Partial loft conversion - insulation issues

Bendy_House

All I know about the three inches is from when I looked under the loft floor from the eaves. Sticking a tape measure down (where you can see the cables laying) is three inches. Whether there’s another layer under that I can’t see I don’t know?

I am thinking now it’s not that big of a job to take up the carpet tiles, unscrew the boards and put more insulation down (new and better stuff maybe), make sure it’s all filled.  I did wonder if i can just screw on some more 2inch high wood batons onto the joists and then have a 5inch gap to insulate, and just replace floor on top. Don’t know if that would be stable and strong enough to walk on though. (I’m quite capable of cutting wood, screwing and unscrewing etc. )


I don’t think I’m fully understanding the physics though.

Why would underfloor soft insulation work to keep heat in the house below, but insulation board would heat up the loft? Only if anyone can be bothered explaining…

Tee-hee - that will be down to my attempt at an explanation :-)

If you could get insulation board into that 3" gap, then - yes - that would keep the heat in the rooms below, just as stuffing loft insulation in there would. And better, since rigid insulation board has a higher insulation value than 'wool' loft lagging.

What I thought you were saying was, you were going to lift the carpet tiles, lay the insulation on top the loft room floor, and then replace the tiles. If you did this, the 3" gap would remain, so it wouldn't help the rooms below. But it would help the loft room (a wee bit...) by insulating its floor.

Ok, options for things you can do, to help the main house:

1) Stuff 'wool' loft insulation down that 3" gap until it is absolutely filled. Then increase the layer of loft insulation in the eaves, in and over the horizontal joists (ie the joists that make the ceiling of the rooms below) until it is 300mm thick. That should be a huge step forward, and might be all you need to do. NB: as said above, you really need access to that other eaves space too, so you can (a) stuff loft insul in from that side, and (b) also add more to that area if it's needed.

If you do this, then in theory you will have significantly insulated the whole ceiling of the rooms below. This should be 'easy', well within your remit, and cost very little. (The only tricky bit would be making an access hatch in the other knee wall.)

2) Yes, if you were to completely lift the loft floor, add 2" (or more) battens along the tops of the existing 3" joists, then that would increase the amount of loft insulation you could stuff in there. I think this would be more work than you imagine. However, if you do decide to do this, then I wouldn't add any extra height to the joists, but just fit rigid insulation in there instead - that would be just as, possibly more so, effective.

What I would do is to hoover-up, carefully lift the cables out of the way, cut as accurately as possible the Celotex (or similar) to fit snugly down between the joists, and press it down until it's in contact with the downstairs ceiling. Any gaps between the sheets and the joists, gently fill with a expanding foam. Ditto any gaps between the sheets themselves - run a wee bead of foam on the edge, and butt them together. Don't worry about any foam that extrudes out - trim it if it'll interfere with the loft floor when you put it back.

Lay the cables back down on top of the Celotex. 

Make sure the eaves get the full 300mm loft insulation. Superb job done.

2 will be likely better than 1, but a LOT more work, and I'm not sure if it's worth the effort. I wouldn't underestimate the effectiveness of just 3" of loft insulation in this - your specific - case, because it'll (a) be well-stuffed!, (b) have very little air flow over and through it (as it's well stuffed, and in between top and bottom panels), so I think would act more like 6-odd inches of 'normal' loft insulation. And (c), the sandwich of this well-packed 3" of loft insulation and the loft floor above (18mm chipboard?) will, to my mind, be FAR more effective than 3, 4, 5 even 6" of loose-lay loft insulation.

But, you still need to get in to that other eaves space.

Finally... in the eaves, make sure you do not block the very outside edges of the sloping roof, as this is almost certainly providing the ventilation to the loft voids - essential.

markin

In a new loft conversion it would be 100mm Celotex between rafters and 50mm across them making 150mm Celotex that is about = to 270mm of fibreglass, So i defiantly think its worth raising the floor up to 7 or 8 inch for 200mm of fibreglass or 150mm of Celotex board.

