# Forum Home Renovation Structural Renovation  Insulation under timber floor

## topsie

Hello, 
We recently had to pull up the timber floorboards in our back room to do some serious pier work. The room itself gets pretty cold - it has two external walls, two large windows and French doors. Before the floor boards go back down, we want to insulate the floor (the walls and ceiling will be insulated as we go too). I was told that using yellow tongue underneath the timber boards is the quickest and easiest way to go. Is this true? And does the yellow tongue insulate well? 
Cheers,
Leah

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## woodbe

I recently came across this mob: http://www.foilboard.com.au 
Look pretty good, worth a look. 
Michael

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## sundancewfs

Couple of things you are trying to achieve by insulating....
Stop/slowdown air infiltration (drafts) and rug up warmly (R-rating)
Yellow tongue is probably not that high an R value but it would go some way to reducing air infiltration. Batts, foam sheet/boards or an Aircell type product would give you the insulation R value. If you've got the floor up, do it right. It maybe a long time before you get a chance like this again.....  :Biggrin:

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## Compleat Amateu

I've done this  - I had reasonable access from below, no lifting of floors - using batts supported by packing tape (the plastic non-sticky blue strip tape that goes around heavier cartons) stapled to underside of the joists.  Slow and uncomfortable fitting batts above your head lying on the ground, to say the least.  But it worked.  Use stiffened wall batts. 
I looked at the Foilboard product, but it is
a) Expensive
b) Has to be sealed carefully at the ends to produce an insulating effect - not that easy in practice 
Compleat

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## pawnhead

> I recently came across this mob: http://www.foilboard.com.au 
> Look pretty good, worth a look. 
> Michael

  That looks like an interesting product Mick.  :2thumbsup: 
I'll keep it in mind if I need something like that. 
Edit; Actually, I can think of a use right now. I have laserlite skylights in my family room and it's lovely in winter, but it gets unbearably hot on some summer days with direct sunlight streaming in. You can't sit under them. I'm going to put opaque perspex diffusers under them to spread the light, but it would still be nice to have some insulated panels that I could drop in place of the perspex when it gets really hot. A sheet of plasterboard with 10mm edge spacers, then a sheet of that foilboard would insulate the skylights really well, and the panels wouldn't take up too much room in a cupboard for winter.  :Smilie:

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## woodbe

Pawnhead, it might be even easier than that. 
Try making an infil for the skylight out of any material. 3-ply would be fine. Cover it in sisalation or any other RFL product with the shiny surface on the 'out' side. Paint the interior face to match your room. 
Bet it removes most of the problem as soon as you put it up. 
An even more effective, but slightly more hassle approach would be to tie a space blanket outside the skylight in summer. This would stop all the radiant heat from entering. 
woodbe.

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## seriph1

if you add yellow tongue to the equation it will raise the level of your floor, which might cause other problems  -  I reckon fixing aircell directly to the floor joists and nailing your floor on top will do the trick nicely  -  possibly their insulair product but you can see it all on their site. I enquired about this or a similar product through eBay a couple of months ago  -  from memory it was around $5 per square metre  www.air-cell.com.au

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## Bloss

Cheapest and best bang for buck with such easy access is batts - look for them on special and get the R-value that fits neatly to the depth of the joists without too much compression - that will be around R-2.5. Add the timber flooring at around R-0.6 and you have good insulation for the underfloor. Adding another layer of flooring whether yellow tongue or other will raise the floor heights and only add another R0.6 or so to the thermal barrier - not cost effective at all. 
To hold the batts in place use the strapping tapes used by A/C heating guys to hold up ductwork or any wire or tape that will not tear or rot - just twist in a zig zag winding onto soft sheet nails tapped into this lower sides of the joists. Just to stop the middle sagging really (which I could do that for my middle!  :Biggrin: ) - the sides should be an interference fit against the joists.  :2thumbsup:

