Manual 'J' and Foam Insulation

16 years ago

New house under construction. Live in South Louisiana. Most windows on North side of home. Foam under the roof deck(5-6 inches) and in the walls(3-4 inches) of a two story 6000 sq. ft. home. Manual J performed by several HVAC contractors...all provide for 5 ac units (varying tonnage)for a total tonnage of 16-18 tons! Told that the Manual J program does not always take into account the significance of the foam insulation....told program makes no allowance for anything with R value greater than 19.

Told by foam contractor that 16-18 tons of ac way high...suggest we could get away with as little as 6-8 tons. BTW, going with American Standard 18 SEER units with variable speed handlers.

What gives? Would think that we could get away with much less tonnage and number of units.

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Comments (6)

mikenew
16 years ago
Manual J only knows what it is told and they don't know what to tell there is an R value to your structure period and that is what you need to know the foam contractor needs to talk to your HVAC guys. Kinda discouraging that five companies can't figure that out, SCARY!
lsufan
Original Author
16 years ago
Yes, it is very discouraging. No confidence that it will be done right. Now I am second guessing every decision on the HVAC system.
Welcome more comments from others.
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This is from Manual J 8th edition version 2 ACCA Improper Practices 2-4 Do not use "rules of thumb". The idea that the required equipment capacity equals the floor area multiplied by some magic number has resulted in many customer complaints and legal actions. Heating and cooling loads depend on individual circumstances. Floor area to tonnage ratios for the U.S. housing stock can range from less than 500 sq ft per ton to more than 1,200 sq ft per ton. Efficient single family detached homes with a normal amount of well distributed glass typically fall in the 700 to 1,200 range. Limted exposure dwellings with concentrated glass (that produces a time-of-day peak)may fall into the 500 to 800 range. Homes with exceptional features can be all over the map in this regard. Just rotating a home on the site can change the ratiio by 100 to 400 sq ft per ton. I edited out the replacement equipment paragraph... Comfort system performance is only as good as the accuracy of the load calcuation. Efforts to "adjust the load" to provide a "safety factor or to produce a solution that is compatible with the " I have been doing it this way for thirty years" syndrome only produces designs that generate customer complaints about poor temperatureand humidity control, or more serious problems, such as mold and mildew. _______________________________________________________ and Bob...please- this isn't a DIY or best case house we are talking about, but a homeowner trying to get his house right.. save your diy projects for your indulgent threads on that topic. As he is trying to decide to spend xxx dollars insulating his roofline and the timeline for payback your project does not project the numbers he will be paying someone to do his job as yours is diy. I have yet to see a foam insulation company do a job that has a payback of less than 20 years, and I do these projections often. depending on the pitch of the roof the cost of the foam insulation is a consideration. having a complete thermal barrier at the attic floor is a much more conservative approach if detetmined to use foam, although this thermal barrier can be achieved with out foam with attention to sealing of ALL penetrations into the conditioned space (wire, plumbing, recessed lights and knee walls where ceilings change heights). I would recommend the latter, sealing and insulating well with conventional insulation for same performance at a lesser cost and faster payback. the load calcs are only as good as the inputs and the information provided to the person doing the load calcs. I would question the information that has been provided by taking the house plans, a copy of the current load calc to another more experienced hvac company for comparism.
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Going from 4 ton to 5.5 ton on 2500 sq ft in Jersey seems excessive. Around here, and we have similar heat in the summer, we'd probably have 1 system and 3 tons, 3.5 tons max. I find it hard to believe the house has a duct system sufficient to upsize the equipment like that! Most systems are starved for airflow as it is. I don't see going 2 to 3 downstairs. Cool air from above falls and warm air down goes up. Maybe the 2.5 ton upstairs makes sense. Oversized can mean humidity control issues. I wouldn't do anything without an accurate load calc being done. Then someone should check the static pressures of the duct systems and be sure there is the capability of moving enough air. You could spend all this money and find you aren't any more comfy than before.
