Reasonable retrofit efficiency goals
As I battle with an hvac solution for our home, we are leaning more and more towards trying to reduce the load further. I’ll mention some details of my own house a bit below but I’m actually asking in hopes of a more generalized discussion of what a reasonable retrofit btu and ACH50 goal is.
I did some reading tonight in the hopes of answering this question but didn’t find any useful rules of thumb. I know, the old HVAC rules of thumb have caused a lot of grief. I’m not asking for a rule of thumb about BTUs/sf for heat but rather what is rationally achievable in a retrofit scenario.
What’s rational? I’d argue that in a DER remodel most folks probably want to either do the outside or the inside but not both. If you are remodeling inside anyways, an owner might be willing to strip the drywall and do various things from the interior: air sealing, blow foam or cellulose in the walls, blow cellulose etc in the attic, modify the air barrier, replacement windows. Alternatively, if the interior is not being remodeled, I’d guess most owners interested in DER type work might look to the outside: air sealing, add foam outsulation, blow cellulose in the attic, replace windows. But few would do both. Oh, I’m talking heating and cooling, not net PV.
But in the end, what kind of BTU loads can be achieved? I realize we have to talk climate here but there are vast swathes of the country that run 5000 to 7000 HDD. Individual variation in site, exposure, and opportunity probably vary more than that.
So what kind of retrofit projects have people undertaken and what kind of modeled or measured results have you achieved? What did you have to do to achieve that? Bonuses for links to your construction blog etc.
We are currently at BTUS/sf
Heating 17
Cooling 15
ACH50 3.8
We’d like to achieve more like 11 and 8 in order to open the option of 3 and 2 ton equipment.
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Replies
Keith,
It's hard to answer your question about "reasonable goals." As you may know, the cost of a deep energy retrofit is usually so high that the cost of the retrofit work can't be justified by the energy savings.
For an article detailing the work needed to obtain energy savings of 60% to 65%, see The High Cost of Deep-Energy Retrofits.
To read about some of the lessons that Paul Eldrenkamp and Mike Duclos have learned from their deep energy retrofit projects, see Deep Energy Retrofits.
To read a discussion of whether this type of work makes any sense, see Deep Energy Retrofits Are Often Misguided.
Martin,
I'd read the two gba articles before. While excellent reading, the scenarios are a bit different than a homeowner retrofit.
The home energy article is great and gives some realistic ideas about what can be achieved. 11 or 12 btus sounds achievable but not without cost and serious air sealing.
Thank you very much for that link.
I do understand the point of the pv article. In our case, thoughts of lowering the load are driven by poorly installed hvac. Our nearly 5 ton load limits our options.
One question that arises is 'good enough' windows. Under 0.30 is affordable achieved. Under 0.25, particularly without vinyl, is expensive. Under 0.2 a fortune. How good is good enough?
Keith,
For an energy retrofit, it's usually easy to answer your question about windows ("How good is good enough?").
That's because the existing windows that you already have are almost always good enough. It's almost impossible to justify the cost of window replacement with future energy savings.
In cold climates, however, the installation of low-e storm windows makes sense if your existing windows are single-glazed.
Keith, you asked a great question and framed it well (pun intended). I'm often on this site because I'm doing a deep energy retrofit on my 1940s built house in Northern California. I come here to not only get tips, but to also gain overall perspective and insight on what I'm attempting to accomplish. I would say that if you are trying just to save money on energy then just work from the outside, i.e. , strip your siding and put up foam board in at least two layers. I think this is the most cost effective way to both air seal and insulate most homes. It's quite straight forward, though admittedly still a lot of work. If you are doing it yourself then this seems to me definitely the way to go.
In my case I did not go that way because I wasn't at all satisfied with the internal layout of my house. There was just too many hallways eating into living space. Also, my kitchen was cavernous and took a huge amount of space from my long narrow living room. It just wasn't a house that felt good to be in even though the overall square feet met my needs. The answer I came to was to knock out internal walls and reposition them. While doing that I've torn down all the old plaster walls that were too highly textured and in terrible shape. Cracks were showing up everywhere.
This is a case where a deep energy retrofit works. It was part of a general remodel. But I have to tell you that many times I've thought to myself that doing this is more work than just building a whole new house. I'm doing it all myself so it's a labor of love but I would never advise someone who does not like this kind of work to take it on. If I had someone else doing it then it would be extremely expensive.