Also do you have ridge tile vents?

barnaclebill

I have just read this post with interest as I am in a similar poition except mine is a bungalow and loft has a full staircase with a large open landing, two bedrooms with radiators and a toilet with hand basin. The larger bedroom has a hatch in the end wall and I can see the rest of the loft extending to the gable end is standard with rolls of insulation between joists, cant see what is behind the knee walls. The whole foor area is capeted wth underlay.

The bugalow is 1950's with cavity wall insulation and I understand the conversion was addded in the  1980's with 3 velux windows.

I was thinking if I closed the door to the downstairs lobby for the bottom of the stairs and closed off the two upstairs radiators and only lived in the original bungalow area would this save on heating costs.

The downstairs is mostly laminate in the hall, lounge and bathroom, tiled kitchen and carpeted main bedroom.

Sorry for jumping on this post but it is very similar to my situation.

Brebal

Bendy_House said:

All I know about the three inches is from when I looked under the loft floor from the eaves. Sticking a tape measure down (where you can see the cables laying) is three inches. Whether there’s another layer under that I can’t see I don’t know?

I am thinking now it’s not that big of a job to take up the carpet tiles, unscrew the boards and put more insulation down (new and better stuff maybe), make sure it’s all filled.  I did wonder if i can just screw on some more 2inch high wood batons onto the joists and then have a 5inch gap to insulate, and just replace floor on top. Don’t know if that would be stable and strong enough to walk on though. (I’m quite capable of cutting wood, screwing and unscrewing etc. )


I don’t think I’m fully understanding the physics though.

Why would underfloor soft insulation work to keep heat in the house below, but insulation board would heat up the loft? Only if anyone can be bothered explaining…

Tee-hee - that will be down to my attempt at an explanation :-)

If you could get insulation board into that 3" gap, then - yes - that would keep the heat in the rooms below, just as stuffing loft insulation in there would. And better, since rigid insulation board has a higher insulation value than 'wool' loft lagging.

What I thought you were saying was, you were going to lift the carpet tiles, lay the insulation on top the loft room floor, and then replace the tiles. If you did this, the 3" gap would remain, so it wouldn't help the rooms below.

Oh, I was saying that first 🙈. Sorry for the confusion!

I think I’ll get a joiner to make a door into the other eaves. More sensible than me knocking it through. I’ll fully insulate both sides like you describe. 

Then maybe reassess. I get that the rest of it may be a bigger job than I think. I have been known to start jobs then wish I hadn’t 🤣. I like the idea of using boards under there instead of the soft insulation but looks like it will be pricy. Would have to measure up. If I’m going to do it/have it done, I want decent stuff, not just the cheapest that won’t be worth it. 

Brebal

markin said:

In a new loft conversion it would be 100mm Celotex between rafters and 50mm across them making 150mm Celotex that is about = to 270mm of fibreglass, So i defiantly think its worth raising the floor up to 7 or 8 inch for 200mm of fibreglass or 150mm of Celotex board.

Also do you have ridge tile vents?

I had to Google ridge tile vents but I’m not sure if I have them or not. I can’t tell by looking at a photo of the outside (I’m not home at the moment).

markin

barnaclebill said:

I have just read this post with interest as I am in a similar poition except mine is a bungalow and loft has a full staircase with a large open landing, two bedrooms with radiators and a toilet with hand basin. The larger bedroom has a hatch in the end wall and I can see the rest of the loft extending to the gable end is standard with rolls of insulation between joists, cant see what is behind the knee walls. The whole foor area is capeted wth underlay.

The bugalow is 1950's with cavity wall insulation and I understand the conversion was addded in the  1980's with 3 velux windows.

I was thinking if I closed the door to the downstairs lobby for the bottom of the stairs and closed off the two upstairs radiators and only lived in the original bungalow area would this save on heating costs.

The downstairs is mostly laminate in the hall, lounge and bathroom, tiled kitchen and carpeted main bedroom.

Sorry for jumping on this post but it is very similar to my situation.

I think its 50/50 if they left the original 50's insulation in place, you would have to get in and have a look, But yes closing of un-used rooms will save a little gas.

barnaclebill

Thanks for that Markin I have now posted this as a separate post about my Bungalow, I have taken photos and can see there is  good insulation behind the knee walls but no chance to see what is under the floor.