PERSIST vs REMOTE vs SIP ?
We are building a new house in MN (6b), considering various wall systems, and trying to get a handle on the pros and cons of each. Thus far we have:
SIP Pros
– Better known construction method, easier to find carpenters, …
– Well documented construction details
PERSIST Pros
– Seems like it would be easier construction than SIP.
– Easier to get all details correct.
– Provides significant interior space in attic for ducting, extra room, storage areas, etc.
– Chemical elements (foam, osb, etc) on outside of air/vapor barrier
REMOTE Pros
– Most of the benefit of PERSIST for considerable less cost?
– Uses standard and known roof construction techniques
– Well documented construction details from CCHRC
SIP Cons
– Chemical elements (foam, osb) inside the air/vapor barrier.
– OSB (ply possibly available from ??).
– Relies on adhesion for structural integrity and longevity while PERSIST & REMOTE rely on a standard 2×4 mechanical fastened structure and mechanical fasteners for skin to structure.
PERSIST Cons
– Will there be sagging problems with exterior skin since skin & foam are effectively cantilevered out over block wall? Will mechanical fasteners hold up for 100+ years?
– Any problems with long screws that go through all foam layers causing any condensation (??) problems? Seems they could xmit cold (and moisture?) to sheathing & stud. Or is there enough distance and protection that cold from outside and moisture from inside never interact?
– Uninsulated 2nd floor ceiling allows heated air to rise to attic and is thus wasted (vs having insulation between ceiling and attic space)?
– No known construction details similar to REMOTE.
REMOTE Cons
– More difficult to get wall/roof seal done properly?
– Allows thermal bridging at wall/roof junction
– Requires additional framing to provide interior space for overhead ducting?
Thank you,
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Replies
Walker,
OK, you've done your homework. I notice that you don't have a question.
So all that remains is for you to make a decision. Good luck with your project.
Walker, you certainly have done your homework.
I haven't built one, but have long been considering a PERSIST or REMOTE retrofit of my home. From what I've learned some of the cons you list shouldn't worry you too much.
The long screws could allow condensation to form, but only if you miss a stud and the fastener just penetrates the sheathing - it's recommended to cover this penetrating screw tip with a scrap of foam or a glob of spray foam. This requires a walk through to inspect the bare sheathing from the inside before interior finishes are applied. I believe this is shown in the CCHRC REMOTE videos and its shown on page 36 of the REMOTE Manual. Here's the video Part 1, and Part 2 is also on the CCHRC You Tube channel. I found these to be excellent resources!
https://www.youtube.com/watch?v=1leyQtqVV-A
REMOTE doesn't have to allow a significant thermal bridge at the top plate. There are 2 ways to address this spot with a raised heel truss. You can continue the exterior foam up beyond the top plate to a height equal to the top of the ceiling insulation (but still leaving an inch or two below the roof deck for ventilation air.) Or you can carrying the ceiling insulation out and over the top plate, to align with the outer edge of the foam. This requires some blocking or baffles that are pushed out to align with the outside edge of the foam. These methods are both discussed in the Framing section and shown visually on page 4 of the REMOTE Manual.
I think you've got a pretty good list. I really like the idea of PERSIST but agree that REMOTE seems a bit more cost effective with more standard roof construction. I'm confident either of these methods are safe as they match up with Building Science Corp's 'Perfect Wall' concept.
If you haven't watched the CCHRC's videos, I'd strongly recommend them. Good luck!
Which raises a question of if Martin hasn't had his cup of coffee this morning and is feeling extra snarky or if I should have had one more cup before posting and made sure to be more specific. BTW, I counted five questions. :-)
My bigger question is how accurate is my list, have I left anything significant off, and have I included things that don't need to be included?
Walker,
My answer was sincere; I praised you for doing your homework and wished you well. Absolutely no snarkiness was intended (although it's quite possible that I haven't yet had enough coffee.)
Q. "Does REMOTE have most of the benefit of PERSIST for considerably less cost?"
A. Yes.
Q. "Are SIPs available with plywood facings?"
A. No, not to my knowledge.
Q. "Will there be sagging problems with PERSIST exterior skin, since skin & foam are effectively cantilevered out over the block wall?"
A. There is no evidence that this is a problem. For more information on engineering calculations for furring strips screwed through foam, see Fastening Furring Strips to a Foam-Sheathed Wall.
Q. "Will mechanical fasteners hold up for 100+ years?"
A. Plenty of 150-year-old houses in New England have sheathing that is held in place with the original fasteners.
Q. "Any problems with long screws that go through all foam layers causing any condensation problems?"
A. No, not that I've heard of -- especially if the screws hit solid framing rather than sheathing.
Q. "Is there enough distance and protection [of fasteners] that cold from outside and moisture from inside never interact?"
A. The most important thing is to embed the fasteners in lumber rather than having them hanging in the air.
Q. "In a PERSIST house, does the heated air rise to the attic, through the uninsulated 2nd floor ceiling, thus wasting heat (vs having insulation between ceiling and attic space)?"
A. A PERSIST house has a higher interior volume and a greater building envelope area, and therefore should have slightly higher energy needs than a REMOTE house. But you are getting more square feet of interior space.
Q. "Is it more difficult to get the wall/roof seal done properly in a REMOTE house than a PERSIST house?"
A. Yes, but it is a solvable problem. You just have to think ahead.
Q. "Is it true that a REMOTE house requires additional framing to provide interior space for overhead ducting?"
A. Only if you choose to heat the house with a forced-air system, and only if you don't have enough room in your basement to run adequate ductwork.
Thank you. Sorry for the delay, I wasn't aware I'd be living without internet for a week. And sorry for the snarky assumption. Off to get a Scotch.
There had been a number of problems with rot of SIPs, particularly in roofing applications. Have these been resolved?
Walker,
There are two ways to reduce the chance of OSB rot on a SIP roof:
1. Make sure that you install air-sealing tape on the interior side of the SIP seams. For more information on this issue, see Air-Sealing SIP Seams.
2. Include a 1.5-inch-deep ventilation gap above the SIPs, followed by a second layer of roof sheathing. (If you go this route, make sure that you specify a vapor-permeable roofing underlayment like asphalt felt -- not one of the new synthetic roofing underlayments.)
Thanks Martin. Assuming all are done properly (based on current known best practices), are all three systems similar in terms of likely longevity or is one or another more likely to last longer. It seems like SIP has a higher likelihood of failure due to rot or adhesive breakdown than PERSIST/REMOTE but perhaps not if done properly?
Walker,
I guess that it's fair to say that some builders wonder about the longevity of OSB. Most SIPs include OSB facings.
We don't really have good data to show how long OSB will last if it stays dry.
If you are worried about OSB longevity, I suppose you should gravitate to a REMOTE or PERSIST house with plywood sheathing.