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## GraemeCook

Hi Leah 
Bloss knows about insulation and cannot fault his logic.   Although I have never seen an adverse case, I would worry about the long term viability of stretched wires or straps under the batts - over an extended period od time will/may the batts sag past the wires and cause heat leaks?   I do not know the answer to that. 
A possible refinement would be to staple sisalation under the joists to hold up the batts - this would add a further R=0.6 to the insulation value and would also reduce the infiltration mentioned by Sundance. 
I do not know about cooling mode, but in heating mode :Redface: 
* If underfloor space is enclosed then underfloor insulation is of marginal value in terms of heat saved per $ spent.
* If the underfloor space is open (eg house on stumps) then it is quite cost effective, especially if you can minimise infiltration.
In either any case, underfloor insulation increases the floor contact temperature and makes the room feel more cozy - I think that is because it reduces in-room drafts caused by a cold floor. 
Cheers 
Graeme

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## Gaza

we used foil board once, turned out crap. 
to hard to cut in around the old joists & the stick tape has no thermal rating at all. 
it also built up moiuster between the underside of the flooring and the foilboard.

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## Bloss

> Hi Leah 
> Bloss knows about insulation and cannot fault his logic.   Although I have never seen an adverse case, I would worry about the long term viability of stretched wires or straps under the batts - over an extended period od time will/may the batts sag past the wires and cause heat leaks?   I do not know the answer to that. 
> A possible refinement would be to staple sisalation under the joists to hold up the batts - this would add a further R=0.6 to the insulation value and would also reduce the infiltration mentioned by Sundance.

  I've used and seen other use plastic straps that have been in place over 20 years now and my own has part doen with wire in a hard to get to part with low clearance and that has been in for 12 years with no sagging at all - the zig zagged wire is at approximately 45 degrees so batts are sitting only on 3 or 4 pieces. I used pretty light wire - around 12g I think, but maybe only 16g - I'll check. 
Batts are only holding themselves up and they are most air  :Smilie:  and supported at the sides by friction against the joists. In my own house I have about half the underfloor held up using  R-0.6 sisalation stapled to the bottom of the joists as Graeme describes and it has been very effective - and cheap as chips. I used tape to ensure that all joins were sealed too. How much better is not apparent from using the house areas above, but as the air is the main factor   
[QUOTE=GraemeCook;760767
I do not know about cooling mode, but in heating mode :Redface: 
* If underfloor space is enclosed then underfloor insulation is of marginal value in terms of heat saved per $ spent. 
* If the underfloor space is open (eg house on stumps) then it is quite cost effective, especially if you can minimise infiltration. 
In either any case, underfloor insulation increases the floor contact temperature and makes the room feel more cozy - I think that is because it reduces in-room drafts caused by a cold floor. 
Cheers 
Graeme[/QUOTE]   Underfloor space has to be well sealed not just closed to not get a very measurable change by adding insulation - few houses are. But as Graeme says simple gap sealing from above or below is a very effective and cheap way of stopping heat loss. Underfloor insulation is not great value in hot summer climates - it is for cool temperate ones like Canberra and anything south of there. Doesn't make it worse although as I have pointed out before if you create a well insulated box to keep heat in the winter and to reduce heat getting in from roof and walls in the summer you MUST follow the rule that you open all windows and doors and get air flowing as soon as the outside temperature in the summer evening is at or approaching the inside temp. If it is a hot night then get air flow going at night anyway - and in the mornings you close heavy curtains, drop down external shades etc.

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## jatt

Thinking about using concertina foil batts under my floor, although my concern is the same issue Gaza bought up   

> we used foil board once, turned out crap. 
> to hard to cut in around the old joists & the stick tape has no thermal rating at all. 
> it also built up moiuster between the underside of the flooring and the foilboard. 
> <!-- / message --><!-- sig -->

  Havent done any costings yet, but from casual observation it looks fairly simple, especially with the obvious probs one encounters when insulating this area of the house.  Happy to hear feedback, both good and bad on this product.

