*
I’m in the planning stages with my GC and Timberframer and I’ve told them I want to use 7″ SIPs on the walls and 9″ on the roof of our timberframe house. The 7″ SIPs are rated at R26 and the 9″ at R36. The house will be build in N. Central OH, and we do get cold spells in the winter in the sub-zero’s.
Both the GC and TF’er are questioning me on why I want to have such a high R value wall and roof. Evidently the homes they usually build only use 5″ walls (R17) and 7″ on the roof. We will have a HRV(?) so that we can keep fresh air circulated in/out of the house. I’m sure they’re thinking is probably that this is overkill, but with the insignificant cost to go with these thicker panels, am I wrong to go with the thicker option and higher insulative value? Each year the ante keeps going up on R value, so what is so ludicrous of my decision.
I appreciate your input, thanks.
Matt
Replies
*
Hi Matt,
Your proposal for insulation is not overkill. Unless you want the look of timberframe, the timberframe is overkill since SIPs are much stronger than stick building by themselves. Make sure that the SIPs go over the timbers, not between them, to prevent thermal bridging.
*I doubt that there is such a thing as too much insulation. However the extra cost involved in the overage is an issue. For your structure you can plan on spending an extra .50 a square foot for the thicker material. With the higher R value your mechanical needs to heat/cool should be less therefore a smaller unit? Less baseboard? Whatever. Figure these savings over a 30 year mortgage and punt. Just remember the customer is always right.And Ron, the product that should be used on Matt's Timberframe is actually an SSP (Stress Skin Panel). These panels rely on the structure the timberframe provides as they are only rated as semi-structural and provide shear strength and an easy way to skin out a frame. I have never seen anyone use these panels as infill but I have not seen it all yet either-- I wouldn't be really worried about the thermal bridging though as an 8" timber does not conduct very fast, at least if you listen to the log home industry.By the way I am glad that you think that SIP construction is so much stronger than conventional but do you have proof? Or do you just suscribe to the dogma that SIPA and their members spew? How is it that the IRC in Canada does not recognise SIP's as structural? see: http://www.nrc.ca/irc/newsletter/v2no1/panels.htmlhttp://fox.cisti.nrc.ca/irc/newsletter/v2no4/panneaux_e.html
*
Bob/Ron,
Thanks for the replies. To answer some of your questions, the panels will be installed on the outside of the timberframe. I don't yet have the exact cost to go with the thicker panel, but am inquiring about it. I know with a previous panel mfr (Insulspan) we looked at, the extra cost was nominal/worth it.
*
Matt. As with any building product the question for SIPs is how long will they last and how well will they hold up. The major problem with SIP panels is when they are used for roofing. Because the expanded polystyrene (ESP) core is a thermoplastic it softens at high temperatures, such as are typical of roofsfound on roof panels. The softening causes the inner and outer structural skins to move relative to each other. This movement is called creep.Humidity also pay a part.
This nothin new. As far back as 1987 researchers observed SIP roof panels concaving--arching upward--1-inch in 8 feet. The outer skin contain more moisture than the inner skin when the upper surface is cold and the inner skin is warm. When the outer skin exppands relative to the inner skin the panel bows. As the outer skin expands it closes the outer joints or deforms them by crushing.The crushing causes telegraphing of the shingles.
Best bet is to (a) not use SIPS for roof. (b) if you do use SIPs for roofs reduce the clear span distance or (c) increase the SIP's thickness. I understand that in southern hot humid regions the edges of the skins thicken as they absorb moisture. Gene L.
*Gene raises some good points about SIP's for roof applications in regards to their shortcomings as far as he goes but there are ways to overcome these problems with a little research and some common sense.First of all the panels that are known for creep if studied, are spanning a greater distance than they should. Which matches quite well with Gene's suggestion to lessen the clearspan. However adding thickness to the foam core is not a great idea as the foam is not structurally rated. Will give you a greater R-value though. However if you actually add structural content to the system ie: 2x's on 24" centers, which means each 4' wide panel would have two "rafters" the same depth as the core encapsulated within, the 2x's would be engineered to span using conventional guiedlines set forth by BOCA etc. and at the same time stop the "creep". This type of SIP is currently available but not well known. I personally don't care about the thermal bridging issue for this application as any truly structural system will TB and I am more concerned with structure. Those of you who are offended by this just go ahead and do it your way, please.The more efficient EPS (not ESP) foam insulation is I feel a better product than Fiberglass for any building purpose, sidewall or roof. The lack of need to ventilate due to the encapsulated air is in itself worth it in my book. Look ma, no vents! Of course deleting vents or the need for them or where to put them etc. will take away a good share of banter on this forum though---A properly installed roof system built of actual structural insulated panels is a joy to behold and a breeze to maintain. Hot or cold. Imagine a 40 sq. roof with through dormers and pitch changes from 8/12 to 12/12 installed in 6 days with pine t&g interior surface and eaves/soffits ready for fascia. I personally will take that over the old way. And do.
