There are some folks who appear to have it all in the housing department. They’ve got the location-location-location, they’ve got great grounds, a terrific view, southern exposure, all energy-star appliances, first-rate windows, and a bone dry basement. Just sittin’ on top of the housing world!
Can this old Stick Beauty find true love and happiness with a SIP addition?
Can we economically add a SIP one or two story wing? Can we lop off the existing gable roof from the garage and put a SIP second floor addition up there? And the biggest dream of all---boy, I wish I could put a second floor on my ranch!
Well, we’ve been around all those blocks and I’m here to tell you that basically, if you thought you could build any of those ideas out in sticks, you can certainly build ‘em with SIPs. We’ve had great success with all of those projects and have a few more like them just out to bid now. Let’s look at some of the issues, working from the simplest to the trickiest:
1. SINGLE STORY ADDITION ALIGNS WITH NOTHING ON THE EXISTING
This is almost as easy as building a small SIP stand-alone, like a garage. We do have to watch where we connect. We find the easiest strategy to follow is to order the SIPs that are to connect a little short, say by 1 ½ inch. It is easier to block, foam and seal up this connection gap than to scribe cut an oversized SIP back to fit.
2. ADDITION THAT HAS FLOOR AND ONE WALL ALIGN WITH EXISTING
Now we are getting a little more complex. That new floor, which might be of SIPs, should be dropped an extra half inch to allow for an additional layer of OSB so that the finish wood flooring is nailed down through a double layer (one inch) of OSB, a single layer won’t do. Aligning a wall requires a bit more attention. The SIP outer surface should align with the outside face of sheathing of the existing structure. If the wall has to be flush on the inside as well, and furring out the existing is not an option, you might want to use a SIP manufacturer who will furnish you with some custom made SIPs to match. Or you might use the thinnest you can easily get and fur it out as required.
3. ADDITION WITH FLOOR, WALL, AND ROOF PLANE ALIGNMENTS
What I want to say is, “Don’t even think about it!” But the same is true of stick framing. Here the rule would be: Measure seventeen times because you can only cut once. Slow and careful measurements must be recorded before SIP drawings are prepared. I would still order pre-cuts and not have raw bulk panel delivered to the site, that always slows you down and should be avoided if at all possible.
4. ADDITION OVER GARAGE
As you can see, it’s the flush alignments that you’ve got to watch out for. The addition we did over an existing garage had overhangs on all three sides – except of course on the side that abutted the existing. We used SIPs for flooring that cantilevered out a foot or so which was installed over the existing stick walls after the gable roof was torn off. The rest went up easily, with the walls stood up on top of the edge of the SIP deck.
5. FULL SECOND FLOOR SIP ADDITION
What happens here is similar to the SIP addition over a garage, but it’s a whole roof tear-off…and we mean the whole roof, not just the roofing. Most likely the existing ceiling joists will not be strong enough to be the new second floor, but we don’t want to tear them off with the attached drywall ceiling as people may still be living there through the whole process. Though it might be nice to have a SIP floor – and it will go down very quickly – what must be done is the installation of new deeper floor joists inserted along side (but not touching) the existing framing members. A proper job would require that you set the new floor joist framing members on ¾” blocking set on the existing plates so that they never touch the existing ceiling. This is good in case the existing is out of level, but is really done to allow for great sound and vibration isolation between the two floors.
Most plans will require the new walls to align with those of the first floor below, but again, if an overhang can be worked into the solution it will make life much easier. If the first floor is slightly out of square and/or the walls are not true the overhang will disguise this difference. The second floor cut drawings will be easier to prepare as there will be no need for precise coordination between the existing and the new.
A project we are currently working on will do this all around, with 10” composite joists cantilevering out past the existing plate everywhere. The SIPS are being designed to be fastened along side the new framing with the bottoms of the SIPs aligning with the bottom of the joists. This will enable us to make a strong shear connection.
Once completed, you will notice that the thermal performance of the existing first floor is vastly improved as most of it’s conductive and infiltration losses occurred through the old roof. Now we have a snug, efficient SIP hat on top of our old home with all the standard thermal SIP benefits that we are familiar with. In all three of the cases we have been involved with, the existing heating equipment was able to carry the load of the new whole even when the square footage and volume was more than double that of the original structure. We know you weren’t surprised to hear that!
All in all, the standard SIP advantages enjoyed by a new SIP project are magnified with the use of SIPs for an addition. Don’t forget, most people continue to live in their home while the addition construction is taking place. The short time that the roof is off and the envelope completed is greatly appreciated. Of course, if the addition is alongside the existing you wouldn’t break through until the new work is almost done, but for a full second floor job…speed really counts!
As mentioned above, the excellent thermal performance of the SIPs “spills over” to enhance the performance of the existing. In many instances also, the structural “marriage” of the new to old enhances the stiffness of the existing.
All in all, if you are contemplating an addition to the old homestead, I would encourage you to consider building it with SIPs. You do want a Sip-erior addition, don’t you?