Beyond repositioning internal walls I've added 2x3s to all the 2x4 perimeter framing so that there is very little thermal bypassing with the cellulose I've blown in. I'm doing that myself also.
So, to your question: if you are satisfied with your house layout then go external. It's a lot less work and you'll have a very easy air seal of the 2 layer foam board on the outside. Doing the inside is a LOT more work and to me only makes sense if it part of a general remodel. Of course, if there is only one room of the house that is cold or creating a draft then it might make sense to do an internal retrofit, depending on the circumstances.
Edit: I should add that the advice to go external does not apply to insulating roofs. If you have a vented uninsulated attic it always make sense to try to air seal the ceiling from the attic and then just add insulation on the ceiling, unless you are going for a conditioned attic or have a cathedral ceiling.
I don't think the goal should be framed as something like BTU/sq. ft. Rather, I'd frame it as an assessment of each possible upgrade, and consider the cost/benefit analysis for each. Sadly, few beyond the basics will have a reasonable payback time, but they might be worthwhile in some expanded consideration of values and objectives.
The expanded considerations could include:
* Improving health and comfort of the occupants, by providing better air quality and thermal comfort.
* Solving problems that are damaging the building as is, such as moisture, ice dams, etc., or taking the opportunity to upgrade performance while replacing things that need to be replaced anyway.
* Renovations that are wanted for reasons other than energy savings, with energy savings rolled into the package.
* As well as concern about CO2 emissions.
Given the wide variety of situations with regard to those expanded considerations, it makes more sense to me to assess not just each project but each measure individually, rather than set an arbitrary general goal.
Reasonable would be hitting zone insulation values and 1.5 arch @ 50 pascals on the blower test.
I do wish GBA had a reply notification system. I missed these replies.
Charlie: I think comfort and damaging or deferred maintenance opportunities do represent the best opportunities to execute DER style work cost effectively while 'getting' the owners something for their money.
In the case of deferred maintenance, you are probably talking about siding or windows primarily. These things often deteriorate as a set IMHO while interior damage and problems are more often addressed piecemeal. It's a rare owner I think who undertakes a full interior remodel.
But part of my question was in regards to what should a homeowner push for?
A lot of insulation contractors looked at the R-19 in my previous house and said 'good enough' in zone 5b. No, it isn't.
A stucco contractor I talked to recently said 1" of XPS was enough (not by code in my area and not by my sensibilities).
HVAC contractors aren't at all bothered by a furnace that is twice the modeled load. In fact, several wholly rejected the idea of replacing the existing unit with a Manual J/D sized furnace.
I guess what I'm getting at is the "Pretty Good Remodel", akin to the Pretty Good House but for existing structures.
I know: remodels are hard because every remodel is different. Yet, I still think that a model house and model target could be created.
For example, in my area a lot of housing stock was 'tract' built in the 50s, 60, and 70s. A very typical house is 2200 sf in a two story rectangular box with an attached two car garage on a crawl space or sometimes an additionally 1100 sf basement.
Unremodeled, a lot of these house have these characteristics:
ventilated crawl spaces with no rim joist insulation, floor insulation, vapor barrier, or foundation insulation
80% efficient single stage single blower speed furnaces
55% atmospheric vented water heaters
R-11 fiberglass in 2x4 walls with no outsulation
U 0.8-1.3 aluminum single or double pane windows
R-11 to R-19 fiberglass batts in joist bays in the ventilated attic
Poor air sealing details like window cripples open to attached porch roofs
Unsealed and uninsulated attic hatches
Given this house, you are probably looking at ~80k to 90k heating load which results in a 100k to 120k furnace.
If you have this house, your BTU load is perhaps 35-40 BTUs/sf. What can you achieve? With what ROI? Where should you spend your money? When? Some items will be never, some will be right now.
It really seems like there is a void of information out there.
Before anyone says energy audit, I'll say I had one. It was junk. The kind of thing that says I'll reduce my total electrical usage by 20% by getting a new energy star dishwasher instead of my 5 years old energy star dishwasher.
Anyways, thanks for the replies.
You can harvest modest energy gains as part of the regular maintenance/replacement cycle, or you can turn your house into a passivehaus. It's really up to you. All depends on what you're willing to spend, do yourself, change, etc. An old structural brick house in Minneapolis might have a 25,000 BTU heat loss through the walls. Fixing that would entail losing the brick exterior or some interior floor space, depending on where you put the insulation. Just a bunch of trade-offs. It all depends on your expectations, which are informed by your desires, budget, and length of expected occupancy.