*Bob, I have seen SIPs homes hit by violent storms and trees that survived when their neighbors went down. Ask any trade that has put siding on one and they will tell you how solid they are too (like allways hitting a stud). I have had roof SIPs routed on the long sides to recieve TJIs if I need a long span with minimal thermal bridging (1/2" osb every 4 feet isn't much). Also Bob, I think allmost any insulation is better than fiberblass. When I don't use SIPs, I build my walls with TJIs 24" O.C. as wall studs with a caulked poly vapor retarder and blown in cellulose insulation or Icynene.
*Bob. Your points are well taken. I forgot to mention that creep is noit a problem with urethane foam cores.Gene L.
*Interesting in that Urethane cores are introduced into panels by foaming in place and the reaction of the chemical process creates the bond to the OSB. Unfortunatly there are emissions of CFC's during this same chemical reaction. But is a Urethane core more sturctural than an EPS core? Not to my knowledge. Kind of makes me wonder if creep is not caused by the glue joint itself rather than the core material and that the urethane bond is stronger. I have never really dealt with Urethane but I know that Poly-Iso cores that depend on the bond produced during manufacture can delam pretty easily.And Ron, I guess you must live in Michigan where the cover photo for SIPA's site is. Still is not proof to me that SIP's are "much stronger than conventional construction" though I am afraid I am a hard sell. Also I think it is strange that the reports I have heard of siding on SIPs delaminating due to the inadequate nailbase provided by the OSB has anything to do with structural strength? Also I was unaware that TJI's had any structural value used as a column with stresses parallel to the grain? How do you keep the cellulose from settling which would create an airspace at the top of the wall? Even with a vapor barrier shouldn't you in theory ventilate the wall like you would a roof? But 9 1/2" of cellulose, wow talk about alot of insulation!
*Hi Bob, I live in NY now but used to live in Ohio. TJIs are not rated by their manufacture as columns, but their strength can be calculated worst case from the flanges alone. Some have 2x4 flanges which we all know can hold up a house. Both the sheathing and the flange keep them from buckling. Dense pack Cellulose won't settle. My own house is 11 7/8" TJIs for the walls and I don't keep the oil man rich.
*
Let's get back to evaluating R versus more R. Assume that you loose about two thirds your heat through your roof and one third through the walls. your R values yield a potential decrease in heat loss of 53% for the roof (R 17 vs 26) and 38% less through the walls (R 26 vs 36). Now it is important to take your projected heating bill, annually, cause this will reflect the realities of living in Ohio and check out how many dollars you'll save per year on this increased cost. The decision boils down to how many years in payback it will take to recoup your expense. Having been a "retrofitter" during the energy crisis, I will tell you that most people won't spring for something without a 2 or 3 year payback, minimum. I would say it is worth it if you get a payback in a maximum of 5 to 7 years.
*
Payback? Whats everybody think about that?? I personally think it's very much used to support one's preconceived views much the same way polls and medical studies are viewed as being interesting but often not scientifically of real long term value or truth. The statistic people now say butter's better than Parkay! Have you switched again?
*You also have to factor in the cost of the heating /cooling system that will provide a comfortable level of heat/cooling for your envelope. If you can heat your house with a light bulb and cool it with a glass of ice left on the counter because you have so much insulation the extra cost of insulation is offset by the lower cost of your heating/cooling system.Also your annual bills will be lower and through the Energy Star program that is available you can qualify for a larger mortgage on an efficient home because they factor in the energy cost for the time of the mortgage.The biggest problem we all face in our search for the right balance though are the openings in the envelope that provide for doors and windows. Even the most efficient door or window cannot hold a candle to the wall it is attached to. Also the cost of the former is out of hand and though people try to do their best the quality of doors/windows across the board are pretty pathetic.
*
I'm in the planning stages with my GC and Timberframer and I've told them I want to use 7" SIPs on the walls and 9" on the roof of our timberframe house. The 7" SIPs are rated at R26 and the 9" at R36. The house will be build in N. Central OH, and we do get cold spells in the winter in the sub-zero's.
Both the GC and TF'er are questioning me on why I want to have such a high R value wall and roof. Evidently the homes they usually build only use 5" walls (R17) and 7" on the roof. We will have a HRV(?) so that we can keep fresh air circulated in/out of the house. I'm sure they're thinking is probably that this is overkill, but with the insignificant cost to go with these thicker panels, am I wrong to go with the thicker option and higher insulative value? Each year the ante keeps going up on R value, so what is so ludicrous of my decision.
I appreciate your input, thanks.
Matt