My Beef With One-And-One-Half Story Houses
May 11th, 2010 | 11 comments
I’ve lived in a one-and-one-half story house in Minneapolis for the last six years, and I feel like I’ve earned the right to complain about them. My main beef with them is insulation and ventilation; they’re a pain in the butt and they’re expensive to get right.
There are two primary ways to insulate a one-and-one-half story house: traditionally, or with a hot roof.
Hot Roof
Hot roofs are actually pretty simple. A hot roof will have foam insulation sprayed against the roof decking, and won’t have any ventilation. If a closed-cell foam is used, it will act as a perfect vapor barrier, and will prevent any air leakage. Simple. I’m a big fan of this method, and I even did it at my own house.
The problem with spray foam is that it’s hideously expensive, and the installers need to have access to the roof boards; that means the attic space needs to be gutted before the work can happen. If you’re doing a big remodel, great… otherwise, it’s just not practical.
Traditional
The traditional way to insulate and ventilate a one-and-one-half story home is to insulate right up against the first floor ceiling, the knee walls, the vaulted roof sections, and again at the second floor ceiling. The diagram below illustrates this nicely.
The cold spaces shown above are all supposed to be ventilated. The ventilation will help to keep these spaces cool during the winter, which helps to reduce the potential for ice dams at the exterior and condensation in the attic. The illustration below shows one way to do this.
In this illustration, soffit vents are installed at the eaves, baffles are installed between the lower and upper attic spaces, and gable end vents are installed. There are other ways to achieve a similar venting strategy, such as using a continuous ridge vent at the top section instead of gable end vents, but the main idea remains the same.
The problem with traditional insulation is that it’s very difficult to retrofit an existing installation. Some homes have access to all three of the attic spaces, while others don’t have access to any of the attic areas – and there is never access to the vaulted roof sections between these attic spaces. To get at these areas, it often involves gutting the upper level. Sure, more insulation can easily be added at the knee wall attic areas, but that’s just a fraction of the total heat loss that’s occurring here.
With traditional insulation, attic bypasses also need to be sealed… and these houses have a ton of them. Perhaps the largest bypass is the one that occurs right below the knee wall, which is illustrated below. This area needs to be sealed off to prevent warm air from leaking in to the unheated attic areas.
What this all boils down to: if you’re buying a one-and-one-half story home that hasn’t been properly insulated and ventilated, you’ll probably have to gut your upper level if you want to correct it.
Oh, and another thing… these homes will often have just one supply and one return register from the furnace at the upper level; combine that with poor insulation, and you have a cool space in the winter and a hot space in the summer.
Oh, and another thing… they’re prone to ice dams.
Oh, and another thing… no, that’s enough whining for today.
Reuben Saltzman, Structure Tech Home Inspections - Email - Home Inspector Twin Cities
Note: The diagrams in this blog entry came from www.nrcan.gc.ca. I marked up the diagrams to help illustrate my points.
Reuben Collins
May 11, 2010, 8:53 am
Oh jeez. Don’t even get me started. Hot roof is totally the way to go. There are even more complicating factors you haven’t mentioned:
1. in a lot of these homes, the roof rafters are 2×4′s, which means that on the sloping portions of the roof, you’re trying to fit insulation and ventilation within a mere 4″. You’re either not gonna have much insulation or you’ll have poor ventilation. I’ve heard of some people tacking 2×2′s to the bottom of the rafters just to give them 2″ extra space.
2. In a lot of these homes, the eave end of the 2×4 roof rafters terminate right on top of the 2×4 double top plate on the walls. If there are eaves/soffits, it’s a totally separate structure and air space. This means that there’s no passageway for air to get from the soffits to the space between the rafters. In some homes, some sort of flared eave system was created, which allows for a passageway. But flared eaves just create more problems – especially for ice dams.
In my house, I have pretty much given up on the idea of proper ventilation. I’d rather have insulation and I’m just gonna take my chances with condensation, I guess….
Also, this is just a personal opinion, but I think knee walls are dumb and waste valuable space in these small 1.5 story homes. I’m seriously considering removing mine – or at least cutting some holes through them to access that space. I’m still trying to figure out the structural status of the knee walls, but I’ll probably be breaking into them sometime this summer.
Reuben Saltzman
May 11, 2010, 5:22 pm
Reuben – I completely agree.
On point number 1 – if I hadn’t done a hot roof on my house, I definitely would have added 2×2 on to my rafters for more space. No question about it.
On number 2 – you’re exactly right. It’s very difficult to get the ventilation correct.
Good luck with the knee walls – be sure to post photos.
Graham Allan
October 31, 2010, 10:21 am
Seeing the same problem, having tried to both insulate and ventilate, I see there’s no way to pass any ventilation from the eaves, and it’s impossible to prevent ice dams etc.
Do you have any opinion about retrofitting a cold roof on top of the existing one – there’s a good explanation at http://www.danperkinsroof.com/1108_JCL_Perkins_A.pdf
Seems like this wouldn’t be so hard to do, at the same time as replacing the shingles.
