Charleston home inspector discusses general insulation requirements.

Submitted by Ray Thornburg on Wed, 01/16/2013 - 12:43


General Insulation Requirements

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This is a quick synopsis of the current insulation requirements for homes in the Charleston lowcountry area. South Carolina uses the 2006 IRC code so our discussion will be limited to that application. Charleston is in the climate zone 3-a – category warm moist and “a” is the warm humid designated area.

On newer homes an energy certificate is required on the energy distribution panel. This label will tell you how much insulation is installed on the home at time of constrution.

·         Ceiling R-value 30

·         Wood framed walls R-value 13

·         Floor R-value 19

·         Mass Wall R-value 5 ( mass walls are generally concrete or concrete block type walls)

·         Crawlspace walls R-value 5/13 .  ( 5 for mass wall,13 other types)


Fenestration – This is code speak for windows – Window energy efficiency is called the U-factor. The lower the U-factor the less the heat loss so a lower number is better. To Convert the U-factor to R-factor take the number 1 and divide it by the u factor.  For example 1/u=R.  Current standard is .40 so 1/.4 = 2.5 R factor.

·         Minimum glazed window U factor is .40.

SHGC- Solar Heat Gain Coefficient – the lower the number the better - it is a measure of a windows ability to reduce heat gain.

Low – E – Low E coatings reflect direct heat radiation and thereby keep the inside surface of the glass cooler for instance. These types of coatings can be region specific which means that they way they coat the glass can vary by their geographic needs.

DP- rating- stands for design pressure and has to do with the windows ability to withstand hurricane force winds. The higher number means a stronger window.  The window size and wind speed and jurisdictional requirements vary so your code official can help here. Generally DP-35 or higher are what you will see around here.  A DP-35 window 20sf or less is good for the 130 mph wind zone for instance.  A larger window might not meet this standard. This is not really an energy efficiency rating but you will want to know what is required if you are ordering windows.

Generally replacement windows need to meet the new energy codes.

Ductwork Insulation

Supply and return ducts insulated to R-8. Ducts in floor trusses shall be insulated to R-6. All other ducts R6 except ducts inside the thermal envelope of the building.

Mechanical system piping – R-2(if it carries fluid over 105 or below 55 degrees)

Water Piping

Hot water Piping- R3 for 3/4 piping with a run greater than 10'. All other water piping shall be insulated if subject to freezing. See 2012 IRC 1103.4.2 for more on hot water pipe insulation.

About The Crawlspace Under Floor Insulation

Floor insulation shall be installed so that it maintains permanent contact with the underside of the subfloor.

Concerning the paper facing on fiberglass roll insulation. Vapor retarder shall be placed on the warm in winter side of the thermal insulation if used. Unfaced insulation is allowed to insulate the under floor areas of crawlspaces in climate zones 1-4 (Charleston area) (Recommended)Crawl space floor vapor retarders not excepted. See irc 1102.5 e., reference irc 2006 318.1 exception 2,3. References IRC 1101- 1103




insulation upside down In this photo the insulation is upside down. The paper facing is designed to be against the warm in winter side of the building. Also the instructions printed on each piece say the paper facing should be in substantial contact with the subfloor. It needs to be turned around and properly supported with insulation rods or other means. Although the manufacturer warns against this because the paper facing can burn in this climate zone it's more likely the paper will become damp and moldy.





window label

   Here is an example of the information you'll find on the window label. Click image to enlarge. It includes DP ratings and energy efficiency ratings. This window has a DP 50 rating.


example of blown in cellulose insulation

        At left is an example of blown in cellulose insulation. Typically made of plant fibers like ground up newspapers and magazines or denim cloth it has good insulation properties. It is somewhat heavier and denser than fiberglass and stays in place better. I've noticed also that it's easier on your lungs as it doesn't become airborne as easily as fiberglass can.   It's treated with chemicals to resist rot and fire. If you look closely you can see the ground up print material. It has a R value of about 3.8 per inch.


Blown in fiberglass insulation example.    At left is blown in fiberglass insulation which is very popular with many builders today. A very good insulator it is nearly fireproof, rot proof and won't absorb moisture. The R value of fiberglass can vary somewhat depending on how it is fluffed up. On older homes the fiberglass is denser and stays in place better than what you see here. This kind is easily airborne and trampled by workmen etc. thus reducing its R value in those places.  Because it is so light there will often be bare spots in the attic next to cornice vents etc. where the wind has blown it back.

loose fill mineral wool insulation

Here is an example of mineral wool or rock wool insulation. It's mostly made from slag which is a type of stony waste product left over from smelting metals.  It has an insulation value of about 3.7 R per inch. It's heavier than the fluffy white fiberglass and stays in place better. It has good insulation and sound deadening properties. It's resistant to mold and is fire resistant. It can be blown in like this or in rolled batts with paper facing just like fiberglass insulation. It is also frequently used around ductwork.


asbestos pipe insulationHere is some asbestos pipe insulation found in the crawl space of a home which was built in 1950. Asbestos is a health hazard if it becomes friable. It appears to be a corrugated type of product here.





asbestos corrugated sample


Here is a close up view of the corrugated  asbestos pipe insulation above.