Regardless, it sounds like you've discovered the old motto: "if you want something done right, do it yourself!" There are simply too many crooks out there to trust this kind of work to the supposed pros unless you can find someone who actually knows energy efficiency as opposed to simply installing commodity equipment or doing work common the the area. If you are to find someone who knows what they are doing, it will likely be through word of mouth and personal recommendations from other people in your area who also care a thing or two about energy efficiency and successfully hired the firm. But in general, concern for efficiency is uncommon (sadly) so your typical contractor is likely to be clueless, even if they are capable of good craftsmanship.
I am retrofitting my own house and have run into the same thing. Here's what I did/am doing:
1. Do your own Manual J modeling. You can buy the software ($$$$), or, if you're cheap like me, you just can make a spreadsheet that calculates it all out given the relatively simple equations for heat loss, and sanity-check your work with a rough estimate from http://www.loadcalc.net/. It doesn't have to be perfect, just pretty good.*
2. Make a graph of the results and see which major building components have the most heat loss/gain associated with them.
3. Estimate the cost to improve each element, either DIY or hiring a pro, accounting for paying the "asshole tax" due to supervising and micromanaging the work to make sure it actually gets done right.
4. Calculate the BTUs per dollar spent for each building component.
5. Write off the ones that will be too expensive, impossible to do without wrecking other things in the process, impossible to DIY but unwilling to trust to a pro, etc.
6. Tackle the remaining projects in order of cost-effectiveness.
Keep in mind that air infiltration is likely to be your worst enemy, especially during the heating season. Prioritize sealing those up.
In your tract home example, where most things are wrong or insufficient (like my house) honestly I would say just start wherever you want. If the house has horrible sticky aluminum single pane windows, replace those with pretty good vinyl windows (surprisingly cheap if you do the install yourself). Drafts you can feel are easy to locate and seal. Floor and rim joist insulation isn't too hard to do yourself provided your crawlspace isn't full of Morlocks. Blown attic floor cellulose is hard to mess up too badly and can often be hired out very cheaply. If the furnace is breaking down, replace it with a high efficiency model sized for the modeled load that you can achieve in a year or two, and wear a sweater until then. If the siding is deteriorating and needs to be replaced anyway, take the opportunity to stick a few inches of rigid insulation over the sheathing. Forget about getting a better kind of water heater unless you are hot water pigs. The savings will exceed the equipment lifespan if you are efficient (and you probably are if you're posting here; your possible teenage children, on the other hand...).
* FWIW, I did my own Manual J modeling and calculated my heating load to be 30k BTUs. The previous owners' HVAC guy put in a 125k BTU 80% efficiency furnace. It runs for about five minutes like a jet engine and then shuts off for 20. Go figure. Taking into account the actual delivered BTUs, it still oversized by more than a factor of 3!
Nathaniel,
Ha! I could have written your post. I've had the same experiences on a lot of fronts. And I have a 2x load furnace that runs about 10% of the time as well.
I had hopes this post to be a metric guideline discussion; that is, what # of BTUS/hr is standard, efficient, or hyper efficient for a given climate zone. For example, in my zone 5b, a standard older home uses X therms a year which is Y BTUS and similar guidelines for new energystar homes or perhaps a stab at the PGH (Pretty Good House) bar to meet.
I haven't the money (or really interest) to make my house into a passive house but I'd like to achieve a pretty good efficiency. It's just hard to know what BTUS/sf that is.
From some limited investigation with a Manual D engineer and playing with loadcalc.net, it looks like 10 BTUs/sf might be the practical limit but I'm surprised by that. It seems like large amount of outsulation should produce better results than the models are suggesting.
Anyways, thanks for the validating post. You've had many of the same experiences I've had I think.
The models should be able to tell you where the heat is going. If your windows are 50% of the heat loss before outsulation doubling the wall performance they will totally dominate the load numbers unless up upgrade the windows.
Rarely is it cost effective to strip fully functional shingles, or siding, or replace windows purely for the purpose of upgrading the thermal performance of the house, but If you're doing and of those projects anyway, it's an opportunity moment that shouldn't be passed up.
At the projected price curves for grid tied solar, it may become cost effective to re-roof a "not quite dead yet" roof to be able to cash in on much-lower-than retail lifecycle cost power, which can be further leveraged with heat pumps. In high electricity price markets it's already there.