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## Bloss

The best underfloor I have installed in terms of ease of install and effectiveness is the polystyrene. If you are a DIYer and have time batts are cheaper and work well, but the polystyrene work well, but are dearer.
The type I helped install is from this mob: http://www.expol.com.au/index.htm but there are others. It's been in place a couple of years and has been very effective. Very easy to remove for access etc too. They are R-1.4 so with a bare timber floor that gives around R-2 - for most parts of Australia all you need anyway.   :2thumbsup:

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## ausdesign

Retrofitting ( or even initially installing) insulation to timber subfloor areas is a major problem which still needs to be effectively addressed by insulation manufacturers.
The foilboard system with joist brackets seems to be favourable with many designers.
A major problem with 'batts' is the sagging that can occur plus the opportunity for mice etc to set up home in the insulated space.
The addition of extra flooring, even though it may have an additional R value, does not come into the equation.
In cooler climates I would be looking at a total R value of around 2 for effective insulation which can be achieved with batts, extruded poly or a board type product.
The main problem is how to keep it there.

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## seriph1

I do like the concertina foam idea  -  may use it for our place depending on cost and our capacity to actually get under our damn house!  
I researched using spray foam insulation and while I think it would do the job best for retrofitted underfloor insulation being a total seal solution, from memory the price was around $30-$35 per square metre which seemed extremely high to me

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## Gravy258

I put these under my floor about  18 months ago. 
They work well in winter, like a heated floor (house is all floorboards) 
there has to be a gap between the floorboard and the foil for it to work. 
they are stapled taught to the bottom of the rafter, so nice big gap.  
theres a 50mm overlap on each batt, theres a gap to allow air in and moisture out. 
I crawled under the house at Easter to fix leaks the plumber had left me with, no prob's with sagging. 
Aircell also make a product called retroshield. this is like a foam camping mat foil coated. The retro bit being tiny holes punched in to allow moisture control.  
Renmark make concertina foil batts. Google em and have a look.  
Theres been plenty of similar posts to this one so have a search of the forum for more solutions. 
cheers 
Gravy

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## Richard_1983

I just spent 2.5 under my house (stumped) yesterday insulating one room (my reno is room by room).
I decided to use batts held up with chook wire stapled to the joists.
Bought an electric Ozito stapler for $36 which held up great. 
But my God, the job was horrific!!! I'm still getting flashbacks! Clearance varied from 300 - 500mm. Had a spider crawl across my face! Must a swallowed about a kilo of dust. 
Dreading doing the rest. might look into strapping idea next time cause I guess you could do it in 2 stages... loosely strap all the joists first then get the batts in and tighten, cause I was finding it hard to hold the batts place while getting the wire underneath to staple... 
Made a massive difference with drafts though... so it will all be worth it... as soon as the nightmares go away...

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## bugsy

> Dreading doing the rest. might look into strapping idea next time cause I guess you could do it in 2 stages... loosely strap all the joists first then get the batts in and tighten, cause I was finding it hard to hold the batts place while getting the wire underneath to staple... 
> Made a massive difference with drafts though... so it will all be worth it... as soon as the nightmares go away...

  mate
staple through the batts into the floor or side bearers first
that holds them up :Clap2:

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## Bloss

> mate
> staple through the batts into the floor or side bearers first
> that holds them up

   :What he said:  works well and is easy to do by one person.

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## Richard_1983

yeah I figured this out towards the end....
wouldn't go as far to say it is an easy thing to do  :Smilie:

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## Ramoun

I used expol about 12 months ago, there is a noticable difference in the house.  Installing was a pain in the rear.  Probably would not do a retrofit again though, unless the under floor space was a lot bigger. If it was a new floor and you have easy access to the joists I would definitely do it as long as you have unsualtion in the walls and roof. 
I measured a 2 degree difference between areas with insulation and areas that wern't

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## Nifty1

I went through the same thing last year - floorboards up, limited budget, not much underfloor space. I used the concertina foil, which can be fastened from above (staple it to the sides of the joists) and I think it cost $400 for about 7 squares. It's worth buying a power stapler though. 
One tip. I found that the foils would "fly" off the bearers that they rest on in certain breezes, and then land again making a distinctive sound that quietly rippled across the house. Not loud, but still intrusive. I ended up crawling under and putting some gobs of liquid nails where it mattered and this fixed the problem. For you, it would be easier to simply staple the foil down to the bearers as well as to the joists when fitting. I must write to the manufacturer and recommend that they add this info to their fitting instructions....