 Fiberglass Warning



At left is a warning label on an Air handler. It says that fiberglass can cause cancer (in California) so be careful and protect yourself especially around the fluffy stuff.











Submitted by Ray Thornburg on Wed, 01/16/2013 - 12:43


Anonymous | Tue, 08/24/2021 - 08:41

We've been told to get rid of our insulation in our crawl space in Goose Creek. If we sell our home, can we pass an inspection with no insulation? We do have a vapor barrier on our 21 year old home.

Ray Thornburg | Thu, 08/26/2021 - 08:46

Yes for a ventilated crawlspace system R19 is required.  Withoug insulation the subfloor sheathing will get moldy and rot especially if the occupants like cool AC in the summer. Think of it like this. A nice ice cold glass of tea condensates on the outside of the glass. So this is what happens to your subfloor sheathing without insulation. If you like your thermostat turned down in the summer....the faster the damage will occur. If you install moisture resistant floor covering, then the AC won't be able to dry the subfloor out from the inside as the damage occurs faster. In the Charleston lowcountry area many homes can benefit from an encapsulated crawlspace system. Contact a professional about installing this type of system because special knowledge is required.

Anonymous | Mon, 10/14/2019 - 07:57

We are planning for a new home build in Bluffton SC and we are conflicted about which type insulation is best for the attic so that it doesn’t heat up too high especially during the hot summer months. Builders are telling us different aspects of the proper insulation. We want to prevent the air handlers in the attic to not be working extra hard to cool the house. But, we also want the attic to breath. How can we accomplish both? Is foam insulation in the roof rafters good to keep attic cool? If so, then how dies the attic exhaust the moisture build up? We want to do the right thing in the beginning to prevent any future problems. We feel that we need to know just as much and if not more than the builder who is trying to sell their contract. Thanks for your honest advice and recommendations.

Ray Thornburg | Fri, 10/18/2019 - 14:40

Thank you fo your question.  Modern insulation standards are enough for for attics in this area and the HVAC system should be sized and calculated by your HVAC technician based on the size of the home, insulation and other factors. There is a lot of science in this. If the builders are giving you a choice of insulation types then you may get different opinions from contractors based on their business model. My personal preferance is blown in cellulose applied on top of the top floor ceiling. It stays in place better than the blown in fiberglass ( which floats everywhere and tramples easily)  and it doesn't irritate my lungs when I crawl into the attic. Again my opinion here is to avoid foam on the underside of rafters (creating an encapsulated attic). In my opinion it serves no useful purpose.  Foam will hide the condition of the roof sheathng if there is a leak. Most roofs do experience a leak eventually. Without the drying effect of a ventilated attic a small leak can be hidden (behind the foam) and a lot more sheathing damage can occur. Also most of the encapsulated attics I've seen have had issues with moisture buildup in that space making a dehumidifier necessary. However this isn't discovered until after problems arise. Hope this helps......

Anonymous | Wed, 07/17/2019 - 04:23

Hello. I have a marsh facing home on Saint Helena Island, SC, that was built to code in 2001-2002. The existing fiberglass insulation is ragged and the staples have rusted and the tape is loose. Condensation forms and drips from the underside of the ducts. In addition, squirrels have gotten in from time to time and just shredded the existing fiberglass insulation, so I would like to avoid a repeat of that. What is your opinion of closed cell foam insulation for duct work? Thanks in advance for you response. Great page you have here.

Ray Thornburg | Mon, 07/22/2019 - 19:27

My guess is that you're talking about duct work in the crawlspace.....My recommendation is to contact an HVAC technician so he can remove and/or re-insulate the ducts to modern standards. Condensation dripping from duct work means it isn't insulated well enough and depending on the type of duct,  replacement may be an option (metal ducts for instance could be rusted). Also wet insulation is not much good as long as it's wet and then there's the question of microbial growth. After the repairs are done there may be some benefit to spray foam in this instance especially at the boots. In general I'm not huge fan of spray foam. If this is a tight crawlspace placing a quality vapor barrier (6 mil or better) on the ground will reduce the amount of moisture in the air so will reduce condendation that way.

Hope this helps....

Anonymous | Mon, 07/08/2019 - 06:17

Good Morning, I am currently renovating my bathroom, I am installing a schluter products shower and re-tiling the foor and walls up 47", where I will switch to drywall. I am also installing a new window. Currently, I am completely down to the studs on the wall and ceiling and I am installing Rockwool comfortbatt on the exterior walls and ceiling, as well as using Rockwool safe n' sound on the interior walls. I live in Summerville, is it neccessary to install a vapor barrier on the interior of the exterior walls? If so, what is the process here? Thank you for your time!

Ray Thornburg | Tue, 07/16/2019 - 14:10

I apoligize for the delay in answering.

