Adding underfloor insulation

[ntl142]

The kitchen of my house is an extension out the back of a 1900 cottage. The suspended timber floor in the cottage is fine, but the extension floor is painful to walk on when it's cold. If you're barefoot it's like walking on ice, it stings your feet after about 10 seconds standing on it.

There is a composite tile floor on top of something like 30mm OSB. Between the two is 2mm of useless white insulation material. I dug a hole through under a cupboard to see what was underneath, hoping to find no insulation. What I found is shown in the photos. Is the blue stuff valid insualtion, or completely useless? Will ripping out out and putting something better in make a significant difference? One of the photos shows the hole I have to wriggle through to get under, so I'm a bit limited in what else I can fit through it as well. I'll also have to knock a few bricks out to get to the other side of the supporting wall.

There is a lot of ventilation under the floor, possibly too much. Should I consider blocking some airbricks? I don't think that would make much difference unless I totally sealed the void and that would probably cause other problems.

 

[endecotp]

There is a composite tile floor on top of something like 30mm OSB.

Looks like chipboard to me.

Is the blue stuff valid insulation

Can you read any of the writing on it? I'm not sure what it is, but maybe it's extruded polystyrene (XPS).
It does seem to have been fitted from below. It might have been more effective if sealed to the joists to stop draughts.

One of the photos shows the hole I have to wriggle through to get under

Got any small children? (Semi-serious suggestion.)

Lag the rest of the pipes while you're down there.

 

[ntl142]

Hi, thanks for your message. I agree that it appears to have been fitted from below. Couldn't see the writing on it, but I'll have another look and report back.

I can just about fit through the hole, but wouldn't be able to get any insulation boards through it unless in small pieces. If I had a child then they would be helping me for sure!

Yes, the pipes need lagging. This house seems to be one that has had a lot of care put into apperances, but as soon as you look behind the scenes then there's a lot to be desired...

Do you/anyone else know what thickness/type of insualtion should be in place in this application? I want to see how far off the mark the current insualtion is. If it meets guidelines/regualtions then I'd be better off investing time/effort elsewhere. If it falls far short then this would be a valuable place to invest.

 

[JP_]

I'd vote for more insulation - don't block air bricks - this part of the house might have more airflow than needed, but what about the 1900 part?

I am putting in 25m solid board insulation, sealed with foam, with 50mm rockwall on top, to make 75mm total. But, if cost was not an issue, I'd put in as much as I can fit.

 

[ntl142]

This part of the house doesn't seem to have a direct connection to the 1900s part, there are no obvious gaps. I'm assuming for now that it's a separately ventilated space. But yes, I agree with you and would rather not block off airbricks if possible.

I'm almost certain the blue foam is XPS (thanks endecotp). It's probably about 50mm thick, but I'll measure a bit.

Assuming I need to add some more insulation, are there any recommendations of what to use? Assume I have to do it from underneath, and I have to get everything in through that hole...

Should I seal between the blue foam and the joists? Would normal expanding foam work, or is there something neater for just filling in small gaps?

 

[JP_]

Hmmm, I am surprised that the floor is freezing cold with 50mm insulation, I'd expect it to be bearable at least. I'd guess that its either not very good insulation, or badly fitted with lots of gaps. If freezing, I guess the best option is to out the thickest boards in that will fit under the existing. Maybe fix them with small battens at right angle to joist - you could leave the screw a little loose so you can easily turn the battens to hold the insulation up. Then, seal with expanding foam. Would be a hell of a job from underneath though. How big is the room? How important to keep the existing tiles? Maybe rip all up, insulate well, put in some thicker boarding and cover with something like Karndean that is warmer than tile - I had this in my last house, was great, socks not needed in winter.

 

[endecotp]

The underfoot feel of a floor depends not only on its actual temperature; a carpet will feel much warmer than a stone tile at the same temperature because the stone conducts heat away faster. So changing the floor covering - I like cork, but not necessarily in a kitchen - may make sense.

If the cold air can easily move above the insulation into a gap below the actual floor then the effect of the insulation will be much reduced. Sealing with foam would be one choice but it might not be the most pleasant thing to use in a confined space. An alternative would be taping.

