Thread: Sub-Floor Insulation under floorboard - Good Idea?

I have a few bags of R2.5 and R3 batts that I am thinking of using under my subfloor.

I have tassie oak polished floorboards and good under floor space. I was thinking of using these batts under the floor by simply tucking them in between the joists and holding them in place using either metal strips, wire, or masonite strips.

I can see that type of insulation method is relatively popular, but I have also read some reasons why not to do it. Is this a good idea? Do the floorboards need to "breath"?

The way I see it is that it may take away some of the benefits of the sub-floor cooling in Summer. However it should be quite advantageous in winter to eliminate some of the cold and drafts getting in between some small gaps in the T&G joins.

Any opinions?

Yep good idea. See here for reasons:

http://www.yourhome.gov.au/technical/fs47.html

Not sure what you mean about floorboards needing to 'breathe' - in any case batts are full of air (that's how they work - the fibres maintain a trapped layer of still air) and even in very moist climates will not effect timber moisture content.

Just do it . . ./

I'm with Bloss, I cannot see any reason not to do it

Regards Bradford

Well this is good to hear considering I have the batts sitting under the house in bags and that it is a simple job to do.

I was just wondering if there were any other implications in doing this that I may not have thought of. For example any moisture implications by effectively "sealing" the floorboards with the batts. Pest issues? Will accidental spills sit under the floorboard and eventually rot the timber? Fire risks. Etc. If not, then I may just be under the house next weekend.

I will probably try and do some of the bedrooms first to see if I can actually tell the difference in these rooms compared to uninsulated rooms. At the moment the floors certainly seem quite cold to the touch.

Thanks for the responses. Much appreciated.

The only thing that might be a problem is electrical cables.
Insulation prevents the heat build up in the cable from dissapating, therefore when cables are surrounded by insulation they are derated. If no electric cables no problem.

Regards Bradford

Originally Posted by BRADFORD

The only thing that might be a problem is electrical cables.
Insulation prevents the heat build up in the cable from dissapating, therefore when cables are surrounded by insulation they are derated. If no electric cables no problem.

Regards Bradford

Yeah.. something I will have to look at. Perhaps just leave about 50mm gap around cables. Anyone know of a standard?

Good advice from Bloss and Bradford, Gooner.

You can get very cheap "top hat shaped" plastic extrusions - about 20 - 25 mm square, that you simply staple or screw over existing cables to protect them from insulation. Best not to leave holes in insulation coverage as the effect is much greater than the size of the gap. Heat always flows from hot to cold.

Unless your underfloor area is very open and windy then R=2.5/3.0 may be more than recommended for Melbourne, but you never can have too much insulation.

The effect of underfloor heating on in-room comfort is somewhat similar to the effect of double glazing in that it increases the comfort level much more than the R-level of the insulation would suggest. The reason is that the in-room surfaces - of insulated floor and/or double glazing - remain warm. Cold interior surfaces in a warmed room help create and maintain in-room drafts. Drafts are movements of air, otherwise known as wind, and moving air contains a wind-chill factor even when the air is warm.

This air movement or draft effect is also why it is so important to seal all leaks in a room to stop infiltration. If you do have significant air movement through your floor boards then it might be worthwhile looking at putting sarking on your underfloor joists.

Cheers

Graeme

Originally Posted by GraemeCook

Good advice from Bloss and Bradford, Gooner.

You can get very cheap "top hat shaped" plastic extrusions - about 20 - 25 mm square, that you simply staple or screw over existing cables to protect them from insulation. ......

Thanks for the response Graeme. Any idea where to obtain these "top hat shaped" extrusions and what their "proper" name is? I am assumming my local electrical retailer would sell them?

Originally Posted by Gooner

Thanks for the response Graeme. Any idea where to obtain these "top hat shaped" extrusions and what their "proper" name is? I am assumming my local electrical retailer would sell them?

I just got them from an electrical wholesaler when I upgraded my insulation about ten years ago. They were quite cheap and flimsy but did the job. Initially I tried to staple them but my ceiling joists are 130 year old hardwood and the staples bent - had to drill and screw in 8 mm screws. They look like this, but wholesaler had several designs, all more expensive.

Cheers

Graeme

Whats a .SKP file?

Originally Posted by Gooner

Whats a .SKP file?

Sorry, its Google Sketchup.

Its a free download that does architectural/engineering drawings. Quite easy to learn and very useful.