Reuben Saltzman
October 31, 2010, 1:39 pm
Graham – I think that retrofitting a cold roof on top of an existing one would be an excellent plan.
I actually just read an article this morning from the March 2010 edition of JLC titled “An Ice Dam Analyzed” , where the solution was to do exactly what you’re asking about.
I’ve never inspected a house that has had this done, much less had the opportunity to evaluate how well this system works, but it certainly makes sense.
tami
October 1, 2011, 7:55 am
i’m about to undergo having open-cell foam applied to my attic which has a furnace in it to create a hot roof. any comments on using open celled icynene instead of closed cell?
Reuben Saltzman
October 1, 2011, 8:00 pm
Hi Tami – I would definitely go with closed cell foam. It has about twice the insulating value, it won’t allow moisture in, and it costs about the same as Icynene, which is an expensive brand name for open cell foam. You can read more about the differences here – http://getfoaminsulation.com/?p=142 . The guy who wrote that article, Kevin Klein, is a very knowledgeable insulation contractor who I’ve recommended dozens of times.
Graham Allan
October 8, 2011, 6:00 pm
Well, almost exactly a year after my previous comment, I’m about 90% of the way through retrofitting a cold roof atop my existing one, using the Dan Perkins article as a guide. It’s been a lot of work, and I have yet to install the metal before the snow arrives, but it will be interesting to see what difference it makes this winter.
As an aside, your point about recessed lights (elsewhere) was spot on. We *tried* to insulate adequately above these on the upstairs when they were installed several years ago, but I pulled some “sample” sheathing of the top of the roof to inspect, and could see the tops of the cans poking through the fiberglass. Took enough boards off the roof during the project to stuff a couple layers of R-30 on top of them from above, which should take care of it…
Tim
October 28, 2011, 10:05 pm
I am this exact situation right now… we live in a 1.5 story in minneapolis and are renovating the upstairs. We have gutted the entire space and have access to all of the rafters. We contracted a spray foam company to do a hot roof however after talking with him and the sheetrockers they decided they did not want to mess around with doing the space behind the knee walls. Apparently if foam is used behind the knee walls it also needs to be sheetrocked for fire code reasons. I was initially okay with not foaming behind the knee walls because it cut the cost by more than a third. However now that I’ve been investigating how to ventilate and insulate the upstairs i think it might be nearly impossible to do it sufficiently. There is not enough space at the eaves to add any soffits any therefore it is pointless to add any exhaust vents. Is this correct? What has your experience been with your hot roof? Are the temperatures manageable in the upstairs? Do you have any problems with ice dams?
Reuben Saltzman
October 29, 2011, 4:47 am
@Graham – thanks for sharing your experience. I’m interested to hear how the rest goes.
@Tim – I strongly recommend you foam behind the knee-walls. Part of the benefit to doing a hot roof in a 1.5 story home is that you completely eliminate all of the attic bypasses. It’s not big deal to add drywall in the knee-wall spaces; you don’t need to make it look pretty. I think you can actually even get away with 3/8″ drywall in those spaces.
Yes, you’re correct with your theory on vents, mostly. If you didn’t do a hot roof you could at least add gable end vents to knee-wall attic spaces, but this is a far less desirable option than making the entire space warm.
I was very glad I did the hot roof, and I would certainly do it again. If I could do it again, I would have made my 2×4 rafters a little thicker by adding 2x2s, in order to have two more inches of foam insulation. I also would have added 1″ foam sheets of insulation all over the interior when done, to help eliminate thermal bridging. The wood rafters are a poor insulator; when frost showed up on my roof on cold mornings, I could see exactly where every rafter was when standing outside, because they created warm ‘streaks’ where frost wouldn’t appear. That really bugged me.
Nevertheless, the hot roof was great. The upper level was instantly much cooler in the summer, and stayed very comfortable in the winter – I was able to turn off all of the heat registers going to the upper level in the winter, because the walls were so warm. It was great.
I still had some small ice dams, but nothing like what I had before re-doing the roof. I’m pretty sure that I could have completely eliminated the ice dams had I done what I described above. Good luck.
Steve Rasmusson
January 10, 2012, 11:10 am
We have an old story and a half farmhouse – balloon framed with plastered ceilings, 2×4 rafters and woefully inadequate insulation. I have been considering Dan Perkin’s idea for insulating, but am concerned about trapping moisture at the point of the old roof boards. My greatest concern is for the sloping roof/ceiling section with the rafters. Have you heard of any way (short of removing all the plaster and lath) of controlling the moisture penetration from inside the house into the rafters and roof boards. This issue might be moot, since the damage may already be doen after 120 years…
Reuben Saltzman
January 10, 2012, 8:20 pm
Two ways – one would be with a straight hot roof, and the other looks like this – http://www.fairandsquareremodel.com/2011/03/22/unusual-ways-to-insulate-your-roof/
- Reuben