Yes install a vapor barrier on the warm in winter side of the wall. Sometimes the insulation comes with paper backing which acts as the vapor barrier. The edges of the paper are stapled through to the studs.  If not they sell thin pastic sheeting for this. Just staple it on. Hope this helps....

Anonymous | Thu, 04/18/2019 - 05:54

Great write up Ray. I am new to this climate/area. What are your thoughts on wall and ceiling vapor barriers/retarders? Lots of opinions on this ...I value yours. Thank you, Eric

Ray Thornburg | Fri, 04/19/2019 - 18:24

Thanks Eric....It is not necessary or desirable to have a vapor barrier on the ceiling in this area. Most attics have blown insulation directly on top of the sheetrock which is fine..... and the attic is ventilated so moisture can dry out via the attic. If you have to use roll insulation unfaced is better....if it has a paper facing it's ok but it must face the warm in winter side of the ceiling. On walls the paper facing is always towards the warm in winter side of the wall. Some bulilders will put a thin sheet of plastic up instead which is ok.

On floors some of the more modern simulated hardwood flooring products are very restrictive as far as vapor transmission is concerned. So if you have a crawlspace watch out. For instance the old fashioned hardwood floor over a crawlspace would allow drying from both sides. If faced insulation was installed it could only dry from the inside for the most part. Add the simulated hardwood laminate and now the subfloor has no way to dry out. Trapped by the paper facing on the insulation from underneath and either the foam backing or top coating of the simulated hardwood flooring above. For this reason a house can do fine for many years and then undergo remodeling with modern AC and laminate floor coverings and start to have problems with trapped moisture in the crawlspace. Anyway hope this helps....Ray

Anonymous | Mon, 12/03/2018 - 10:53

I've got a marsh front condo on Johns Island built in the 80s and the crawlspace insulation is in need of replacement. Does the code allow/recommend to not use a vapor retarder in our area? I'm planning on pulling out the deteriorated fiberglass and replacing with rockwool for better moisture and pest resistance. And is a ground vapor barrier recommended? Thank you.

Ray Thornburg | Mon, 12/10/2018 - 13:33

I would say most places on Johns Island would benefit from a vapor barrier installed on the ground. The lower the home (floor framing) is to grade the more important it becomes. As far as the paper facing on the insulation is the crawlspace it can be eliminated...this can be considered if restrictive floor coverings (to moisture movement) is installed over the subfloor as a finish flooring. In my opinion anything that stops the natural drying process can trap moisture and cause moisture issues. The paper facing acts as a vapor retarder and sometimes moisture is trapped between the paper facing and a restrictive floor covering....add that to the desire to cool a modern home with air conditioning which exacerbates the issue. Each case if different so all options should be explored.

Anonymous | Wed, 10/31/2018 - 14:44

Could you please tell me both the code requirement and your recommendation for Open Cell Spray Foam Insulation in a renovated home? We'll be gutting down to the stud walls and using foam insulation in the ceiling of the attic space. Will also be removing the shingle roof and applying a vapor barrier before applying a corrugated metal roof. thank you!

Ray Thornburg | Wed, 10/31/2018 - 16:54

Thank you for your question.

My answer to this is my personal opinion based on my observations. I'm not a big fan of spray foam insulation (though I know it is marketed heavily) in wall, ceiling, and roof systems.  Under the roof it hides the sheathing which makes checking for leaks impossible.....leaks happen in most homes at one time or another and this leaking can continue until major damage has occured because not only is it difficult to see but the foam holds the moisture from evaporating as quickly. Also if your attic is unfinished it's a waste as the ceiling is the proper place for the insulation. I have inspected homes with spray foam under the attic rafters and those homes have needed a dehumidifier in the attic to control the moisture. Moisture is everywhere and in every breath we take. Proper home design takes this moisture from the ground, people, pets, plants, cooking, showers etc into consideration and allows for a natural process for this moisture to escape. With moderern floor finishes and air conditioned homes this process needs even more consideration. In walls moisture issues can occur too especially in areas where plumbing is located. I do like the application of foam in the crawlspace however just because it stays in place better. Though sometimes in a few instances it's application appeared a little thin. But again places were plumbing is located could be an issue if a leak developes. A small amount of water trapped in behind foam evaporates so slowly damage is amplified when it is finally discovered. The solution might be to stop the foam in places where piping emerge from under the floor for inspection purposes. Stuff some fiberglass in those places.

There also an issue with certain types of roof coverings, roof covering of all types do better if the roof deck is cool or can breath from underneath. The manufactures of wood shakes for instance advise against foam under the roof deck.

Controlling moisture in homes is a complex issue and caution is advised in any system which may stop the natural drying processes. Foam of all types can hinder moisture movement so you should take this into consideration.

The good thing about foam is it does appear to seal around penetrations etc. better. But even that is contriversial. Now that your house is sealed like a drum you'll want some make up air to breath.

Anonymous | Thu, 10/18/2018 - 04:21

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Ray Thornburg | Tue, 10/23/2018 - 05:42

Thank's alwasy nice to hear a kind word....