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## LeoAU

> The best underfloor I have installed in terms of ease of install and effectiveness is the polystyrene. 
> The type I helped install is from this mob: Expol Underfloor Insulation from Foamex Group Pty Ltd - For New & Existing Homes but

  I am unclear about what happens at the end of the joists? That is, on the perimeter walls, where bottom plates sit on the joists, using expol, aren't we end up with having gaps there? And the cold draft just blows through under the house? Hope this makes sense- the profile of these panels creates an air gap, which runs from one end of the house to the other end, between any given two joists. But then this gap 'extends' to the outside world, doesn't it??  :Shock:

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## blute

Aircell 
This is fast and easy to install, especially if you have the floor up. Would definately recommend it. Have also used the concertina foil stuff but I much prefer the air cell for ease of use.

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## RumpledElf

I'm fishing to build a new house and underfloor insulation is compulsary - I'll be building a timber frame weatherboard transportable on concrete stumps. 
From the quotes, it looks like they use R1.5 batts held on with plywood, and are going to charge me $2000 for the privelege.

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## Roadhouse

> Cheapest and best bang for buck with such easy access is batts - look for them on special and get the R-value that fits neatly to the depth of the joists without too much compression - that will be around R-2.5. Add the timber flooring at around R-0.6 and you have good insulation for the underfloor. Adding another layer of flooring whether yellow tongue or other will raise the floor heights and only add another R0.6 or so to the thermal barrier - not cost effective at all. 
> To hold the batts in place use the strapping tapes used by A/C heating guys to hold up ductwork or any wire or tape that will not tear or rot - just twist in a zig zag winding onto soft sheet nails tapped into this lower sides of the joists. Just to stop the middle sagging really (which I could do that for my middle! ) - the sides should be an interference fit against the joists.

  Sorry to hijack this thread but Bloss, I am interested in your thoughts on compressing the batts. I am thinking of putting batts under our main bedroom which has floorboards and I have access to R3.5 batts, but they are 185mm, which I reckon will be anything up to 50mm too thick for my joist depth.  
I am taking it from your response that you shouldn't compress the batts much/at all? Can you explain to a newbie why not? I have read that compressing changes the R value but is that a good or bad thing (I'm guessing it would increase??). Does it not allow enough ventilation if compressed?  :Confused:  
Would compressing a 185mm thick batt to say 130mm be the wrong thing to do?  
Cheers

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## Gooner

The purpose of batts is to trap air in the fibers thus providing the insulation effect. Same thing as wool or fur on animals. By compressing the batts you reduce the amount of air trapped within the batts and therefore reduce the insulation properties. Extend this concept further and imagine you compress them down to pancake width. Now no air is trapped within the fibers and you are basically left the insulation properties of a thin layer of fiberglass. 
I have insulated my subfloor in various rooms and personally I do not see a difference. (I have 19mm floorboards and good clearance under the house). I have an infrared thermometer capable of 0.1 deg readings and I see virtually no difference in floor temperature between the rooms that are and are not insulated. I used R2 batts.  
Good insulation in the ceiling is another story. I had the roof re-insulated with R4 batts and the difference has been very significant. 
If you fit them with straps nailed/stapled to the joists you probably will not need to compress them all that much, and even if you did, I doubt it will make too much difference.