XPS is about equal to mineral wool and about half as effective as PIR (i.e. celotex) for the same thickness.

Here's another thread that you'll enjoy: https://www.diynot.com/diy/threads/underfloor-project-ventilation-and-insulation.474794/

 

[Doggit]

Unfortunately, adding more insulation isn't going to do a thing, as the problem is the tiles themselves. You'd put the insulation underneath if you were going to use electric UFH beneath the tiles, and that's stop the heat going downwards, but you've got no heating source, so they're cold.

You're best bet is to remove the times, and replace them with something warmer. If you're lucky, you can scrape the adhesive off of the chipboard, otherwise you'd need to overboard with 6mm ply, and then start from there - the choice will depend on your budget of course.

 

[ntl142]

Thanks for all your comments so far.

Just to confirm a few things, the insulation is 50mm thick (Dow Wallmate), so it's normal XPS. It's held in place with nails and there are definitely gaps that would benefit from filler/tape.

Unfortunately as it's in a kitchen I'd have to remove the units to redo the floor, which I'm not prepared to do at this stage. That would have to be a summer job. Even getting one tile up (under the fridge) was hard because they're tongue/groove and all the exposed edges have kitchen units on them. If underfloor insulation would help I'd do that now. I think if anything, I'd lay a new cork/tile floor covering over the top, but would have to skim the bottom of two doors to allow them to open. On the plus side the tiles aren't glued down if I did go down that route.

I'm trying to reduce heatloss from the extension. It's an insulated flat roof (not changing that just now). The only other thing I can tackle is the floor. Even with the heating on full and oven on, the floor is still freezing*. I thought that was because of heat loss through it. Perhaps not?

Maybe I'll put a carpet down for a few days and see if that makes a difference...


* I almost mean that literally! I spend a lot of time in the ice and snow so I'm used to the cold. If it's 0 deg C outside, this floor burns your feet!

 

[endecotp]

Better slippers?

 

[ntl142]

Perhaps. They work fine in the rest of the house though.

 

[Doggit]

If the tiles aren't glued down, then you just need to cut them with an angle grinder that has a diamond blade, you use a club hammer to break them. You could raise one of the front cupboard base legs, break out the tile, and then wind the leg down. The new floor only needs to go as far as the legs, and you might need to take a bit off of the plinth, but it should all be manageable.

 

[ntl142]

Just an update on this...

I got under the floor and added pipe lagging to the heating pipes. Much needed! I suspect there's none in the rest of the house, so might need to do that as well.

Also went around and taped every joint between insualtion and joists, and where insualtion boards butted up against each other. Thanks for the suggestions to tape - it worked.

Standing around barefoot on the floor before doing this, I was slightly suspicous that one half of the room was colder than the other, but wasn't really sure. Now I've taped the joints, I'm certain of it. And just to make sure, I had a look through the sleeper wall gaps, and guess what - no insulation on that side of the room! At least whoever was in the house before was consistent in their lack of attention to detail!

So the next stage will be to get through the sleeper wall and put some boards up on that side, and more pipe insulation. The photo shows the middle of the wall. Above this is an island in the kitchen so no one walks over the top of the wall. I'm sure that a few missing bricks there will be ok until I get around to putting them back in. Any recommendations for how to knock out the bricks (I have grinders, chisels, lump hammers, etc), and which/how many bricks to knock out? I'll need 4 bricks out (horizontal length), and as many rows as possible for me to squeeze through. Is that safe? Do I need to prop it up once I've got the bricks out?

Thanks.

 

[Doggit]

Work out where the centre of the island is, and take the bricks out below that. Hopefully, it'll be above just one joist, and with a bit of luck, it'll be above the joist shown in the centre of the picture. Now if that's the case, then after you've got through, I'd be inclined to just put in a piece of 4x3 or 4x4 to span the gape you've created, rather than trying to put the bricks back in.

 

[ntl142]

Thanks, sounds similar to my plan. I was going to knock some wooden uprights in after instead of replacing the bricks. Sounds like you're suggesting a horizontal bridge/lintel of wood, so I'll do that an add some uprights too. Will also add some under the washing machine as it's a bit wobbly...

As for knocking the bricks out, would a hammer and chisel be the best approach?