I’m down here in Houston and a lot of our houses have uninsulated crawl spaces. The houses usually have a lattice surrounding the crawlspace to keep the dogs out, but to let air in.
In the old days nobody insulated the crawl space as energy was cheap. I have a client who just bought a house with a 3′ high crawl space and I suggested it would be better insulated. He asked me to calculate how much he would save by insulating this crawl space?
Is there a simple formula to calculate this? His walls are R15 and ceilings R-30, windows are doulble pane low-e. And it is really comfortable under the house during the hot Houston summer.
Thanks
john
Replies
In a Residential Manual J calculation, there is no BTU gain for spaces/walls below grade. Btu gain = need for cooling.
If the space is above grade, there is a btu gain.
So, if your crawlspace is entirely above grade, then there is a btu gain through the floor, and money will be saved by insulating it.
You will have to figure out the btu gain through the floor, and once you know your cost per cooling btu, you will have your answer. I do not know of an "easy way" to do this.
There are also air infiltration issues through holes in the floor for things like plumbing and electrical.
Indoor comfort will also be improved with proper insulation, sometimes that is an easier sell than money saved.
Around here, the ground holds plenty of moisture, so improperly insulating and a lack of air movement will create huge mold and moisture problems. I would think the vapor barrier would need to go towards the floor of the house, and a vapor barrier/cement slurry coat on grade.
Conduction formula's at the bottom of the first page, explained on the second page.
PAHS Designer/Builder- Bury it!
thanks for the formulas, these are fun. Thanks again
john
You're welcome.
That's "Other Homes and Garbage" ISBN 0-87156-141-7 if you want a copy. Lots of used ones around, cheap. Engineering manual written for non-engineers. Variety of subjects. Highly recommended.PAHS Designer/Builder- Bury it!
thanks again, when you said cheap I didn't know that meant16 cents plus $4.00 postage.
thanks again
john
LMAO... that surprised me too. It's a very good book if you don't mind cranking out the calcs. From the number of used copies available it would appear that most do mind. PAHS Designer/Builder- Bury it!
I love how they alway calulate how much insulation cost compared to the energy used. hell juust go ahead and do it and be comfortable. no telling how much insulation or energy will cost in twenty years. Just bite the bullet and planned on spending money at beginining for insulation.they told me I just needed r30 in my attic. I told them I wanted r80. well you wasting money. so, its my money. I really had to fight them on it.
they told me I just needed r30 in my attic. I told them I wanted r80. well you wasting money. so, its my money. I really had to fight them on it.
That's so true. The funny thing is over the life of the building that "extra" insulation will have paid for itself many times over.
Beer was created so carpenters wouldn't rule the world.
>> they told me I just needed r30 in my attic. I told them I wanted r80. well you wasting money. so, its my money. I really had to fight them on it.
Must have been the cousin of my insulation contractor.
It's genetic, I tell ya!
Dave
The real trick is to balance how you invest your dollars. Above some R value, it becomes more cost effective to infest the dollars somewhere else. For my climate, investing in a savings bond will be a better investment that going beyond about R50 or so.
So, instead of adding more insulation on top of the R50, other things need to be looked at as possible investments.
Some of the many, many other choices are: going with a variable speed blower in the air handler, increasing the size of the duct work and adding more filter area to decrease the back pressure on the blower, putting all the duct work in conditioned space, investing in a more cost efficient refrigerator and other appliances, higher quality windows and glass, casement windows instead of double hung, larger overhang from the roof, better air sealing all around the house, vents that seal better for the bathroom and dryer exhausts, strategically placed trees and shrubs, etc...
Well, given that the house is functionally "floating" in ambient temperature and humidity, the "answer" is probably to get 2-3" of spray foam in the bays and 1" on all the framing.
As you will have already seen, floors are not often well covered in heating/cooling calculations. Some of that is because, historically, the calculations were heat-based, and any warmth rising up through a floor was "good." Partially, too, was that "everybody knew" that you do not insulate floors, you ventilate the space.
Ok, so, letting more 68% RH 90+º air "cool" one's house is now understood to be less-than ideal; "the system" just has to "catch up" to the reality of it.
This is not helped at all by some "floor" calcs presuming that there is a perimeter wall in ground contact either. I've run into this a time or two with houses on various forms of "stilts" (coastal & riverside).
So, what can be needful is a sharp ashrae type who can codify what reducing infiltration will net you, along with increasing the R-value between the warm air in the c/s versus the finished floor. Since a majority mud-bed tile floor is unlikely, you probably could assume that the existing R value is in the 4-5 range, and since it is permanently shaded, has no solar gain, and cipher from there (so, getting R17-19 of foam in place cannot but help).