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## Roadhouse

Thanks Paul - makes sense. Thanks for the explanation  :2thumbsup:  
I read a couple of other threads about the possible negligible effects of floor insulation so might think about this one a bit more. Are you talking about negligible difference when heating rooms, cooling rooms or both?  
Cheers

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## Gooner

> Thanks Paul - makes sense. Thanks for the explanation  
> I read a couple of other threads about the possible negligible effects of floor insulation so might think about this one a bit more. Are you talking about negligible difference when heating rooms, cooling rooms or both?  
> Cheers

  Both. Once again I often go around with my infrared thermometer thingy-mi-giggy out of interest sake. Whether it is summer or winter, the floor temperature is the same for insulated and un-insulated floors. Technically insulating your sub-floors should be more effective for winter months to stop the cold air under the house cooling your floors.  
I believe they say that putting in sub-floor insulation should contribute about 10% to your insulation energy savings, compared to around 50-60% for ceiling insulation. In my case, I certainly see where the 50-60% is coming from in the ceiling, but can't seem to quantify the 10% in the subfloor.

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## Bloss

Gooner is tight about batts - compressing reduces effectiveness and R-value - it is the volume trapped still air that is doing the work to prevent thermal transfer. 
Can't agree with Gooner on the under floor insulation and I too use a thermal imager - which shows a significant improvement (but I am in Canberra so my under house is much colder the the Melbourne climate so that might explain it). But as he says - not nearly as much as roof then walls and in most houses glass is the biggest culprit - windows in all seasons if exposed to sunlight coming in or room heat going out in winter. So shades externally or deciduous tress etc for sumer and heavy curtain with pelmets in winter - especially overnight. Venetians, vertical blinds, roman blinds etc might be pretty, but they are useless for thermal control. 
And in moderate climates I would simply use foil (sisalation) stapled to joists under floor - easy cheap and effective. I have batts and foil in my own home.

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## Roadhouse

Thanks guys - some good opinions. For the minimal cost of doing the rooms I need to do, might give it a go. Cheers

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## teknostar

I have a supply of R3.5 batts lying around. I am thinking of stapling them to the underneath of my timber floor in between the joists. 
The batts are free, so no cost there other than my time. 
Stapling them seems like it would be the easiest option. 
I understand that this would compress the batts where ever there are staples, and that this would obviously decrease the R Value. 
Has anyone done this? Is it worth the effort of crawling around under the floor to do?

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## ausdesign

A major problem would be vermin control.
the amount of stapling would not have a huge effect but obviously the best would be to keep the batts expanded as much as possible.

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## bugsy

> I have a supply of R3.5 batts lying around. I am thinking of stapling them to the underneath of my timber floor in between the joists. 
> The batts are free, so no cost there other than my time. 
> Stapling them seems like it would be the easiest option. 
> I understand that this would compress the batts where ever there are staples, and that this would obviously decrease the R Value. 
> Has anyone done this? Is it worth the effort of crawling around under the floor to do?

  i have done this
i stapled along the insides of the joists and a couple into the floor
more of a noise thing for me

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## teknostar

thanks for the replies, i will give it a try

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## Bloss

I've had batts under for yonks. Vermin control is same as always - ya gotta use baits and regularly.

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## teknostar

batts attached no problems at all with staple gun, the batts were not to crushed either. 
There was a noticeable difference in the temperature of the floor boards the next morning. They were warm to the touch rather than the cold that they were before insulating. 
I expect that this will make the living area of my house much warmer over winter - looking forward to it!

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## estwing

if  cellotex insulation is available over here it would answer your problems it comes in a range of thicknesses from 25mm to 150mm sheet size 2.4x1.2 it is compressed foam with foil on both faces . it is easy to cut just use an old saw.  :2thumbsup:

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## paulinspace

I've got plenty of space underneath as on a slope in the Dandenongs, Melbourne.  I'm keen to insulate under timber floors.  Its an older place so draughty, cold boards. 
My concerns are around:
1.  closing the gaps.  being an unenclosed subfloor there is a bit of wind gets under there on those windy days.  are there any products I should avoid that will not last (eg. concertina foil flapping around.
2.  vermin.  anything that creates space is going to attract vermin right?  any products that seem to keep them away (probably only the spray on stuff)
3.  termites.  i may need to be able to rip out the insulation to inspect for termites.  i use an inspector that has a dog so it would be only if the dog sniffed something out through the floor.
4.  ease of installation.  sounds like the polysterene is easy but someone else raised the question re how to seal the end of the gaps.  i had the same thought on my mind. 
bunnings sells a roll of silverbatts product.  cheapish at around $80 for 30m2 i think.  but it looks like it night be hard to retrofit as you would need to get the roll itself through past bearers and needs to be taut.  i am not an octopus so not sure about how to hold taut while stapling under floor.  also looks a bit fragile for exposed underfloors.

