unobstructed sunlight pouring in your south-facing windows during sunny or partly sunny Winter and Fall days. Solar heat gain likely would keep your interior warm during daylight hours and into the evening. Have you computed the solar heat gain for average January days? Or the average January heat losses for your home?
(For more info please see: https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/quantitative-look-solar-heat-gain )
Its unlikely you'd need a wood stove for backup heating unless you lost electric service for an extended period.
Lisa and Paul,
The biggest issue (in my opinion) is that the proposed house has "no roof overhangs." That's a problem.
Robert Opaluch has mentioned one result of this decision -- your "large south-facing windows" will be unshaded year-'round, and may cause overheating. Carefully designed roof overhangs can mitigate (but not eliminate) this potential overheating.
More importantly, your siding will be unprotected from rain and sun, and will therefore have a much shorter life than the siding installed on a house with roof overhangs. I urge you to start paying close attention to the siding condition of houses in your neighborhood. You'll notice a direct correlation between siding condition and the width of the roof overhang. Exposed siding fares poorly.
Moreover, it's unpleasant to stand on a stoop and fumble with door keys, or to wait for someone to answer the door, if the house has no roof overhangs, and it's raining.
Robert pointed to another potential problem: It's often difficult to operate a wood stove in a home with a tight thermal envelope. When the stove is operating, it often struggles to find a source of combustion air. When the stove isn't operating, the flue becomes the biggest hole in the home's thermal envelope, and therefore the source of makeup air whenever a fan is operating. The air that enters by this path smells of soot.
For more information on the first problem, see this article: "Every House Needs Roof Overhangs."
For more information on the second problem, see this article: "All About Wood Stoves."
Hi Malcom,
Thank you for replying! We will be using the Huber Liquid Flash product which is part of their system, so I think we will be OK. The solar panels will be attached to the metal roof panels using a s-5-n mini clamp for standing seam. I think you are right the engineer needs to address the question about the foam, weight of the panels and OK the roof attachment system.
Lisa and Paul
Hi Robert,
Thank you for the link! We are right on the cusp of north and south and the closest city would be Washington DC which is not on the list. Philly is the next best bet and they have slightly colder winters. We will have good solar gain in winter and need good shade over the south facing windows in summer which is why we are using the exterior shutters/shades. This area has so much humidity as well—really swampy. It’s a tricky climate.
Lisa and Paul
Hi Martin,
Thank you so much for replying to our post! We were inspired by Matt Risinger’s videos on the Perfect Wall house in Texas which does not have over-hangs and avoids bird blocking penetrations. We are OK with the siding weathering as that is a part of the aesthetic. The lower level has a cart port with decks for cover where we enter. The upper level will have sliding exterior shades on the larger windows for when it gets hot. We have our fingers crossed that these shutters are going to do the trick.
We are required to have a second source of heat by code so we chose a wood stove to avoid the expense of gas. We read your article on stoves and thought we could choose a stove that has an exterior air intake. Will that be OK?
We also debated the issue with the cooktop fan/ventilation as it is another penetration can create an issue with negative pressure. We are planning for an induction cooktop, however we wanted to protect the ERV from cooking grease, so we decided on a recirculating fan with a plasma filter which is supposed to capture all the particulates.
Paul and I are enjoying puzzle that a “pretty good house” presents and so grateful for this forum and check it for updates every day.
Lisa and Paul
No overhangs -
I like Matt Risinger's work as well, but my first thought when I saw that house a couple years ago was that they forgot the roof overhangs (although they did put a separate porch roof on one side). Even if your siding is durable, or if you like a weathered look, to me the bigger risk is that you are stress testing the flashing around every opening whenever you get rain. Since flashings are rarely done well, roof overhangs are your biggest protection against leaks and subsequent damage.
Shade is a separate issue, also important.
Martin has written on the topic of overhangs several times, both in Q&As and in the linked articles he pointed to above. I'd think carefully about this decision.
I do like the sounds of the rest of the details of your house; it sounds like my dream house. Good luck!
The no-overhang look is popular on the west coast where there is little rain; it makes no sense on the east coast. Water damage is the primary enemy of residential construction; roof overhangs are the first line of defense. Eliminatings overhangs doesn't guarentee damage, but it does ensure a lifetime of addressing wall issues where the water has caused issues or penatrated the siding.
BSC has published a chart showing the direct correlation of siding damage to overhang depth, but I could not locate the chart. Instead, here are two articles addressing the issue:
Document # RR-0103: Water Management by Joseph Lstiburek; principle at BuildingScience.com:
The fundamental principle of water management is to shed water by layering materials in such a way that water is directed downwards and outwards out of the building or away from the building. The key to this fundamental principle is drainage.
Gravity is the driving force behind water management and drainage. The "down" direction harnesses the force of gravity and the "out" direction gets the water away from the building enclosure assemblies, openings, components and materials. In general, the sooner water is directed "out" the better.
and, here at GBA:
https://www.greenbuildingadvisor.com/blogs/dept/musings/every-house-needs-roof-overhangs
Lisa and Paul,
Can you tell us more about the recirculating fan with plasma filter? I am getting ready to swap out a bunch of kitchen appliances and was wrestling with what to do about the exhaust fan. Installing a ducted exhaust would be optimal but also a major undertaking.
If you are trying build the house with the best return on investment for insulation and systems. You may want to model your home with a computer program call BEopt it was developed with tax dollars and is free to users. The program uses your local weather data, your interest rate, and your inflation projections with your cost per square foot to compare wall, roof and floor insulation options. It also will model cost for HVAC systems and roof overhangs. It has a steep learning curve plan on about 40 hours to watch the training videos and do the model.
https://beopt.nrel.gov/
I think you should consider using the Zip tape for the flat joints between the panels and save the liquid for the 3 dimensional joints around windows and doors.
Walta
Hi Steve,
The technology is new to the US but here is a link to the product: https://www.plasmamade.com/
Cheers! Lisa and Paul
Thank you for all the comments on over-hangs! We are taking them very seriously. Love the idea of using zip tape for the flat seams--very practical. Cheers! Lisa and Paul
Lisa and Paul,
I guess the questions I would have are
1. I get using the recycled XPS for insulating the foundation, but maybe use that on the exterior walls, and go to ICF's? Maybe you have already checked it out, but if not, might be worth it?
2. Part two of #1 - will this be a crawl space? If so, how are you planning on dealing with that?
3. So you will essentially have an insulated "pocket" in your roof? Batts on bottom, and polyiso on the exterior? I am skeptical of this being effective for you? I would also encourage boosting to at least an R-50 total. I don't really know your climate, and maybe this is sufficient, but the additional cost to go from 38 to 50 would not be that great.
4. Further with the "pocket" - are you planning on running duct work in that zone? Is that zone ventilated?
5. Wall insulation. I would upgrade from batts to a blown in. Very hard to really get an R-24 out of a batt.
That is my two cents. Good Luck
Lisa and Paul:
You won't have a problem with combining exterior rigid insulation with interior fluffy insulation if you follow the rules. (See here: https://www.greenbuildingadvisor.com/articles/dept/musings/how-install-rigid-foam-top-roof-sheathing).
You may want to consider installing R-49 insulation (minimum) for the roof. Your area may not require that level of insulation, but it is more in line with current performance recommendations. You can reduce your rigid foam costs (and environmental impact) by sourcing reclaimed foam, which typically costs one-half to one-third the price of new foam.
Also... Do you have any cathedral ceilings in your design? If so, you need to make sure your rafters are deep enough to ensure there is room for a safe amount of insulation.