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I can imagine that being in any jobber biz you're eventually going to get shopped after quoting a job. In my business, we take that risk by submitting our best ideas on multimillion dollar public works projects. They may like your idea but not your price, and you spend tens, maybe hundreds of thousands in the pursuit over multiple years. Think that's just a lack of ethics that pops up across the business community. Energyrater- I've been participating in a discussion on FH's Breaktime forum. Someone putting in vinyl windows where the mfr instructions said not to foam them in. I was shocked by the number of folks who agreed that you should follow the directions and use fiberglass. I can't imagine putting windows in and not using the window/door poly foam to seal them in. dallasbill- the only thing that you'd need to wing it on is the ACH. If the house remodel is in progress, you won't really know how leaky it's going to end up. We did a blower door test before construction and then based the sizing on an estimate of how leaky it'd be after doing lots of improvements. Still have work on weatherstripping old windows/doors to do. I'd intended to get it tested again afterward, but we'll see if that actually happens. yoyoma- ideally you want to test the new system with a "duct blaster" test to see how well sealed up the ductwork is. Not as big a deal if it's all w/in the conditioned space, but most of the time it isn't. A well-installed new system will leak .05 cfm/sq ft of conditioned space. Here is a link that might be useful: Southface fact sheet on blower door & duct blaster testing
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When it comes to new home builds, the discussion is almost always entirely centered around capacity or size of the equipment. While that 'could be' an issue the chances of that are extremely rare cases. They had to have not sized the equipment right. Certainly that is possible. But a larger issue that comes with new construction is knowledgeable people that perform the work of installing the system and designing it properly. Design: think ductwork, think refrigerant circuit (line set), think air returns and air return placement / sizing. Architectural limitations: the architect forces the unimaginable due to layout of the structure. How is a newbie installer going to work out those challenges? (Uh, he won't. --- don't kid yourself.) Even a properly sized, 'improperly' installed --- 'improperly' designed HVAC system will not work. Not by any stretch of the imagination. Due to construction labor shortages, proper design is thrown out the window. We don't have the skilled labor, because those have changed careers think $$. Money in new construction is on a razor wire. You know it's too easy to blame it on sizing. Let's not pay a pro to come in and diagnose a problem. Because this will cost us more money. Until you rule everything out, you're no better off. These things can't be done from the luxury of a forum board. HVAC is an in person type business. I service the Katy, Texas area.
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ryanhughes
16 years ago
Five a/c units may be 1 or 2 too many. I suggest you keep calling around until you find a competent dealer who really does a good load calculation. Ask to see their calculations. 16-18 tons for a 6000 square foot home is just unheard of in my opinion. My home is around 3500 sq. ft. and uses a 3.5 ton unit (soon to be 4 ton hopefully).
henryd
16 years ago
We are certified home energy raters and do quite a few homes here in Florida that have foam insulation. We do a Manual J8 up front for our Energy Star builders for planning. You are in a hot humid climate and dehumidification is the controlling factor. This said, first on the list are variable speed air handlers. Secondly, by foaming things in you have reduced the air infiltration rate considerably, ACH's for cooling and heating used in the Manual J8 should be .15 air changes per hour or "very tight" construction if that is the only option. Next, if the widows are Low-e (really should be) they should have a label or spec with a SHGC (Solar Heat Gain Coefficient) of .4 or less. All overhangs and orientations should be accounted for. Also the ductwork will all be in quasi conditioned spaces -- meaning there is no load on the ducts. Air handlers should be located inside. If you proceed with the 16-18 tons projected - humidity control problems are almost a certain. We get numerous calls to diagnosis large multi-million dollar homes and oversizing is always one of the problems. Rarely will you undersize. Although we don't suggest using rules of thumb - it sounds like you probably need 5-7 tons total depending on the other variables. Any good contractor should do a room-by-room Manual J8 (this way they know where the heat loads are predominant - generally roofs with western window exposures)followed by Manual D (used to size and design the ductwork) and finally a Manual S to select the proper equipment.Another factor on loads is the type of roof you have. What you are buying is a "system" - you want well designed ductwork, with plenty of returns (remember we are removing heat not blowing cold air). The variable speed air handlers provide "low volumes" of air (or heat removal and moisture-humidity removal)over longer periods of time. 5AC units - unless the 6,000 square feet is a bunch of detached bungalows this sound ridiculous unless there are problems with routing the duct work. The only suggestion would be to have a Manual J8 room-by-room done independently by a Home Energy Rater or someone you trust. Try going to the Energy Star website and download info on heat pump sizing etc. - Also there is a website you can find call HVAC Calc where you can download the load analysis program and do your house yourself for about $50, and probably 5-6 hours to do a room by room. Good Luck
henryd
16 years ago
I posted an earlier response. If we can be of any help or you have any questions, please feel free to email them to me at henry@usahers.com
lsufan
Original Author
16 years ago
Thanks to all for the replies....very informative.

Manual 'J' and Foam Insulation

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Manual 'J' and Foam Insulation

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mikenew

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