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## bigblue

Forgive me if this is a silly question, but along the same lines of this thread, if you were to put sisalation/sarking foil paper underneath your floorboards ( before they are nailed on to the joists ) .......would it actually do anything with regard to R value?? It _MAY help with the drafts but will it help insulate at all ???_

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## r3nov8or

With sarking the R-value is reduced as the draping effect goes from zero (between floor and joist) to say 90mm between floor and product then back up to zero at the next joist.  
The concertina foil batts discussed up in post #16 are better than sarking because they have a more consistent (although saw-tooth) gap between floor and product. They are best install before the floor goes down. I recommend them - have half a house insulated with foil batts and we feel the difference between the areas especially in winter.  
In general terms, any product that touches the internal barrier (e.g. floor boards, gyprock wall) will conduct a proportion of the heat, or cold, to the internal area and so reduce effectiveness. Concertina foil batts don't touch the floor...

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## bigbilko

> Pawnhead, it might be even easier than that. 
> Try making an infil for the skylight out of any material. 3-ply would be fine. Cover it in sisalation or any other RFL product with the shiny surface on the 'out' side. Paint the interior face to match your room. 
> Bet it removes most of the problem as soon as you put it up. 
> An even more effective, but slightly more hassle approach would be to tie a space blanket outside the skylight in summer. This would stop all the radiant heat from entering. 
> woodbe.

  Try 25mm flyscreen frames(cost about $30..Remove mesh and attach whatever you wish.We use roller blind fabrics,available in many colours.Will fit inside recess over summer and store away over winter.Simple and economical.  Regards Big Bilko

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## lawsmeister

> 4. ease of installation. sounds like the polysterene is easy but someone else raised the question re how to seal the end of the gaps. i had the same thought on my mind. 
> .

  I've just started installing the polystyrene stuff.  I've been using pieces of offcuts to form a seal at the open ends.Really only need one piece at the external wall and where ever you meet another obstruction.  Other than that the pieces all line up and form a continuous seal.

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## gray71

> I have insulated my subfloor in various rooms and personally I do not see a difference. (I have 19mm floorboards and good clearance under the house). I have an infrared thermometer capable of 0.1 deg readings and I see virtually no difference in floor temperature between the rooms that are and are not insulated. I used R2 batts.

  Mate you have it all wrong. Heat travels from hot to cold. You must measure the temperature difference of the floor under the house to measure heat loss(provided you are trying to reduce heat loss in winter) The insulated floor will not have as much heat transfer but will be hard to measure through the insulation,

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## donR

I used this Reflecta-Cell® greeninsulation.com.au foil/bubble cell insulation to make a near perfectly sealed air cell between the floor boards and the joists. Used silicon along the bottom of the joists and stapled the foil insulation to the joists. Worked a treat.  
I considered the polystyrene stuff but figured I wouldn't be able to get the stuff down the man hole.

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## woodbe

> With sarking the R-value is reduced as the draping effect goes from zero (between floor and joist) to say 90mm between floor and product then back up to zero at the next joist.

  Just to be clear - Sarking on its own does not have an R Value. If a manufacturer quotes one, they are measuring the effect of the airgap created by installing the product.  
Sarking works by high reflection and low emittance. It does not work like bulk insulation.  
Products like aircell and foil batts combine sarking and an airgap giving them an inherent R Value. 
woodbe.

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## indy_swift

Stick to a Foil based product under your floor. It most likely will act as a moisture barrier and an insulation material.

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