Cheers

Graeme

I have just done mine using R2 wallbatts (the stiffened ones) and packaging tape attached to the underside of the joists as support. A pr*ck of a job!

I think every bit of insulation makes a marginal difference; it isn't until all surfaces are adequately insulated that the total effect is noticeable ... starting with the ceiling/roof, of course.

I note that CSR/Bradford are now advertising a rockwool/fibreglass blend for post-construction underfloor insulation, the idea is that they are stiff enough to jam between joists. Don't know how well they'd work without additional support.

Don't worry too much about the cabling - just ensure that you have correctly rated circuit breakers not fuses.

Originally Posted by Bloss

Don't worry too much about the cabling - just ensure that you have correctly rated circuit breakers not fuses.

Not too much cabling running across the joists anyway. Most of it is along the bearers. I wouldn't think it would have made much of a difference.

I just came back up from under the house. Doing a little bit at a time. It's not the most pleasant job, especially when the batts are old ones that were on the roof and therefore covered in dust.

Compleat Amateu, how are you using packing tape? How exactly are you securing them in with packing tape? The way I see it, may as well not use anything at all. Isn't it a bit flimsy?

Another thing is that I have some old glasswool/fiberglass batts and some polyester ones. The polyesters ones are a joy to work with compared to the old fiberglass ones. Mind you, the polyester ones are only about 5 years old whereas the fiberglass ones are probably 30. I think I am going to ditch the old fiberglass ones as they are extremely aggravating to work with.

Hi, I am bringing this thread back to life because it is relevant to my situation but I don't really understand everything that has been said

I have relocated an old farm house with baltic pine boards, and want to insulate the sub floor.

There are many wires running underneath the house, everything has been checked by the electrician, do I still need to 'protect' the wiring?

I had a quote from a company who spray on a hot liquid and it converts to a foam. Does anyone have any ideas on this product?

Thanks in advance for any help!

Covering houses wiring with insulation is generally OK so long as the protection is correct (covering 'derates' the circuit. That means using contact breakers of the correct type and rating not fuses. If in doubt get a sparky to have a look. The foam insulation works well, but is an expensive option for underfloor.

I agree with Bloss, spray foam is a good option, but quite expensive up front. One of the advantages that it does have over other methods though, is that it stops drafts by effectively sealing the underfloor where it is sprayed

Whereabouts can you buy subfloor insulation from in Melbourne

I dont think sub floor insulation is any different to ceiling insulation.
The spray foam is brilliant cos it flls up gaps and stops drafts, but whenever you need to rewire , it will be almost impossible to do without surface condiut. Even adding a power point later will be difficult. Thats the only downside of spray foam.

I used concertins foil batts as under floor insulation during construction of my extension and recommend it. Our home is all poilshed Tas oak, and the extension floor is quite noticably warmer in winter and cooler in summer than the original house. Vew the Full Story video for a quick summary . Concertina FOIL BATTS - foil insulation - main page. Made in Cheltenham, Melbourne.

Hi, I notice some of these posts are a little old but thought I would add my 2cents worth on a couple of things that spring to mind.

The question previously with regards to glass wool Vs polyester bats. When we purchased them the fellow said it was more problematic around wiring and more flammable in the event there was a problem.

In addition, we recently purchased some Green Insulation (glass wool style) and there's a notice on it with regards to putting it over wiring pre-1989. So I can only assume that there was a change to the way house wiring was done - something to do with the circuit breakers perhaps. So in summary if the wiring is pre 1989 I wouldn't think placing the batts too snugly around them would be advisable.

Hope this helps a little. Atleast now that the housing insulation program has gone belly up the price of batts has plummetted. R4 batts for around $35 to $45 which is a nice change

Edited to fix grammar.

The simplest way to fit bats between floor joists is to use old 2" venetian blind slatting.Can be stapled or nailed and being aluminium will last. Good luck. BIG BILKO

An electrician saying it is ok to insulate - that's what I value.
Then I would just go ahead.
I understand they check the thickness of the cable, and the value of the circuit breaker, and then look up a table; and perhaps replace the circuit breaker with a lower rated one if needed.

Can anyone tell me the name of a few companies that supply spray on subfloor insulation, and the estimated cost compared to foam etc..

Google is your friend - Home Insulation, Insulfoam Solutions :: Spray Foam Insulation :: Sealection500 and Underfloor Thermal Insulation